Reapply "Add more functionality to LLFile and cleanup LLAPRFile""

# Conflicts:
#	indra/llcrashlogger/llcrashlock.cpp
#	indra/llrender/llshadermgr.cpp
#	indra/test/CMakeLists.txt

8 files changed, 1474 insertions, 672 deletions

diff --git a/indra/llcommon/CMakeLists.txt b/indra/llcommon/CMakeLists.txt
index 597a222b32..cb3f0d7842 100644
--- a/indra/llcommon/CMakeLists.txt
+++ b/indra/llcommon/CMakeLists.txt
@@ -322,6 +322,7 @@ if (LL_TESTS)
   LL_ADD_INTEGRATION_TEST(lleventdispatcher "" "${test_libs}")
   LL_ADD_INTEGRATION_TEST(lleventfilter "" "${test_libs}")
   LL_ADD_INTEGRATION_TEST(llevents "" "${test_libs}")
+  LL_ADD_INTEGRATION_TEST(llfile "" "${test_libs}")
   LL_ADD_INTEGRATION_TEST(llframetimer "" "${test_libs}")
   LL_ADD_INTEGRATION_TEST(llhttpdate "" "${test_libs}")
   LL_ADD_INTEGRATION_TEST(llheteromap "" "${test_libs}")
diff --git a/indra/llcommon/llapp.cpp b/indra/llcommon/llapp.cpp
index 73b50d0a50..aa84895fb8 100644
--- a/indra/llcommon/llapp.cpp
+++ b/indra/llcommon/llapp.cpp
@@ -37,7 +37,7 @@
 #endif
 
 #include "llcommon.h"
-#include "llapr.h"
+
 #include "llerrorcontrol.h"
 #include "llframetimer.h"
 #include "lllivefile.h"
@@ -53,6 +53,8 @@
 //
 // Signal handling
 #ifndef LL_WINDOWS
+#include "apr_signal.h"
+
 # include <signal.h>
 # include <unistd.h> // for fork()
 void setup_signals();
diff --git a/indra/llcommon/llapr.cpp b/indra/llcommon/llapr.cpp
index 65ee383c6b..9ea7a34bac 100644
--- a/indra/llcommon/llapr.cpp
+++ b/indra/llcommon/llapr.cpp
@@ -526,226 +526,6 @@ S32 LLAPRFile::seek(apr_file_t* file_handle, apr_seek_where_t where, S32 offset)
     }
 }
 
-//static
-S32 LLAPRFile::readEx(const std::string& filename, void *buf, S32 offset, S32 nbytes, LLVolatileAPRPool* pool)
-{
-    LL_PROFILE_ZONE_SCOPED;
-    //*****************************************
-    LLAPRFilePoolScope scope(pool);
-    apr_file_t* file_handle = open(filename, scope.getVolatileAPRPool(), APR_READ|APR_BINARY);
-    //*****************************************
-    if (!file_handle)
-    {
-        return 0;
-    }
-
-    llassert(offset >= 0);
-
-    if (offset > 0)
-        offset = LLAPRFile::seek(file_handle, APR_SET, offset);
-
-    apr_size_t bytes_read;
-    if (offset < 0)
-    {
-        bytes_read = 0;
-    }
-    else
-    {
-        bytes_read = nbytes ;
-        apr_status_t s = apr_file_read(file_handle, buf, &bytes_read);
-        if (s != APR_SUCCESS)
-        {
-            LL_WARNS("APR") << " Attempting to read filename: " << filename << LL_ENDL;
-            ll_apr_warn_status(s);
-            bytes_read = 0;
-        }
-        else
-        {
-            llassert_always(bytes_read <= 0x7fffffff);
-        }
-    }
-
-    //*****************************************
-    close(file_handle) ;
-    //*****************************************
-    return (S32)bytes_read;
-}
-
-//static
-S32 LLAPRFile::writeEx(const std::string& filename, const void *buf, S32 offset, S32 nbytes, LLVolatileAPRPool* pool)
-{
-    LL_PROFILE_ZONE_SCOPED;
-    apr_int32_t flags = APR_CREATE|APR_WRITE|APR_BINARY;
-    if (offset < 0)
-    {
-        flags |= APR_APPEND;
-        offset = 0;
-    }
-
-    //*****************************************
-    LLAPRFilePoolScope scope(pool);
-    apr_file_t* file_handle = open(filename, scope.getVolatileAPRPool(), flags);
-    //*****************************************
-    if (!file_handle)
-    {
-        return 0;
-    }
-
-    if (offset > 0)
-    {
-        offset = LLAPRFile::seek(file_handle, APR_SET, offset);
-    }
-
-    apr_size_t bytes_written;
-    if (offset < 0)
-    {
-        bytes_written = 0;
-    }
-    else
-    {
-        bytes_written = nbytes ;
-        apr_status_t s = apr_file_write(file_handle, buf, &bytes_written);
-        if (s != APR_SUCCESS)
-        {
-            LL_WARNS("APR") << "Attempting to write filename: " << filename << LL_ENDL;
-            if (APR_STATUS_IS_ENOSPC(s))
-            {
-                LLApp::notifyOutOfDiskSpace();
-            }
-            ll_apr_warn_status(s);
-            bytes_written = 0;
-        }
-        else
-        {
-            llassert_always(bytes_written <= 0x7fffffff);
-        }
-    }
-
-    //*****************************************
-    LLAPRFile::close(file_handle);
-    //*****************************************
-
-    return (S32)bytes_written;
-}
-
-//static
-bool LLAPRFile::remove(const std::string& filename, LLVolatileAPRPool* pool)
-{
-    apr_status_t s;
-
-    LLAPRFilePoolScope scope(pool);
-    s = apr_file_remove(filename.c_str(), scope.getVolatileAPRPool());
-
-    if (s != APR_SUCCESS)
-    {
-        ll_apr_warn_status(s);
-        LL_WARNS("APR") << " Attempting to remove filename: " << filename << LL_ENDL;
-        return false;
-    }
-    return true;
-}
-
-//static
-bool LLAPRFile::rename(const std::string& filename, const std::string& newname, LLVolatileAPRPool* pool)
-{
-    apr_status_t s;
-
-    LLAPRFilePoolScope scope(pool);
-    s = apr_file_rename(filename.c_str(), newname.c_str(), scope.getVolatileAPRPool());
-
-    if (s != APR_SUCCESS)
-    {
-        ll_apr_warn_status(s);
-        LL_WARNS("APR") << " Attempting to rename filename: " << filename << LL_ENDL;
-        return false;
-    }
-    return true;
-}
-
-//static
-bool LLAPRFile::isExist(const std::string& filename, LLVolatileAPRPool* pool, apr_int32_t flags)
-{
-    apr_file_t* apr_file;
-    apr_status_t s;
-
-    LLAPRFilePoolScope scope(pool);
-    s = apr_file_open(&apr_file, filename.c_str(), flags, APR_OS_DEFAULT, scope.getVolatileAPRPool());
-
-    if (s != APR_SUCCESS || !apr_file)
-    {
-        return false;
-    }
-    else
-    {
-        apr_file_close(apr_file) ;
-        return true;
-    }
-}
-
-//static
-S32 LLAPRFile::size(const std::string& filename, LLVolatileAPRPool* pool)
-{
-    apr_file_t* apr_file;
-    apr_finfo_t info;
-    apr_status_t s;
-
-    LLAPRFilePoolScope scope(pool);
-    s = apr_file_open(&apr_file, filename.c_str(), APR_READ, APR_OS_DEFAULT, scope.getVolatileAPRPool());
-
-    if (s != APR_SUCCESS || !apr_file)
-    {
-        return 0;
-    }
-    else
-    {
-        apr_status_t s = apr_file_info_get(&info, APR_FINFO_SIZE, apr_file);
-
-        apr_file_close(apr_file) ;
-
-        if (s == APR_SUCCESS)
-        {
-            return (S32)info.size;
-        }
-        else
-        {
-            return 0;
-        }
-    }
-}
-
-//static
-bool LLAPRFile::makeDir(const std::string& dirname, LLVolatileAPRPool* pool)
-{
-    apr_status_t s;
-
-    LLAPRFilePoolScope scope(pool);
-    s = apr_dir_make(dirname.c_str(), APR_FPROT_OS_DEFAULT, scope.getVolatileAPRPool());
-
-    if (s != APR_SUCCESS)
-    {
-        ll_apr_warn_status(s);
-        LL_WARNS("APR") << " Attempting to make directory: " << dirname << LL_ENDL;
-        return false;
-    }
-    return true;
-}
-
-//static
-bool LLAPRFile::removeDir(const std::string& dirname, LLVolatileAPRPool* pool)
-{
-    apr_status_t s;
-
-    LLAPRFilePoolScope scope(pool);
-    s = apr_file_remove(dirname.c_str(), scope.getVolatileAPRPool());
-
-    if (s != APR_SUCCESS)
-    {
-        ll_apr_warn_status(s);
-        LL_WARNS("APR") << " Attempting to remove directory: " << dirname << LL_ENDL;
-        return false;
-    }
-    return true;
-}
 //
 //end of static components of LLAPRFile
 //*******************************************************************************************************************************
diff --git a/indra/llcommon/llapr.h b/indra/llcommon/llapr.h
index 0e1e4277e7..8795a0f2c4 100644
--- a/indra/llcommon/llapr.h
+++ b/indra/llcommon/llapr.h
@@ -35,10 +35,6 @@
 
 #include "llwin32headers.h"
 #include "apr_thread_proc.h"
-#include "apr_getopt.h"
-#include "apr_signal.h"
-
-#include "llstring.h"
 
 #include <mutex>
 
@@ -184,20 +180,7 @@ private:
     static apr_file_t* open(const std::string& filename, apr_pool_t* apr_pool, apr_int32_t flags);
     static apr_status_t close(apr_file_t* file) ;
     static S32 seek(apr_file_t* file, apr_seek_where_t where, S32 offset);
-public:
-    // returns false if failure:
-    static bool remove(const std::string& filename, LLVolatileAPRPool* pool = NULL);
-    static bool rename(const std::string& filename, const std::string& newname, LLVolatileAPRPool* pool = NULL);
-    static bool isExist(const std::string& filename, LLVolatileAPRPool* pool = NULL, apr_int32_t flags = APR_READ);
-    static S32 size(const std::string& filename, LLVolatileAPRPool* pool = NULL);
-    static bool makeDir(const std::string& dirname, LLVolatileAPRPool* pool = NULL);
-    static bool removeDir(const std::string& dirname, LLVolatileAPRPool* pool = NULL);
-
-    // Returns bytes read/written, 0 if read/write fails:
-    static S32 readEx(const std::string& filename, void *buf, S32 offset, S32 nbytes, LLVolatileAPRPool* pool = NULL);
-    static S32 writeEx(const std::string& filename, const void *buf, S32 offset, S32 nbytes, LLVolatileAPRPool* pool = NULL); // offset<0 means append
 //*******************************************************************************************************************************
 };
 
-
 #endif // LL_LLAPR_H
diff --git a/indra/llcommon/llerror.cpp b/indra/llcommon/llerror.cpp
index 8ae0fabcc6..5b42c83a14 100644..100755
--- a/indra/llcommon/llerror.cpp
+++ b/indra/llcommon/llerror.cpp
@@ -481,13 +481,9 @@ namespace
 
         std::string file = user_dir + "/logcontrol-dev.xml";
 
-        llstat stat_info;
-        if (LLFile::stat(file, &stat_info)) {
-            // NB: stat returns non-zero if it can't read the file, for example
-            // if it doesn't exist.  LLFile has no better abstraction for
-            // testing for file existence.
-
-            file = app_dir + "/logcontrol.xml";
+        if (!LLFile::isfile(file))
+        {
+             file = app_dir + "/logcontrol.xml";
         }
         return * new LogControlFile(file);
             // NB: This instance is never freed
diff --git a/indra/llcommon/llfile.cpp b/indra/llcommon/llfile.cpp
index a539e4fe28..ff87d8ce10 100644..100755
--- a/indra/llcommon/llfile.cpp
+++ b/indra/llcommon/llfile.cpp
@@ -29,22 +29,17 @@
 
 #include "linden_common.h"
 #include "llfile.h"
-#include "llstring.h"
 #include "llerror.h"
 #include "stringize.h"
 
 #if LL_WINDOWS
-#include "llwin32headers.h"
-#include <vector>
+#include <fcntl.h>
 #else
 #include <errno.h>
+#include <sys/file.h>
 #endif
 
-using namespace std;
-
-static std::string empty;
-
-// Many of the methods below use OS-level functions that mess with errno. Wrap
+// Some of the methods below use OS-level functions that mess with errno. Wrap
 // variants of strerror() to report errors.
 
 #if LL_WINDOWS
@@ -79,6 +74,7 @@ static errentry const errtable[]
     { ERROR_CURRENT_DIRECTORY,      EACCES    },  //   16
     { ERROR_NOT_SAME_DEVICE,        EXDEV     },  //   17
     { ERROR_NO_MORE_FILES,          ENOENT    },  //   18
+    { ERROR_SHARING_VIOLATION,      EACCES    },  //   32
     { ERROR_LOCK_VIOLATION,         EACCES    },  //   33
     { ERROR_BAD_NETPATH,            ENOENT    },  //   53
     { ERROR_NETWORK_ACCESS_DENIED,  EACCES    },  //   65
@@ -109,22 +105,25 @@ static errentry const errtable[]
     { ERROR_NOT_ENOUGH_QUOTA,       ENOMEM    }   // 1816
 };
 
-static int set_errno_from_oserror(unsigned long oserr)
+static int get_errno_from_oserror(int oserr)
 {
     if (!oserr)
         return 0;
 
     // Check the table for the Windows OS error code
-    for (const struct errentry &entry : errtable)
+    for (const struct errentry& entry : errtable)
     {
         if (oserr == entry.oserr)
         {
-            _set_errno(entry.errcode);
-            return -1;
+            return entry.errcode;
         }
     }
+    return EINVAL;
+}
 
-    _set_errno(EINVAL);
+static int set_errno_from_oserror(unsigned long oserr)
+{
+    _set_errno(get_errno_from_oserror(oserr));
     return -1;
 }
 
@@ -136,69 +135,8 @@ std::string strerr(int errn)
     return buffer;
 }
 
-inline bool is_slash(wchar_t const c)
-{
-    return c == L'\\' || c == L'/';
-}
-
-static std::wstring utf8path_to_wstring(const std::string& utf8path)
-{
-    if (utf8path.size() >= MAX_PATH)
-    {
-        // By prepending "\\?\" to a path, Windows widechar file APIs will not fail on long path names
-        std::wstring utf16path = L"\\\\?\\" + ll_convert<std::wstring>(utf8path);
-        // We need to make sure that the path does not contain forward slashes as above
-        // prefix does bypass the path normalization that replaces slashes with backslashes
-        // before passing the path to kernel mode APIs
-        std::replace(utf16path.begin(), utf16path.end(), L'/', L'\\');
-        return utf16path;
-    }
-    return ll_convert<std::wstring>(utf8path);
-}
-
-static unsigned short get_fileattr(const std::wstring& utf16path, bool dontFollowSymLink = false)
-{
-    unsigned long  flags = FILE_FLAG_BACKUP_SEMANTICS;
-    if (dontFollowSymLink)
-    {
-        flags |= FILE_FLAG_OPEN_REPARSE_POINT;
-    }
-    HANDLE file_handle = CreateFileW(utf16path.c_str(), FILE_READ_ATTRIBUTES,
-                                     FILE_SHARE_DELETE | FILE_SHARE_READ | FILE_SHARE_WRITE,
-                                     nullptr, OPEN_EXISTING, flags, nullptr);
-    if (file_handle != INVALID_HANDLE_VALUE)
-    {
-        FILE_ATTRIBUTE_TAG_INFO attribute_info;
-        if (GetFileInformationByHandleEx(file_handle, FileAttributeTagInfo, &attribute_info, sizeof(attribute_info)))
-        {
-            // A volume path alone (only drive letter) is not recognized as directory while it technically is
-            bool is_directory = (attribute_info.FileAttributes & FILE_ATTRIBUTE_DIRECTORY) ||
-                                (iswalpha(utf16path[0]) && utf16path[1] == ':' &&
-                                 (!utf16path[2] || (is_slash(utf16path[2]) && !utf16path[3])));
-            unsigned short st_mode = is_directory ? S_IFDIR :
-                                     (attribute_info.FileAttributes & FILE_ATTRIBUTE_REPARSE_POINT ? S_IFLNK : S_IFREG);
-            st_mode |= (attribute_info.FileAttributes & FILE_ATTRIBUTE_READONLY) ? S_IREAD : S_IREAD | S_IWRITE;
-            // we do not try to guess executable flag
-
-            // propagate user bits to group/other fields:
-            st_mode |= (st_mode & 0700) >> 3;
-            st_mode |= (st_mode & 0700) >> 6;
-
-            CloseHandle(file_handle);
-            return st_mode;
-        }
-    }
-    // Retrieve last error and set errno before calling CloseHandle()
-    set_errno_from_oserror(GetLastError());
-
-    if (file_handle != INVALID_HANDLE_VALUE)
-    {
-        CloseHandle(file_handle);
-    }
-    return 0;
-}
-
 #else
+
 // On Posix we want to call strerror_r(), but alarmingly, there are two
 // different variants. The one that returns int always populates the passed
 // buffer (except in case of error), whereas the other one always returns a
@@ -245,9 +183,50 @@ std::string strerr(int errn)
     return message_from(errn, buffer, sizeof(buffer),
                         strerror_r(errn, buffer, sizeof(buffer)));
 }
+
 #endif  // ! LL_WINDOWS
 
-static int warnif(const std::string& desc, const std::string& filename, int rc, int accept = 0)
+#if LL_WINDOWS && 0 // turn on to debug file-locking problems
+#define PROCESS_LOCKING_CHECK 1
+static void find_locking_process(const std::string& filename)
+{
+    // Only do any of this stuff (before LL_ENDL) if it will be logged.
+    LL_DEBUGS("LLFile") << "";
+    // wrong way
+    std::string TEMP = LLFile::tmpdir();
+    if (TEMP.empty())
+    {
+        LL_CONT << "No $TEMP, not running 'handle'";
+    }
+    else
+    {
+        std::string tf(TEMP);
+        tf += "\\handle.tmp";
+        // http://technet.microsoft.com/en-us/sysinternals/bb896655
+        std::string cmd(STRINGIZE("handle \"" << filename
+                        // "openfiles /query /v | fgrep -i \"" << filename
+                        << "\" > \"" << tf << '"'));
+        LL_CONT << cmd;
+        if (system(cmd.c_str()) != 0)
+        {
+            LL_CONT << "\nDownload 'handle.exe' from http://technet.microsoft.com/en-us/sysinternals/bb896655";
+        }
+        else
+        {
+            std::ifstream inf(tf);
+            std::string   line;
+            while (std::getline(inf, line))
+            {
+                LL_CONT << '\n' << line;
+            }
+        }
+        LLFile::remove(tf);
+    }
+    LL_CONT << LL_ENDL;
+}
+#endif // LL_WINDOWS hack to identify processes holding file open
+
+static int warnif(const std::string& desc, const std::string& filename, int rc, int suppress_warning = 0)
 {
     if (rc < 0)
     {
@@ -256,319 +235,951 @@ static int warnif(const std::string& desc, const std::string& filename, int rc,
 
         // For certain operations, a particular errno value might be
         // acceptable -- e.g. stat() could permit ENOENT, mkdir() could permit
-        // EEXIST. Don't warn if caller explicitly says this errno is okay.
-        if (errn != accept)
+        // EEXIST. Don't log a warning if caller explicitly says this errno is okay.
+        if (errn != suppress_warning)
         {
-            LL_WARNS("LLFile") << "Couldn't " << desc << " '" << filename
-                               << "' (errno " << errn << "): " << strerr(errn) << LL_ENDL;
+            LL_WARNS("LLFile") << "Couldn't " << desc << " '" << filename << "' (errno " << errn << "): " << strerr(errn) << LL_ENDL;
         }
-#if 0 && LL_WINDOWS                 // turn on to debug file-locking problems
+#if PROCESS_LOCKING_CHECK
         // If the problem is "Permission denied," maybe it's because another
         // process has the file open. Try to find out.
-        if (errn == EACCES)         // *not* EPERM
+        if (errn == EACCES) // *not* EPERM
         {
-            // Only do any of this stuff (before LL_ENDL) if it will be logged.
-            LL_DEBUGS("LLFile") << empty;
-            // would be nice to use LLDir for this, but dependency goes the
-            // wrong way
-            const char* TEMP = LLFile::tmpdir();
-            if (! (TEMP && *TEMP))
-            {
-                LL_CONT << "No $TEMP, not running 'handle'";
-            }
-            else
-            {
-                std::string tf(TEMP);
-                tf += "\\handle.tmp";
-                // http://technet.microsoft.com/en-us/sysinternals/bb896655
-                std::string cmd(STRINGIZE("handle \"" << filename
-                                          // "openfiles /query /v | fgrep -i \"" << filename
-                                          << "\" > \"" << tf << '"'));
-                LL_CONT << cmd;
-                if (system(cmd.c_str()) != 0)
-                {
-                    LL_CONT << "\nDownload 'handle.exe' from http://technet.microsoft.com/en-us/sysinternals/bb896655";
-                }
-                else
-                {
-                    std::ifstream inf(tf);
-                    std::string line;
-                    while (std::getline(inf, line))
-                    {
-                        LL_CONT << '\n' << line;
-                    }
-                }
-                LLFile::remove(tf);
-            }
-            LL_CONT << LL_ENDL;
+            find_locking_process(filename);
         }
-#endif  // LL_WINDOWS hack to identify processes holding file open
+#endif
     }
     return rc;
 }
 
-// static
-int LLFile::mkdir(const std::string& dirname, int perms)
+static int warnif(const std::string& desc, const std::string& filename, const std::error_code& ec, int suppress_warning = 0)
 {
-    // We often use mkdir() to ensure the existence of a directory that might
-    // already exist. There is no known case in which we want to call out as
-    // an error the requested directory already existing.
+    if (ec)
+    {
+        // get Posix errno from the std::error_code so we can compare it to the suppress_warning parameter
+        // to see when a caller wants us to not generate a warning for a particular error code
 #if LL_WINDOWS
-    // permissions are ignored on Windows
-    int rc = 0;
-    std::wstring utf16dirname = utf8path_to_wstring(dirname);
-    if (!CreateDirectoryW(utf16dirname.c_str(), nullptr))
+        int errn = get_errno_from_oserror(ec.value());
+#else
+        int errn = ec.value();
+#endif
+        // For certain operations, a particular errno value might be acceptable
+        // Don't warn if caller explicitly says this errno is okay.
+        if (errn != suppress_warning)
+        {
+            LL_WARNS("LLFile") << "Couldn't " << desc << " '" << filename << "' (errno " << errn << "): " << ec.message() << LL_ENDL;
+        }
+#if PROCESS_LOCKING_CHECK
+        // Try to detect locked files by other processes
+        if (ec.value() == ERROR_SHARING_VIOLATION || ec.value() == ERROR_LOCK_VIOLATION)
+        {
+            find_locking_process(filename);
+        }
+#endif
+        return -1;
+    }
+    return 0;
+}
+
+#if LL_WINDOWS
+
+inline int set_ec_from_system_error(std::error_code& ec, DWORD error)
+{
+    ec.assign(error, std::system_category());
+    return -1;
+}
+
+static int set_ec_from_system_error(std::error_code& ec)
+{
+    return set_ec_from_system_error(ec, GetLastError());
+}
+
+inline int set_ec_to_parameter_error(std::error_code& ec)
+{
+    return set_ec_from_system_error(ec, ERROR_INVALID_PARAMETER);
+}
+
+inline int set_ec_to_outofmemory_error(std::error_code& ec)
+{
+    return set_ec_from_system_error(ec, ERROR_NOT_ENOUGH_MEMORY);
+}
+
+inline DWORD decode_access_mode(std::ios_base::openmode omode)
+{
+    switch (omode & (LLFile::in | LLFile::out))
+    {
+        case LLFile::in:
+            return GENERIC_READ;
+        case LLFile::out:
+            return GENERIC_WRITE;
+        case static_cast<std::ios_base::openmode>(LLFile::in | LLFile::out):
+            return GENERIC_READ | GENERIC_WRITE;
+    }
+    if (omode & LLFile::app)
+    {
+        return GENERIC_WRITE;
+    }
+    return 0;
+}
+
+inline DWORD decode_open_create_flags(std::ios_base::openmode omode)
+{
+    if (omode & LLFile::noreplace)
     {
-        // Only treat other errors than an already existing file as a real error
-        unsigned long oserr = GetLastError();
-        if (oserr != ERROR_ALREADY_EXISTS)
+        return CREATE_NEW; // create if it does not exist, otherwise fail
+    }
+    if (omode & LLFile::trunc)
+    {
+        if (!(omode & LLFile::out))
         {
-            rc = set_errno_from_oserror(oserr);
+            return TRUNCATE_EXISTING; // open and truncate if it exists, otherwise fail
         }
+        return CREATE_ALWAYS; // open and truncate if it exists, otherwise create it
+    }
+    if (!(omode & LLFile::out))
+    {
+        return OPEN_EXISTING; // open if it exists, otherwise fail
     }
+    // LLFile::app or (LLFile::out and (!LLFile::trunc or !LLFile::noreplace))
+    return OPEN_ALWAYS; // open if it exists, otherwise create it
+}
+
+inline DWORD decode_share_mode(int omode)
+{
+    if (omode & LLFile::exclusive)
+    {
+        return 0; // allow no other access
+    }
+    if (omode & LLFile::shared)
+    {
+        return FILE_SHARE_READ; // allow read access
+    }
+    return FILE_SHARE_READ | FILE_SHARE_WRITE; // allow read and write access to others
+}
+
+inline DWORD decode_attributes(std::ios_base::openmode omode, int perm)
+{
+    return (perm & S_IWRITE) ? FILE_ATTRIBUTE_NORMAL : FILE_ATTRIBUTE_READONLY;
+}
+
+// Under Windows the values for the std::ios_base::seekdir constants match the according FILE_BEGIN
+// and other constants but we do a programmatic translation for now to be sure
+static DWORD seek_mode_from_dir(std::ios_base::seekdir seekdir)
+{
+    switch (seekdir)
+    {
+        case LLFile::beg:
+            return FILE_BEGIN;
+        case LLFile::cur:
+            return FILE_CURRENT;
+        case LLFile::end:
+            return FILE_END;
+    }
+    return FILE_BEGIN;
+}
+
 #else
-    int rc = ::mkdir(dirname.c_str(), (mode_t)perms);
-    if (rc < 0 && errno == EEXIST)
+
+inline int set_ec_from_system_error(std::error_code& ec, int error)
+{
+    ec.assign(error, std::system_category());
+    return -1;
+}
+
+static int set_ec_from_system_error(std::error_code& ec)
+{
+    return set_ec_from_system_error(ec, errno);
+}
+
+inline int set_ec_to_parameter_error(std::error_code& ec)
+{
+    return set_ec_from_system_error(ec, EINVAL);
+}
+
+inline int set_ec_to_outofmemory_error(std::error_code& ec)
+{
+    return set_ec_from_system_error(ec, ENOMEM);
+}
+
+inline int decode_access_mode(std::ios_base::openmode omode)
+{
+    if (omode & LLFile::out)
     {
-        // this is not the error you want, move along
-        return 0;
+        if (omode & LLFile::in)
+        {
+            return O_RDWR;
+        }
+        return O_WRONLY;
+    }
+    return O_RDONLY;
+
+    /*switch (omode & (LLFile::in | LLFile::out))
+    {
+        case LLFile::out:
+            return O_WRONLY;
+        case static_cast<std::ios_base::openmode>(LLFile::in | LLFile::out):
+            return O_RDWR;
+    }
+    return O_RDONLY;*/
+}
+
+inline int decode_open_mode(std::ios_base::openmode omode)
+{
+    int flags = O_CREAT | decode_access_mode(omode);
+    if (omode & LLFile::app)
+    {
+        flags |= O_APPEND;
     }
+    if (omode & LLFile::trunc)
+    {
+        flags |= O_TRUNC;
+    }
+    if (omode & LLFile::binary)
+    {
+        // Not a thing under *nix
+    }
+    if (omode & LLFile::noreplace)
+    {
+        flags |= O_EXCL;
+    }
+    return flags;
+}
+
+inline int decode_lock_mode(std::ios_base::openmode omode)
+{
+    int lmode = omode & LLFile::noblock ? LOCK_NB : 0;
+    if (omode & LLFile::lock_mask)
+    {
+        if (omode & LLFile::exclusive)
+        {
+            return lmode | LOCK_EX;
+        }
+        return lmode | LOCK_SH;
+    }
+    return lmode | LOCK_UN;
+}
+
+// Under Linux and Mac the values for the std::ios_base::seekdir constants match the according SEEK_SET
+// and other constants but we do a programmatic translation for now to be sure
+inline int seek_mode_from_dir(std::ios_base::seekdir seekdir)
+{
+    switch (seekdir)
+    {
+        case LLFile::beg:
+            return SEEK_SET;
+        case LLFile::cur:
+            return SEEK_CUR;
+        case LLFile::end:
+            return SEEK_END;
+    }
+    return SEEK_SET;
+}
+
 #endif
-    // anything else might be a problem
-    return warnif("mkdir", dirname, rc);
+
+inline int clear_error(std::error_code& ec)
+{
+    ec.clear();
+    return 0;
 }
 
-// static
-int LLFile::rmdir(const std::string& dirname, int suppress_error)
+inline bool are_open_mode_flags_invalid(std::ios_base::openmode omode)
+{
+    // at least one of input or output needs to be specified
+    if (!(omode & (LLFile::in | LLFile::out)))
+    {
+        return true;
+    }
+    // output must be possible for any of the extra options
+    if (!(omode & LLFile::out) && (omode & (LLFile::trunc | LLFile::app | LLFile::noreplace)))
+    {
+        return true;
+    }
+    // invalid combination, mutually exclusive
+    if ((omode & LLFile::app) && (omode & (LLFile::trunc | LLFile::noreplace)))
+    {
+        return true;
+    }
+    return false;
+}
+
+//----------------------------------------------------------------------------------------
+// class member functions
+//----------------------------------------------------------------------------------------
+int LLFile::open(const std::string& filename, std::ios_base::openmode omode, std::error_code& ec, int perm)
 {
+    close(ec);
+    if (are_open_mode_flags_invalid(omode))
+    {
+        return set_ec_to_parameter_error(ec);
+    }
 #if LL_WINDOWS
-    std::wstring utf16dirname = utf8path_to_wstring(dirname);
-    int rc = _wrmdir(utf16dirname.c_str());
+    DWORD access = decode_access_mode(omode),
+          share = decode_share_mode(omode),
+          create = decode_open_create_flags(omode),
+          attributes = decode_attributes(omode, perm);
+
+    std::wstring file_path = utf8StringToWstring(filename);
+    mHandle = CreateFileW(file_path.c_str(), access, share, nullptr, create, attributes, nullptr);
+    // The dwShareMode = share parameter takes care of locking the file for other processes if indicated,
+    // no need to do anything else for file locking here
 #else
-    int rc = ::rmdir(dirname.c_str());
+    int oflags = decode_open_mode(omode);
+    int lmode = omode & LLFile::lock_mask;
+    mHandle = ::open(filename.c_str(), oflags, perm);
+    if (mHandle != InvalidHandle && lmode && lock(lmode | LLFile::noblock, ec) != 0)
+    {
+        close();
+        return -1;
+    }
 #endif
-    return warnif("rmdir", dirname, rc, suppress_error);
+    if (mHandle == InvalidHandle)
+    {
+        return set_ec_from_system_error(ec);
+    }
+
+    if (omode & LLFile::ate && seek(0, LLFile::end, ec) != 0)
+    {
+        close();
+        return -1;
+    }
+    mOpen = omode;
+    return clear_error(ec);
 }
 
-// static
-LLFILE* LLFile::fopen(const std::string& filename, const char* mode)
+S64 LLFile::size(std::error_code& ec)
 {
 #if LL_WINDOWS
-    std::wstring utf16filename = utf8path_to_wstring(filename);
-    std::wstring utf16mode = ll_convert<std::wstring>(std::string(mode));
-    return _wfopen(utf16filename.c_str(), utf16mode.c_str());
+    LARGE_INTEGER value = { 0 };
+    if (GetFileSizeEx(mHandle, &value))
+    {
+        clear_error(ec);
+        return value.QuadPart;
+    }
 #else
-    return ::fopen(filename.c_str(),mode);
+    struct stat statval;
+    if (fstat(mHandle, &statval) == 0)
+    {
+        clear_error(ec);
+        return statval.st_size;
+    }
 #endif
+    return set_ec_from_system_error(ec);
 }
 
-// static
-int LLFile::close(LLFILE * file)
+S64 LLFile::tell(std::error_code& ec)
 {
-    int ret_value = 0;
-    if (file)
+#if LL_WINDOWS
+    LARGE_INTEGER value = { 0 };
+    if (SetFilePointerEx(mHandle, value, &value, FILE_CURRENT))
     {
-        ret_value = fclose(file);
+        clear_error(ec);
+        return value.QuadPart;
     }
-    return ret_value;
+#else
+    off_t offset = lseek(mHandle, 0, SEEK_CUR);
+    if (offset != -1)
+    {
+        clear_error(ec);
+        return offset;
+    }
+#endif
+    return set_ec_from_system_error(ec);
 }
 
-// static
-std::string LLFile::getContents(const std::string& filename)
+int LLFile::seek(S64 pos, std::error_code& ec)
 {
-    LLFILE* fp = LLFile::fopen(filename, "rb");
-    if (fp)
+    return seek(pos, LLFile::beg, ec);
+}
+
+int LLFile::seek(S64 offset, std::ios_base::seekdir dir, std::error_code& ec)
+{
+    S64 newOffset = 0;
+#if LL_WINDOWS
+    DWORD seekdir = seek_mode_from_dir(dir);
+    LARGE_INTEGER value;
+    value.QuadPart = offset;
+    if (SetFilePointerEx(mHandle, value, (PLARGE_INTEGER)&newOffset, seekdir))
+#else
+    newOffset = lseek(mHandle, offset, seek_mode_from_dir(dir));
+    if (newOffset != -1)
+#endif
     {
-        fseek(fp, 0, SEEK_END);
-        U32 length = ftell(fp);
-        fseek(fp, 0, SEEK_SET);
+        return clear_error(ec);
+    }
+    return set_ec_from_system_error(ec);
+}
 
-        std::vector<char> buffer(length);
-        size_t nread = fread(buffer.data(), 1, length, fp);
-        fclose(fp);
+#if LL_WINDOWS
+inline DWORD next_buffer_size(S64 nbytes)
+{
+    return nbytes > 0x80000000 ? 0x80000000 : (DWORD)nbytes;
+}
+#endif
 
-        return std::string(buffer.data(), nread);
+S64 LLFile::read(void* buffer, S64 nbytes, std::error_code& ec)
+{
+    if (nbytes == 0)
+    {
+        // Nothing to do
+        return clear_error(ec);
     }
+    else if (!buffer || nbytes < 0)
+    {
+        return set_ec_to_parameter_error(ec);
+    }
+#if LL_WINDOWS
+    S64 totalBytes = 0;
+    char *ptr = (char*)buffer;
+    DWORD bytesRead, bytesToRead = next_buffer_size(nbytes);
 
-    return LLStringUtil::null;
+    // Read in chunks to support >4GB which the S64 nbytes value makes possible
+    while (ReadFile(mHandle, ptr, bytesToRead, &bytesRead, nullptr))
+    {
+        totalBytes += bytesRead;
+        if (nbytes <= totalBytes || // requested amount read
+            bytesRead < bytesToRead) // ReadFile encountered eof
+        {
+            clear_error(ec);
+            return totalBytes;
+        }
+        ptr += bytesRead;
+        bytesToRead = next_buffer_size(nbytes - totalBytes);
+    }
+#else
+    ssize_t bytesRead = ::read(mHandle, buffer, nbytes);
+    if (bytesRead != -1)
+    {
+        clear_error(ec);
+        return bytesRead;
+    }
+#endif
+    return set_ec_from_system_error(ec);
 }
 
-// static
-int LLFile::remove(const std::string& filename, int suppress_error)
+S64 LLFile::write(const void* buffer, S64 nbytes, std::error_code& ec)
 {
+    if (nbytes == 0)
+    {
+        // Nothing to do here
+        return clear_error(ec);
+    }
+    else if (!buffer || nbytes < 0)
+    {
+        return set_ec_to_parameter_error(ec);
+    }
 #if LL_WINDOWS
-    // Posix remove() works on both files and directories although on Windows
-    // remove() and its wide char variant _wremove() only removes files just
-    // as its siblings unlink() and _wunlink().
-    // If we really only want to support files we should instead use
-    // unlink() in the non-Windows part below too
-    int rc = -1;
-    std::wstring utf16filename = utf8path_to_wstring(filename);
-    unsigned short st_mode = get_fileattr(utf16filename);
-    if (S_ISDIR(st_mode))
+    // If this was opened in append mode, we emulate it on Windows
+    if (mOpen & LLFile::app && seek(0, LLFile::end, ec) != 0)
+    {
+        return -1;
+    }
+
+    S64 totalBytes = 0;
+    char* ptr = (char*)buffer;
+    DWORD bytesWritten, bytesToWrite = next_buffer_size(nbytes);
+
+    // Write in chunks to support >4GB which the S64 nbytes value makes possible
+    while (WriteFile(mHandle, ptr, bytesToWrite, &bytesWritten, nullptr))
+    {
+        totalBytes += bytesWritten;
+        if (nbytes <= totalBytes)
+        {
+            clear_error(ec);
+            return totalBytes;
+        }
+        ptr += bytesWritten;
+        bytesToWrite = next_buffer_size(nbytes - totalBytes);
+    }
+#else
+    ssize_t bytesWritten = ::write(mHandle, buffer, nbytes);
+    if (bytesWritten != -1)
+    {
+        clear_error(ec);
+        return bytesWritten;
+    }
+#endif
+    return set_ec_from_system_error(ec);
+}
+
+S64 LLFile::printf(const char* fmt, ...)
+{
+    va_list args1;
+    va_start(args1, fmt);
+    va_list args2;
+    va_copy(args2, args1);
+    int length = vsnprintf(nullptr, 0, fmt, args1);
+    va_end(args1);
+    if (length < 0)
     {
-        rc = _wrmdir(utf16filename.c_str());
+        va_end(args2);
+        return -1;
     }
-    else if (S_ISREG(st_mode))
+    void* buffer = malloc(length + 1);
+    if (!buffer)
     {
-        rc = _wunlink(utf16filename.c_str());
+        va_end(args2);
+        return -1;
     }
-    else if (st_mode)
+    length = vsnprintf((char*)buffer, length + 1, fmt, args2);
+    va_end(args2);
+    std::error_code ec;
+    S64 written = write(buffer, length, ec);
+    free(buffer);
+    return written;
+}
+
+int LLFile::lock(int mode, std::error_code& ec)
+{
+#if LL_WINDOWS
+    if (!(mode & LLFile::lock_mask))
     {
-        // it is something else than a file or directory
-        // this should not really happen as long as we do not allow for symlink
-        // detection in the optional parameter to get_fileattr()
-        rc = set_errno_from_oserror(ERROR_INVALID_PARAMETER);
+        if (UnlockFile(mHandle, 0, 0, MAXDWORD, MAXDWORD))
+        {
+            return clear_error(ec);
+        }
     }
     else
     {
-        // get_fileattr() failed and already set errno, preserve it for correct error reporting
+        OVERLAPPED overlapped = { 0 };
+        DWORD flags = (mode & LLFile::noblock) ? LOCKFILE_FAIL_IMMEDIATELY : 0;
+        if (mode & LLFile::exclusive)
+        {
+            flags |= LOCKFILE_EXCLUSIVE_LOCK;
+        }
+        // We lock the maximum range, since flock only supports locking the entire file too
+        if (LockFileEx(mHandle, flags, 0, MAXDWORD, MAXDWORD, &overlapped))
+        {
+            return clear_error(ec);
+        }
     }
 #else
-    int rc = ::remove(filename.c_str());
+    if (flock(mHandle, decode_lock_mode(static_cast<std::ios_base::openmode>(mode))) == 0)
+    {
+        return clear_error(ec);
+    }
 #endif
-    return warnif("remove", filename, rc, suppress_error);
+    return set_ec_from_system_error(ec);
+}
+
+int LLFile::close(std::error_code& ec)
+{
+    if (mHandle != InvalidHandle)
+    {
+        llfile_handle_t handle = InvalidHandle;
+        std::swap(handle, mHandle);
+#if LL_WINDOWS
+        if (!CloseHandle(handle))
+#else
+        if (::close(handle))
+#endif
+        {
+            return set_ec_from_system_error(ec);
+        }
+    }
+    return clear_error(ec);
+}
+
+int LLFile::close()
+{
+    std::error_code ec;
+    return close(ec);
 }
 
+//----------------------------------------------------------------------------------------
+// static member functions
+//----------------------------------------------------------------------------------------
+
 // static
-int LLFile::rename(const std::string& filename, const std::string& newname, int suppress_error)
+LLFILE* LLFile::fopen(const std::string& filename, const char* mode, int lmode)
 {
+    LLFILE* file;
 #if LL_WINDOWS
-    // Posix rename() will gladly overwrite a file at newname if it exists, the Windows
-    // rename(), respectively _wrename(), will bark on that. Instead call directly the Windows
-    // API MoveFileEx() and use its flags to specify that overwrite is allowed.
-    std::wstring utf16filename = utf8path_to_wstring(filename);
-    std::wstring utf16newname = utf8path_to_wstring(newname);
-    int rc = 0;
-    if (!MoveFileExW(utf16filename.c_str(), utf16newname.c_str(), MOVEFILE_REPLACE_EXISTING | MOVEFILE_COPY_ALLOWED))
+    int shflag = _SH_DENYNO;
+    switch (lmode)
     {
-        rc = set_errno_from_oserror(GetLastError());
+        case LLFile::exclusive:
+            shflag = _SH_DENYRW;
+            break;
+        case LLFile::shared:
+            shflag = _SH_DENYWR;
+            break;
     }
+    std::wstring file_path = utf8StringToWstring(filename);
+    std::wstring utf16mode = ll_convert<std::wstring>(std::string(mode));
+    file = _wfsopen(file_path.c_str(), utf16mode.c_str(), shflag);
 #else
-    int rc = ::rename(filename.c_str(),newname.c_str());
+    file = ::fopen(filename.c_str(), mode);
+    if (file && (lmode & (LLFile::lock_mask)))
+    {
+        // Rather fail on a sharing conflict than block
+        if (flock(fileno(file), decode_lock_mode(static_cast<std::ios_base::openmode>(lmode | LLFile::noblock))))
+        {
+            ::fclose(file);
+            file = nullptr;
+        }
+    }
 #endif
-    return warnif(STRINGIZE("rename to '" << newname << "' from"), filename, rc, suppress_error);
+    return file;
+}
+
+// static
+int LLFile::close(LLFILE* file)
+{
+    int ret_value = 0;
+    if (file)
+    {
+        ret_value = ::fclose(file);
+    }
+    return ret_value;
+}
+
+// static
+std::string LLFile::getContents(const std::string& filename)
+{
+    std::error_code ec;
+    return getContents(filename, ec);
+}
+
+// static
+std::string LLFile::getContents(const std::string& filename, std::error_code& ec)
+{
+    std::string buffer;
+    LLFile file(filename, LLFile::in | LLFile::binary, ec);
+    if (file)
+    {
+        S64 length = file.size(ec);
+        if (!ec && length > 0)
+        {
+            buffer = std::string(length, 0);
+            file.read(&buffer[0], length, ec);
+            if (ec)
+            {
+                buffer.clear();
+            }
+        }
+    }
+    return buffer;
 }
 
-// Make this a define rather than using magic numbers multiple times in the code
-#define LLFILE_COPY_BUFFER_SIZE 16384
+// static
+int LLFile::mkdir(const std::string& dirname)
+{
+    std::error_code ec;
+    std::filesystem::path file_path = utf8StringToPath(dirname);
+    // We often use mkdir() to ensure the existence of a directory that might
+    // already exist. There is no known case in which we want to call out as
+    // an error the requested directory already existing.
+    std::filesystem::create_directory(file_path, ec);
+    // The return value is only true if the directory was actually created.
+    // But if it already existed, ec still indicates success.
+    return warnif("mkdir", dirname, ec);
+}
 
 // static
-bool LLFile::copy(const std::string& from, const std::string& to)
+int LLFile::remove(const std::string& filename, int suppress_warning)
 {
-    bool copied = false;
-    LLFILE* in = LLFile::fopen(from, "rb");
-    if (in)
+    std::error_code ec;
+    std::filesystem::path file_path = utf8StringToPath(filename);
+    std::filesystem::remove(file_path, ec);
+    return warnif("remove", filename, ec, suppress_warning);
+}
+
+// static
+int LLFile::rename(const std::string& filename, const std::string& newname, int suppress_warning)
+{
+    std::error_code ec;
+    std::filesystem::path file_path = utf8StringToPath(filename);
+    std::filesystem::path new_path = utf8StringToPath(newname);
+    std::filesystem::rename(file_path, new_path, ec);
+    return warnif(STRINGIZE("rename to '" << newname << "' from"), filename, ec, suppress_warning);
+}
+
+// static
+S64 LLFile::read(const std::string& filename, void* buf, S64 offset, S64 nbytes)
+{
+    std::error_code ec;
+    return read(filename, buf, offset, nbytes, ec);
+}
+
+// static
+S64 LLFile::read(const std::string& filename, void* buf, S64 offset, S64 nbytes, std::error_code& ec)
+{
+    // if number of bytes is 0 or less there is nothing to do here
+    if (nbytes <= 0)
+    {
+        clear_error(ec);
+        return 0;
+    }
+
+    if (!buf || offset < 0)
     {
-        LLFILE* out = LLFile::fopen(to, "wb");
-        if (out)
+        set_ec_to_parameter_error(ec);
+    }
+    else
+    {
+        std::ios_base::openmode omode = LLFile::in | LLFile::binary;
+
+        LLFile file(filename, omode, ec);
+        if (!ec && (bool)file)
         {
-            char buf[LLFILE_COPY_BUFFER_SIZE];
-            size_t readbytes;
-            bool write_ok = true;
-            while (write_ok && (readbytes = fread(buf, 1, LLFILE_COPY_BUFFER_SIZE, in)))
+            if (offset > 0)
+            {
+                file.seek(offset, ec);
+            }
+            // else (offset == 0) file was just opened and should already be at 0.
+            if (!ec)
             {
-                if (fwrite(buf, 1, readbytes, out) != readbytes)
+                S64 bytes_read = file.read(buf, nbytes, ec);
+                if (!ec)
                 {
-                    LL_WARNS("LLFile") << "Short write" << LL_ENDL;
-                    write_ok = false;
+                    return bytes_read;
                 }
             }
-            if ( write_ok )
+        }
+    }
+    return warnif("read from file failed", filename, ec);
+}
+
+// static
+S64 LLFile::write(const std::string& filename, const void* buf, S64 offset, S64 nbytes)
+{
+    std::error_code ec;
+    return write(filename, buf, offset, nbytes, ec);
+}
+
+// static
+S64 LLFile::write(const std::string& filename, const void* buf, S64 offset, S64 nbytes, std::error_code& ec)
+{
+    // if number of bytes is 0 or less there is nothing to do here
+    if (nbytes <= 0)
+    {
+        clear_error(ec);
+        return 0;
+    }
+
+    if (!buf)
+    {
+        set_ec_to_parameter_error(ec);
+    }
+    else
+    {
+        std::ios_base::openmode omode = LLFile::out | LLFile::binary;
+        if (offset < 0)
+        {
+            omode |= LLFile::app;
+        }
+
+        LLFile file(filename, omode, ec);
+        if (!ec && (bool)file)
+        {
+            if (offset > 0)
+            {
+                file.seek(offset, ec);
+            }
+            // else (offset == 0) we are not appending, file was just opened and should already be at 0.
+            if (!ec)
             {
-                copied = true;
+                S64 bytes_written = file.write(buf, nbytes, ec);
+                if (!ec)
+                {
+                    return bytes_written;
+                }
             }
-            fclose(out);
         }
-        fclose(in);
+    }
+    return warnif("write to file failed", filename, ec);
+}
+
+// static
+bool LLFile::copy(const std::string& source, const std::string& target)
+{
+    std::error_code ec;
+    return copy(source, target, std::filesystem::copy_options::overwrite_existing, ec);
+}
+
+// static
+bool LLFile::copy(const std::string& source, const std::string& target, std::filesystem::copy_options options, std::error_code& ec)
+{
+    std::filesystem::path source_path = utf8StringToPath(source);
+    std::filesystem::path target_path = utf8StringToPath(target);
+    bool copied = std::filesystem::copy_file(source_path, target_path, options, ec);
+    if (!copied)
+    {
+        warnif(STRINGIZE("copy failed, to '" << target << "' from"), source, ec);
     }
     return copied;
 }
 
 // static
-int LLFile::stat(const std::string& filename, llstat* filestatus, int suppress_error)
+int LLFile::stat(const std::string& filename, llstat* filestatus, const char *fname, int suppress_warning)
 {
 #if LL_WINDOWS
-    std::wstring utf16filename = utf8path_to_wstring(filename);
-    int rc = _wstat64(utf16filename.c_str(), filestatus);
+    std::wstring file_path = utf8StringToWstring(filename);
+    int rc = _wstat64(file_path.c_str(), filestatus);
 #else
     int rc = ::stat(filename.c_str(), filestatus);
 #endif
-    return warnif("stat", filename, rc, suppress_error);
+    return warnif(fname ? fname : "stat", filename, rc, suppress_warning);
 }
 
 // static
-unsigned short LLFile::getattr(const std::string& filename, bool dontFollowSymLink, int suppress_error)
+std::time_t LLFile::getCreationTime(const std::string& filename, int suppress_warning)
 {
-#if LL_WINDOWS
-    // _wstat64() is a bit heavyweight on Windows, use a more lightweight API
-    // to just get the attributes
-    int rc = -1;
-    std::wstring utf16filename = utf8path_to_wstring(filename);
-    unsigned short st_mode = get_fileattr(utf16filename, dontFollowSymLink);
-    if (st_mode)
+    // As of C++20 there is no functionality in std::filesystem to retrieve this information
+    llstat filestat;
+    int rc = stat(filename, &filestat, "getCreationTime", suppress_warning);
+    if (rc == 0)
     {
-        return st_mode;
-    }
+#if LL_DARWIN
+        return filestat.st_birthtime;
 #else
-    llstat filestatus;
-    int rc = dontFollowSymLink ? ::lstat(filename.c_str(), &filestatus) : ::stat(filename.c_str(), &filestatus);
+        // Linux stat() doesn't have a creation/birth time (st_ctime really is the last status
+        // change or inode attributes change) unless we would use statx() instead. But that is
+        // a major effort, which would require Linux specific changes to LLFile::stat() above
+        // and possibly adaptions for other platforms that we leave for a later exercise if it
+        // is ever desired.
+        return filestat.st_ctime;
+#endif
+    }
+    return 0;
+}
+
+// static
+std::time_t LLFile::getModificationTime(const std::string& filename, int suppress_warning)
+{
+    // tried to use std::filesystem::last_write_time() but the whole std::chrono infrastructure is as of
+    // C++20 still not fully implemented on all platforms. Specifically MacOS C++20 seems lacking here,
+    // and Windows requires a roundabout through std::chrono::utc_clock to then get a
+    // std::chrono::system_clock that can return a more useful time_t.
+    // So we take the easy way out in a similar way as with getCreationTime().
+    llstat filestat;
+    int rc = stat(filename, &filestat, "getModificationTime", suppress_warning);
     if (rc == 0)
     {
-        return filestatus.st_mode;
+        return filestat.st_mtime;
     }
-#endif
-    warnif("getattr", filename, rc, suppress_error);
     return 0;
 }
 
 // static
+S64 LLFile::size(const std::string& filename, int suppress_warning)
+{
+    std::error_code ec;
+    std::filesystem::path file_path = utf8StringToPath(filename);
+    std::intmax_t size = static_cast<std::intmax_t>(std::filesystem::file_size(file_path, ec));
+    if (ec)
+    {
+        return warnif("size", filename, ec, suppress_warning);
+    }
+    return size;
+}
+
+// static
+std::filesystem::file_status LLFile::getStatus(const std::string& filename, bool dontFollowSymLink, int suppress_warning)
+{
+    std::error_code ec;
+    std::filesystem::path file_path = utf8StringToPath(filename);
+    std::filesystem::file_status status;
+    if (dontFollowSymLink)
+    {
+        status = std::filesystem::symlink_status(file_path, ec);
+    }
+    else
+    {
+        status = std::filesystem::status(file_path, ec);
+    }
+    warnif("getStatus()", filename, ec, suppress_warning);
+    return status;
+}
+
+// static
+bool LLFile::exists(const std::string& filename)
+{
+    std::filesystem::file_status status = getStatus(filename);
+    return std::filesystem::exists(status);
+}
+
+// static
 bool LLFile::isdir(const std::string& filename)
 {
-    return S_ISDIR(getattr(filename));
+    std::filesystem::file_status status = getStatus(filename);
+    return std::filesystem::is_directory(status);
 }
 
 // static
 bool LLFile::isfile(const std::string& filename)
 {
-    return S_ISREG(getattr(filename));
+    std::filesystem::file_status status = getStatus(filename);
+    return std::filesystem::is_regular_file(status);
 }
 
 // static
 bool LLFile::islink(const std::string& filename)
 {
-    return S_ISLNK(getattr(filename, true));
+    std::filesystem::file_status status = getStatus(filename, true);
+    return std::filesystem::is_symlink(status);
 }
 
 // static
-const char *LLFile::tmpdir()
+const std::string& LLFile::tmpdir()
 {
-    static std::string utf8path;
-
-    if (utf8path.empty())
+    static std::string temppath;
+    if (temppath.empty())
     {
-        char sep;
-#if LL_WINDOWS
-        sep = '\\';
+        temppath = std::filesystem::temp_directory_path().string();
+    }
 
-        std::vector<wchar_t> utf16path(MAX_PATH + 1);
-        GetTempPathW(static_cast<DWORD>(utf16path.size()), &utf16path[0]);
-        utf8path = ll_convert_wide_to_string(&utf16path[0]);
+    char sep;
+#if LL_WINDOWS
+    sep = '\\';
 #else
-        sep = '/';
+    sep = '/';
+#endif
+    if (temppath[temppath.size() - 1] != sep)
+    {
+        temppath += sep;
+    }
+    return temppath;
+}
 
-        utf8path = LLStringUtil::getenv("TMPDIR", "/tmp/");
+// static
+std::filesystem::path LLFile::utf8StringToPath(const std::string& pathname)
+{
+#if LL_WINDOWS
+    return ll_convert<std::wstring>(pathname);
+#else
+    return pathname;
 #endif
-        if (utf8path[utf8path.size() - 1] != sep)
+}
+
+#if LL_WINDOWS
+
+// static
+std::wstring LLFile::utf8StringToWstring(const std::string& pathname)
+{
+    std::wstring utf16string(ll_convert<std::wstring>(pathname));
+    if (utf16string.size() >= MAX_PATH)
+    {
+        // By going through std::filesystem::path we get a lot of path sanitation done for us that
+        // is needed when passing a path with a kernel object space prefix to Windows API functions
+        // since this prefix disables the kernel32 path normalization
+        std::filesystem::path utf16path(utf16string);
+
+        // By prepending "\\?\" to a path, Windows widechar file APIs will not fail on long path names
+        utf16string.assign(L"\\\\?\\").append(utf16path);
+
+        /* remove trailing spaces and dots (yes, Windows really does that) */
+        size_t last_valid = utf16string.find_last_not_of(L" \t.");
+        if (last_valid == std::wstring::npos)
         {
-            utf8path += sep;
+            return std::wstring();
         }
+        return utf16string.substr(0, last_valid + 1);
     }
-    return utf8path.c_str();
+    return utf16string;
 }
 
-#if LL_WINDOWS
-
 /************** input file stream ********************************/
 
 llifstream::llifstream() {}
@@ -586,10 +1197,8 @@ void llifstream::open(const std::string& _Filename, ios_base::openmode _Mode)
                         _Mode | ios_base::in);
 }
 
-
 /************** output file stream ********************************/
 
-
 llofstream::llofstream() {}
 
 // explicit
@@ -605,30 +1214,4 @@ void llofstream::open(const std::string& _Filename, ios_base::openmode _Mode)
                         _Mode | ios_base::out);
 }
 
-/************** helper functions ********************************/
-
-std::streamsize llifstream_size(llifstream& ifstr)
-{
-    if(!ifstr.is_open()) return 0;
-    std::streampos pos_old = ifstr.tellg();
-    ifstr.seekg(0, ios_base::beg);
-    std::streampos pos_beg = ifstr.tellg();
-    ifstr.seekg(0, ios_base::end);
-    std::streampos pos_end = ifstr.tellg();
-    ifstr.seekg(pos_old, ios_base::beg);
-    return pos_end - pos_beg;
-}
-
-std::streamsize llofstream_size(llofstream& ofstr)
-{
-    if(!ofstr.is_open()) return 0;
-    std::streampos pos_old = ofstr.tellp();
-    ofstr.seekp(0, ios_base::beg);
-    std::streampos pos_beg = ofstr.tellp();
-    ofstr.seekp(0, ios_base::end);
-    std::streampos pos_end = ofstr.tellp();
-    ofstr.seekp(pos_old, ios_base::beg);
-    return pos_end - pos_beg;
-}
-
 #endif  // LL_WINDOWS
diff --git a/indra/llcommon/llfile.h b/indra/llcommon/llfile.h
index 04a2946ac4..0036e896c3 100644..100755
--- a/indra/llcommon/llfile.h
+++ b/indra/llcommon/llfile.h
@@ -35,118 +35,359 @@
  * Attempts to mostly mirror the POSIX style IO functions.
  */
 
-typedef FILE    LLFILE;
-
 #include <fstream>
+#include <filesystem>
 #include <sys/stat.h>
 
 #if LL_WINDOWS
+#include <windows.h>
 // The Windows version of stat function and stat data structure are called _stat64
 // We use _stat64 here to support 64-bit st_size and time_t values
-typedef struct _stat64  llstat;
+typedef struct _stat64 llstat;
 #else
-typedef struct stat     llstat;
 #include <sys/types.h>
+typedef struct stat llstat;
 #endif
 
-#ifndef S_ISREG
-# define S_ISREG(x) (((x) & S_IFMT) == S_IFREG)
-#endif
-
-#ifndef S_ISDIR
-# define S_ISDIR(x) (((x) & S_IFMT) == S_IFDIR)
-#endif
-
-// Windows C runtime library does not define this and does not support symlink detection in the
-// stat functions but we do in our getattr() function
-#ifndef S_IFLNK
-#define S_IFLNK 0xA000 /* symlink */
-#endif
-
-#ifndef S_ISLNK
-#define S_ISLNK(x) (((x) & S_IFMT) == S_IFLNK)
-#endif
+typedef FILE LLFILE;
 
 #include "llstring.h" // safe char* -> std::string conversion
 
-/// LLFile is a class of static functions operating on paths
+/// This class provides a selection of functions to operate on files through names and
+/// a class implementation to represent a file for reading and writing to it
 /// All the functions with a path string input take UTF8 path/filenames
+///
+/// @nosubgrouping
+///
 class LL_COMMON_API LLFile
 {
 public:
+    // ================================================================================
+    /// @name Constants
+    ///
+    ///@{
+    /** These can be passed to the omode parameter of LLFile::open() and its constructor
+
+    This is similar to the openmode flags for std:fstream but not exactly the same
+    std::fstream open() does not allow to open a file for writing without either
+    forcing the file to be truncated on open or all write operations being always
+    appended to the end of the file or failing to open when the file does not exist.
+    But to allow implementing the LLAPRFile::writeEx() functionality we need to be
+    able to write at a random position to the file without truncating it on open.
+
+    any other combinations than listed here are not allowed and will cause an error
+
+    bin    in     out    trunc  app    noreplace  File exists       File doesn't exist
+    ----------------------------------------------------------------------------------
+    -      +      -      -      -      -          Open at begin     Failure to open
+    +      +      -      -      -      -            "                 "
+    -      -      +      -      -      -            "               Create new
+    +      -      +      -      -      -            "                 "
+    -      +      +      -      -      -            "                 "
+    +      +      +      -      -      -            "                 "
+    ----------------------------------------------------------------------------------
+    -      -      +      +      -      -          Destroy contents  Create new
+    +      -      +      +      -      -            "                 "
+    -      +      +      +      -      -            "                 "
+    +      +      +      +      -      -            "                 "
+    ----------------------------------------------------------------------------------
+    -      -      +      x      -      +          Failure to open   Create new
+    +      -      +      x      -      +            "                 "
+    -      +      +      x      -      +            "                 "
+    +      +      +      x      -      +            "                 "
+    ----------------------------------------------------------------------------------
+    -      -      +      -      +      -          Write to end      Create new
+    +      -      +      -      +      -            "                 "
+    -      +      +      -      +      -            "                 "
+    +      +      +      -      +      -            "                 "
+    ----------------------------------------------------------------------------------
+    */
+    static const std::ios_base::openmode app       = static_cast<std::ios_base::openmode>(1 << 1);    // append to end
+    static const std::ios_base::openmode ate       = static_cast<std::ios_base::openmode>(1 << 2);    // initialize to end
+    static const std::ios_base::openmode binary    = static_cast<std::ios_base::openmode>(1 << 3);    // binary mode
+    static const std::ios_base::openmode in        = static_cast<std::ios_base::openmode>(1 << 4);    // for reading
+    static const std::ios_base::openmode out       = static_cast<std::ios_base::openmode>(1 << 5);    // for writing          // for writing and reading
+    static const std::ios_base::openmode trunc     = static_cast<std::ios_base::openmode>(1 << 6);    // truncate on open
+    static const std::ios_base::openmode noreplace = static_cast<std::ios_base::openmode>(1 << 7);    // no replace if it exists
+
+    /// Additional optional flags to omode in open() and lmode in fopen() or lock()
+    /// to indicate which sort of lock if any to attempt to get
+    ///
+    /// NOTE: there is a fundamental difference between platforms.
+    /// On Windows this lock is mandatory as it is part of the API to open a file handle and other
+    /// processes can not avoid it. If a file was opened denying other processes read and/or write
+    /// access, trying to open the same file in another process with that access will fail.
+    /// On Mac and Linux it is only an advisory lock implemented through the flock() system call.
+    /// This means that any other application needs to also attempt to at least acquire a shared
+    /// lock on the file in order to notice that the file is actually already locked. It can
+    /// therefore not be used to prevent random other applications from accessing the file, but it
+    /// works for other viewer processes when they use either the LLFile::open() or LLFile::fopen()
+    /// functions with the appropriate lock flags to open a file.
+    static const std::ios_base::openmode exclusive = static_cast<std::ios_base::openmode>(1 << 16);
+    static const std::ios_base::openmode shared    = static_cast<std::ios_base::openmode>(1 << 17);
+
+    /// Additional lmode flag to indicate to rather fail instead of blocking when trying
+    /// to acquire a lock with LLFile::lock()
+    static const std::ios_base::openmode noblock   = static_cast<std::ios_base::openmode>(1 << 18);
+
+    /// The mask value for the lock mask bits
+    static const std::ios_base::openmode lock_mask = static_cast<std::ios_base::openmode>(exclusive | shared);
+
+    /// One of these can be passed to the dir parameter of LLFile::seek()
+    static const std::ios_base::seekdir beg        = std::ios_base::beg;
+    static const std::ios_base::seekdir cur        = std::ios_base::cur;
+    static const std::ios_base::seekdir end        = std::ios_base::end;
+    ///@}
+
+    // ================================================================================
+    /// @name constructor/deconstructor
+    ///
+    ///@{
+    ///  default constructor
+    LLFile() : mHandle(InvalidHandle) {}
+
+    /// no copy constructor
+    LLFile(const LLFile&) = delete;
+
+    /// move constructor
+    LLFile(LLFile&& other) noexcept
+    {
+        mHandle = other.mHandle;
+        other.mHandle = InvalidHandle;
+    }
+
+    /// constructor opening the file
+    LLFile(const std::string& filename, std::ios_base::openmode omode, std::error_code& ec, int perm = 0666) :
+        mHandle(InvalidHandle)
+    {
+        open(filename, omode, ec, perm);
+    }
+
+    /// destructor always attempts to close the file
+    ~LLFile() { close(); }
+    ///@}
+
+    // ================================================================================
+    /// @name operators
+    ///
+    ///@{
+    /// copy assignment deleted
+    LLFile& operator=(const LLFile&) = delete;
+
+    /// move assignment
+    LLFile& operator=(LLFile&& other) noexcept
+    {
+        close();
+        std::swap(mHandle, other.mHandle);
+        return *this;
+    }
+
+    // detect whether the wrapped file descriptor/handle is open or not
+    explicit operator bool() const { return (mHandle != InvalidHandle); }
+    bool     operator!() { return (mHandle == InvalidHandle); }
+    ///@}
+
+    /// ================================================================================
+    /// @name  class member methods
+    ///
+    /// These methods provide read and write support as well as additional functionality to query the size of
+    /// the file, change the position of the current file pointer or query it.
+    ///
+    /// Most of these functions take as one of their parameters a std::error_code object which can be used to
+    /// determine in more detail what error occurred if required
+    ///@{
+
+    /// Open a file with the specific open mode flags
+    int open(const std::string& filename, std::ios_base::openmode omode, std::error_code& ec, int perm = 0666);
+    ///< @returns 0 on success, -1 on failure
+
+    /// Determine the size of the opened file
+    S64 size(std::error_code& ec);
+    ///< @returns the number of bytes in the file or -1 on failure
+
+    /// Query the position of the current file pointer in the file
+    S64 tell(std::error_code& ec);
+    ///< @returns the absolute offset of the file pointer in bytes relative to the start of the file or -1 on failure
+
+    /// Move the file pointer to the specified absolute position relative to the start of the file
+    int seek(S64 pos, std::error_code& ec);
+    ///< @returns 0 on success, -1 on failure
+
+    /// Move the file pointer to the specified position relative to dir
+    int seek(S64 offset, std::ios_base::seekdir dir, std::error_code& ec);
+    ///< @returns 0 on success, -1 on failure
+
+    /// Read the specified number of bytes into the buffer starting at the current file pointer
+    S64 read(void* buffer, S64 nbytes, std::error_code& ec);
+    ///< If the file ends before the requested amount of bytes could be read, the function succeeds and
+    ///  returns the bytes up to the end of the file. The return value indicates the number of actually
+    ///  read bytes and can be therefore smaller than the requested amount.
+    ///  @returns the number of bytes read from the file or -1 on failure
+
+    /// Write the specified number of bytes to the file starting at the current file pointer
+    S64 write(const void* buffer, S64 nbytes, std::error_code& ec);
+    ///< @returns the number of bytes written to the file or -1 on failure
+
+    /// Write into the file starting at the current file pointer using printf style format and
+    /// additional optional parameters as specified in the fmt string
+    S64 printf(const char* fmt, ...);
+    ///< @returns the number of bytes written to the file or -1 on failure
+
+    /// Attempt to acquire or release a lock on the file
+    int lock(int lmode, std::error_code& ec);
+    ///< lmode can be one of LLFile::exclusive or LLFile::shared to acquire the according lock
+    ///  or 0 to give up an earlier acquired lock. Adding LLFile::noblock together with one of
+    ///  the lock requests will cause the function to fail if the lock can not be acquired,
+    ///  otherwise the function will block until the lock can be acquired.
+    ///  @returns 0 on success, -1 on failure
+
+    /// close the file explicitly
+    int close(std::error_code& ec);
+    ///< @returns 0 on success, -1 on failure
+
+    /// Convenience function to close the file without additional parameters
+    int close();
+    ///< @returns 0 on success, -1 on failure
+    ///@}
+
+    /// ================================================================================
+    /// @name  static member functions
+    ///
+    /// These functions are static and operate with UTF8 filenames as one of their parameters.
+    ///
+    ///@{
     /// open a file with the specified access mode
-    static  LLFILE* fopen(const std::string& filename, const char* accessmode);  /* Flawfinder: ignore */
+    static LLFILE* fopen(const std::string& filename, const char* accessmode, int lmode = 0);
     ///< 'accessmode' follows the rules of the Posix fopen() mode parameter
-    /// "r" open the file for reading only and positions the stream at the beginning
-    /// "r+" open the file for reading and writing and positions the stream at the beginning
-    /// "w" open the file for reading and writing and truncate it to zero length
-    /// "w+" open or create the file for reading and writing and truncate to zero length if it existed
-    /// "a" open the file for reading and writing and position the stream at the end of the file
-    /// "a+" open or create the file for reading and writing and position the stream at the end of the file
+    ///  "r" open the file for reading only and positions the stream at the beginning
+    ///  "r+" open the file for reading and writing and positions the stream at the beginning
+    ///  "w" open the file for reading and writing and truncate it to zero length
+    ///  "w+" open or create the file for reading and writing and truncate to zero length if it existed
+    ///  "a" open the file for reading and writing and before every write position the stream at the end of the file
+    ///  "a+" open or create the file for reading and writing and before every write position the stream at the end of the file
+    ///
+    ///  in addition to these values, "b" can be appended to indicate binary stream access, but on Linux and Mac
+    ///  this is strictly for compatibility and has no effect. On Windows this makes the file functions not
+    ///  try to translate line endings. Windows also allows to append "t" to indicate text mode. If neither
+    ///  "b" or "t" is defined, Windows uses the value set by _fmode which by default is _O_TEXT.
+    ///  This means that it is always a good idea to append "b" specifically for binary file access to
+    ///  avoid corruption of the binary consistency of the data stream when reading or writing
+    ///  Other characters in 'accessmode', while possible on some platforms (Windows), will usually
+    ///  cause an error on other platforms as fopen will verify this parameter
     ///
-    /// in addition to these values, "b" can be appended to indicate binary stream access, but on Linux and Mac
-    /// this is strictly for compatibility and has no effect. On Windows this makes the file functions not
-    /// try to translate line endings. Windows also allows to append "t" to indicate text mode. If neither
-    /// "b" or "t" is defined, Windows uses the value set by _fmode which by default is _O_TEXT.
-    /// This means that it is always a good idea to append "b" specifically for binary file access to
-    /// avoid corruption of the binary consistency of the data stream when reading or writing
-    /// Other characters in 'accessmode' will usually cause an error as fopen will verify this parameter
-    /// @returns a valid LLFILE* pointer on success or NULL on failure
+    ///  lmode is optional and allows to lock the file for other processes either as a shared lock or an
+    ///  exclusive lock. If the requested lock conflicts with an already existing lock, the open fails.
+    ///  Pass either LLFIle::exclusive or LLFile::shared to this parameter if you want to prevent other
+    ///  processes from reading (exclusive lock) or writing (shared lock) to the file. It will always use
+    ///  LLFile::noblock, meaning the open will immediately fail if it conflicts with an existing lock on the
+    ///  file.
+    ///
+    ///  @returns a valid LLFILE* pointer on success that can be passed to the fread() and fwrite() functions
+    ///  and some other f<something> functions in the Standard C library that accept a FILE* as parameter
+    ///  or NULL on failure
 
+    /// Close a file handle opened with fopen() above
     static  int     close(LLFILE * file);
-
-    /// retrieve the content of a file into a string
-    static std::string getContents(const std::string& filename);
-    ///< @returns the content of the file or an empty string on failure
+    ///< @returns 0 on success and -1 on failure.
 
     /// create a directory
-    static  int     mkdir(const std::string& filename, int perms = 0700);
-    ///< perms is a permissions mask like 0777 or 0700.  In most cases it will be
-    ///  overridden by the user's umask. It is ignored on Windows.
-    ///  mkdir() considers "directory already exists" to be not an error.
-    ///  @returns 0 on success and -1 on failure.
-
-    //// remove a directory
-    static  int     rmdir(const std::string& filename, int suppress_error = 0);
-    ///< pass ENOENT in the optional 'suppress_error' parameter
-    ///  if you don't want a warning in the log when the directory does not exist
+    static  int     mkdir(const std::string& filename);
+    ///< mkdir() considers "directory already exists" to be not an error.
     ///  @returns 0 on success and -1 on failure.
 
     /// remove a file or directory
-    static  int     remove(const std::string& filename, int suppress_error = 0);
-    ///< pass ENOENT in the optional 'suppress_error' parameter
-    ///  if you don't want a warning in the log when the directory does not exist
+    static  int     remove(const std::string& filename, int suppress_warning = 0);
+    ///< pass an errno value (e.g., ENOENT) in the optional 'suppress_warning' parameter if you want to
+    ///  suppress a warning in the log when the failure matches that errno (e.g., suppress warning if
+    ///  the file or directory does not exist)
     ///  @returns 0 on success and -1 on failure.
 
     /// rename a file
-    static  int     rename(const std::string& filename, const std::string& newname, int suppress_error = 0);
+    static  int     rename(const std::string& filename, const std::string& newname, int suppress_warning = 0);
     ///< it will silently overwrite newname if it exists without returning an error
     ///  Posix guarantees that if newname already exists, then there will be no moment
     ///  in which for other processes newname does not exist. There is no such guarantee
-    ///  under Windows at this time. It may do it in the same way but the used Windows API
-    ///  does not make such guarantees.
+    ///  under Windows at this time. It may do it in the same way but the used Windows
+    ///  APIs do not make such guarantees.
     ///  @returns 0 on success and -1 on failure.
 
+    /// copy the contents of the file from 'source' to 'target'
+    static  bool    copy(const std::string& source, const std::string& target);
+    ///< Copies the contents of the file 'source' to the file 'target', overwriting 'target' if it already
+    ///  existed.
+    ///  This is a convenience function that implements the previous behavior of silently overwriting an
+    ///  already existing target file. Consider using the function below if you desire a different
+    ///  behavior when the target file already exists
+    ///  @returns true on success and false on failure.
 
-    /// copy the contents of file from 'from' to 'to' filename
-    static  bool    copy(const std::string& from, const std::string& to);
-    ///< @returns true on success and false on failure.
+    /// copy the contents of the file from 'from' to 'to'
+    static  bool    copy(const std::string& source, const std::string& target, std::filesystem::copy_options options, std::error_code& ec);
+    ///< Copies the contents of the file 'source' to the file 'target'. The options parameter allows to
+    ///  specify what should happen if the "target" file already exists:
+    ///   std::filesystem::copy_options::none - return an error in ec and fail
+    ///   std::filesystem::copy_options::skip_existing - skip the operation and do not overwrite file
+    ///   std::filesystem::copy_options::overwrite_existing - overwrite the file
+    ///   std::filesystem::copy_options::update_existing - overwrite the file only if it is older than the file being copied
+    ///  @returns true on success and false on failure.
+
+    /// retrieve the content of a file into a string
+    static std::string getContents(const std::string& filename);
+    static std::string getContents(const std::string& filename, std::error_code& ec);
+    ///< @returns the entire content of the file as std::string or an empty string on failure
+
+    /// read nBytes from the file into the buffer, starting at offset in the file
+    static  S64     read(const std::string& filename, void* buf, S64 offset, S64 nbytes);
+    static  S64     read(const std::string& filename, void* buf, S64 offset, S64 nbytes, std::error_code& ec);
+    ///< @returns bytes read on success, or -1 on failure
+
+    /// write nBytes from the buffer into the file, starting at offset in the file
+    static  S64     write(const std::string& filename, const void* buf, S64 offset, S64 nbytes);
+    static  S64     write(const std::string& filename, const void* buf, S64 offset, S64 nbytes, std::error_code& ec);
+    ///< If a negative offset is provided, the file is opened in append mode and the
+    ///  write will be appended to the end of the file.
+    ///  @returns bytes written on success, or -1 on failure
 
     /// return the file stat structure for filename
-    static  int     stat(const std::string& filename, llstat* file_status, int suppress_error = ENOENT);
+    static  int     stat(const std::string& filename, llstat* file_status, const char *operation = nullptr, int suppress_warning = ENOENT);
     ///< for compatibility with existing uses of LL_File::stat() we use ENOENT as default in the
-    ///  optional 'suppress_error' parameter to avoid spamming the log with warnings when the API
+    ///  optional 'suppress_warning' parameter to avoid spamming the log with warnings when the API
     ///  is used to detect if a file exists
     ///  @returns 0 on success and -1 on failure.
 
-    /// get the file or directory attributes for filename
-    static  unsigned short getattr(const std::string& filename, bool dontFollowSymLink = false, int suppress_error = ENOENT);
-    ///< a more lightweight function on Windows to stat, that just returns the file attribute flags
-    ///  dontFollowSymLinks set to true returns the attributes of the symlink if it is one, rather than resolving it
-    ///  we pass by default ENOENT in the optional 'suppress_error' parameter to not spam the log with
-    ///  warnings when the file or directory does not exist
-    ///  @returns 0 on failure and a st_mode value with either S_IFDIR or S_IFREG set otherwise
-    ///  together with the three access bits which under Windows only the write bit is relevant.
+    /// get the creation data and time of a file
+    static std::time_t getCreationTime(const std::string& filename, int suppress_warning = 0);
+    ///< Different systems have different support for this. Under Windows this is supposedly
+    ///  the actual time the file was created, on the Mac this is the actual birth date of
+    ///  the file which is in fact the creation time. The according ctime entry in the stat
+    ///  structure under Linux (and any other *nix really) is however contrary to what one
+    ///  might expect based on the c in ctime not the creation time but the time the last
+    ///  change to the inode entry was made. Changing access rights to a file will update
+    ///  this value too. In order to have a true creation time under Linux, we would have
+    ///  to use the statx() call which is available since kernel 4.19, but that will require
+    ///  considerable changes to the implementation of above stat() function.
+    ///  @returns the creation time (last status change under Linux) of the file or 0 on error
+
+    /// get the last modification data and time of a file
+    static std::time_t getModificationTime(const std::string& filename, int suppress_warning = 0);
+    ///< @returns the modification time of the file or 0 on error
+
+    /// get the std::filesystem::file_status for filename
+    static std::filesystem::file_status getStatus(const std::string& filename, bool dontFollowSymLink = false, int suppress_warning  = ENOENT);
+    ///< dontFollowSymLinks set to true returns the std::filesystem::file_status of the symlink if it
+    ///  is one, rather than resolving it. We pass by default ENOENT in the optional 'suppress_warning'
+    ///  parameter to not spam the log with warnings when the file or directory does not exist
+    ///  @returns a std::filesystem::file_status value that can be passed to the appropriate std::filesystem::exists()
+    ///  and other APIs accepting a file_status.
+
+    /// get the size of a file in bytes
+    static  S64     size(const std::string& filename, int suppress_warning = ENOENT);
+    ///< we pass by default ENOENT in the optional 'suppress_warning' parameter to not spam
+    ///  the log with warnings when the file does not exist
+    ///  @returns the file size on success or -1 on failure.
+
+    /// check if filename is an existing file or directory
+    static  bool    exists(const std::string& filename);
+    ///< @returns true if the path is for an existing file or directory
 
     /// check if filename is an existing directory
     static  bool    isdir(const std::string& filename);
@@ -161,70 +402,44 @@ public:
     ///< @returns true if the path is pointing at a symlink
 
     /// return a path to the temporary directory on the system
-    static  const char * tmpdir();
-};
-
-/// RAII class
-class LLUniqueFile
-{
-public:
-    // empty
-    LLUniqueFile(): mFileHandle(nullptr) {}
-    // wrap (e.g.) result of LLFile::fopen()
-    LLUniqueFile(LLFILE* f): mFileHandle(f) {}
-    // no copy
-    LLUniqueFile(const LLUniqueFile&) = delete;
-    // move construction
-    LLUniqueFile(LLUniqueFile&& other) noexcept
-    {
-        mFileHandle = other.mFileHandle;
-        other.mFileHandle = nullptr;
-    }
-    // The point of LLUniqueFile is to close on destruction.
-    ~LLUniqueFile()
-    {
-        close();
-    }
-
-    // simple assignment
-    LLUniqueFile& operator=(LLFILE* f)
-    {
-        close();
-        mFileHandle = f;
-        return *this;
-    }
-    // copy assignment deleted
-    LLUniqueFile& operator=(const LLUniqueFile&) = delete;
-    // move assignment
-    LLUniqueFile& operator=(LLUniqueFile&& other) noexcept
-    {
-        close();
-        std::swap(mFileHandle, other.mFileHandle);
-        return *this;
-    }
+    static const std::string& tmpdir();
 
-    // explicit close operation
-    void close()
-    {
-        if (mFileHandle)
-        {
-            // in case close() throws, set mFileHandle null FIRST
-            LLFILE* h{nullptr};
-            std::swap(h, mFileHandle);
-            LLFile::close(h);
-        }
-    }
-
-    // detect whether the wrapped LLFILE is open or not
-    explicit operator bool() const { return bool(mFileHandle); }
-    bool operator!() { return ! mFileHandle; }
-
-    // LLUniqueFile should be usable for any operation that accepts LLFILE*
-    // (or FILE* for that matter)
-    operator LLFILE*() const { return mFileHandle; }
+    /// converts a string containing a path in utf8 encoding into an explicit filesystem path
+    static std::filesystem::path utf8StringToPath(const std::string& pathname);
+    ///< @returns the path as a std::filesystem::path
+    ///@}
 
 private:
-    LLFILE* mFileHandle;
+#if LL_WINDOWS
+    typedef HANDLE        llfile_handle_t;
+    const llfile_handle_t InvalidHandle = INVALID_HANDLE_VALUE;
+#else
+    typedef int           llfile_handle_t;
+    const llfile_handle_t InvalidHandle = -1;
+#endif
+
+    /// ================================================================================
+    /// @name private static member functions
+    ///
+#if LL_WINDOWS
+    /// convert a string containing a path in utf8 encoding into a Windows format std::wstring
+    static std::wstring utf8StringToWstring(const std::string& pathname);
+    ///< this will prepend the path with the Windows kernel object space prefix when the path is
+    ///  equal or longer than MAX_PATH characters and do some sanitation on the path.
+    ///  This allows the underlaying Windows APIs to process long path names. Do not pass such a path
+    ///  to std::filesystem functions. These functions are not guaranteed to handle such paths properly.
+    ///  It's only useful to pass the resulting string buffer to Microsoft Windows widechar APIs or
+    ///  the Microsoft C runtime widechar file functions.
+    ///
+    ///  Example:
+    ///
+    ///  std::wstring file_path = utf8StringToWstring(filename);
+    ///  HANDLE CreateFileW(file_path.c_str(), ......);
+    ///
+    ///  @returns the path as a std::wstring path
+#endif
+    llfile_handle_t mHandle;       // The file handle/descriptor
+    std::ios_base::openmode mOpen; // Used to emulate std::ios_base::app under Windows
 };
 
 #if LL_WINDOWS
@@ -315,17 +530,6 @@ class LL_COMMON_API llofstream : public std::ofstream
               ios_base::openmode _Mode = ios_base::out|ios_base::trunc);
 };
 
-
-/**
- * @brief filesize helpers.
- *
- * The file size helpers are not considered particularly efficient,
- * and should only be used for config files and the like -- not in a
- * loop.
- */
-std::streamsize LL_COMMON_API llifstream_size(llifstream& fstr);
-std::streamsize LL_COMMON_API llofstream_size(llofstream& fstr);
-
 #else // ! LL_WINDOWS
 
 // on non-windows, llifstream and llofstream are just mapped directly to the std:: equivalents
diff --git a/indra/llcommon/tests/llfile_test.cpp b/indra/llcommon/tests/llfile_test.cpp
new file mode 100644
index 0000000000..9f2a9b2988
--- /dev/null
+++ b/indra/llcommon/tests/llfile_test.cpp
@@ -0,0 +1,253 @@
+/**
+ * @file llfile_tut.cpp
+ * @author Frederick Martian
+ * @date 2025-11
+ * @brief LLFile test cases.
+ *
+ * $LicenseInfo:firstyear=2025&license=viewerlgpl$
+ * Second Life Viewer Source Code
+ * Copyright (C) 2025, Linden Research, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation;
+ * version 2.1 of the License only.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ *
+ * Linden Research, Inc., 945 Battery Street, San Francisco, CA  94111  USA
+ * $/LicenseInfo$
+ */
+
+#include <tut/tut.hpp>
+#include "lltut.h"
+#include "linden_common.h"
+#include "llfile.h"
+
+namespace tut
+{
+    static void clear_entire_dir(std::filesystem::path &dir)
+    {
+        std::error_code ec;
+        std::filesystem::remove_all(dir, ec);
+    }
+
+    static std::filesystem::path append_filename(const std::filesystem::path& dir, const std::string& element)
+    {
+        std::filesystem::path path = dir;
+        return path.append(element);
+    }
+
+    static std::filesystem::path get_testdir(const std::filesystem::path& tempdir)
+    {
+        return append_filename(tempdir, std::string("test_dir"));
+    }
+
+    struct llfile_test
+    {
+        std::filesystem::path tempdir = LLFile::tmpdir();
+        std::filesystem::path testdir = get_testdir(tempdir);
+    };
+    typedef test_group<llfile_test> llfile_test_t;
+    typedef llfile_test_t::object   llfile_test_object_t;
+    tut::llfile_test_t              tut_llfile_test("llfile_test");
+
+    template<> template<>
+    void llfile_test_object_t::test<1>()
+    {
+        // Test creating directories and files and deleting them and checking if the
+        // relevant status functions work as expected
+        ensure("LLFile::tmpdir() empty", !tempdir.empty());
+        ensure("LLFile::tmpdir() doesn't exist", LLFile::exists(tempdir.string()));
+        ensure("LLFile::tmpdir() is not a directory", LLFile::isdir(tempdir.string()));
+        ensure("LLFile::tmpdir() should not be a file", !LLFile::isfile(tempdir.string()));
+
+        // Make sure there is nothing left from a previous test run
+        clear_entire_dir(testdir);
+        ensure("llfile_test should not exist anymore", !LLFile::exists(testdir.string()));
+
+        int rc = LLFile::mkdir(testdir.string());
+        ensure("LLFile::mkdir() failed", rc == 0);
+        ensure("llfile_test should be a directory", LLFile::isdir(testdir.string()));
+        rc = LLFile::mkdir(testdir.string());
+        ensure("LLFile::mkdir() should not fail when the directory already exists", rc == 0);
+
+        std::filesystem::path testfile1 = testdir;
+        testfile1.append("llfile_test.dat");
+        ensure("llfile_test1.dat should not yet exist", !LLFile::exists(testfile1.string()));
+
+        const char* testdata = "testdata";
+        S64 bytes = LLFile::write(testfile1.string(), testdata, 0, sizeof(testdata));
+        ensure("LLFile::write() did not write correctly", bytes == sizeof(testdata));
+
+        rc = LLFile::remove(testfile1.string());
+        ensure("LLFile::remove() for file test_file.dat", rc == 0);
+        ensure("llfile_test.dat should not exist anymore", !LLFile::exists(testfile1.string()));
+        ensure("llfile_test.dat should not be a file", !LLFile::isfile(testfile1.string()));
+        ensure("llfile_test.dat should not be a directory", !LLFile::isdir(testfile1.string()));
+        ensure("llfile_test.dat should not be a symlink", !LLFile::islink(testfile1.string()));
+
+        rc = LLFile::remove(testdir.string());
+        ensure("LLFile::remove() for directory llfile_test failed", rc == 0);
+        ensure("llfile_test should not exist anymore", !LLFile::exists(testdir.string()));
+    }
+
+    template<> template<>
+    void llfile_test_object_t::test<2>()
+    {
+        // High level static file IO functions to read and write data files
+        LLFile::mkdir(testdir.string());
+        ensure("llfile_test should exist", LLFile::isdir(testdir.string()));
+
+        std::filesystem::path testfile1 = testdir;
+        testfile1.append("llfile_test.dat");
+
+        std::string testdata1("testdata");
+        std::string testdata2("datateststuff");
+        std::time_t current = time(nullptr);
+        S64 bytes = LLFile::write(testfile1.string(), testdata1.c_str(), 0, testdata1.length());
+        ensure("LLFile::write() did not write correctly", bytes == testdata1.length());
+        ensure("llfile_test.dat should exist", LLFile::exists(testfile1.string()));
+        ensure("llfile_test.dat should be a file", LLFile::isfile(testfile1.string()));
+        ensure("llfile_test.dat should not be a directory", !LLFile::isdir(testfile1.string()));
+
+        bytes = LLFile::size(testfile1.string());
+        ensure("LLFile::size() did not return the correct size", bytes == testdata1.length());
+
+        std::string data = LLFile::getContents(testfile1.string());
+        ensure("LLFile::getContents() did not return the correct size data", data.length() == testdata1.length());
+        ensure_memory_matches("LLFile::getContents() did not read correct data", testdata1.c_str(), (U32)testdata1.length(), data.c_str(), (U32)data.length());
+
+        std::time_t ctime = LLFile::getCreationTime(testfile1.string());
+        ensure_approximately_equals_range("LLFile::getCreationTime() did not return correct time", (F32)(ctime - current), 0.f, 1);
+
+        std::time_t mtime = LLFile::getModificationTime(testfile1.string());
+        ensure_approximately_equals_range("LLFile::getModificationTime() did not return correct time", (F32)(mtime - current), 0.f, 1);
+
+        char buffer[1024];
+        bytes = LLFile::read(testfile1.string(), buffer, 0, testdata1.length());
+        ensure("LLFile:read() did not return the correct size", bytes == testdata1.length());
+        ensure_memory_matches("LLFile::read() did not read correct data", testdata1.c_str(), (U32)bytes, buffer, (U32)bytes);
+
+        // What if we try to read more data than there is in the file?
+        bytes = LLFile::read(testfile1.string(), buffer, 0, bytes + 10);
+        ensure("LLFile:read() did not correctly stop on eof", bytes == testdata1.length());
+        ensure_memory_matches("LLFile::read() did not read correct data", testdata1.c_str(), (U32)bytes, buffer, (U32)bytes);
+
+        // Let's append more data
+        bytes = LLFile::write(testfile1.string(), testdata2.c_str(), -1, testdata2.length());
+        ensure("LLFile::write() did not write correctly", bytes == testdata2.length());
+
+        bytes = LLFile::size(testfile1.string());
+        ensure("LLFile::size() did not return the correct size", bytes == testdata1.length() + testdata2.length());
+        bytes = LLFile::read(testfile1.string(), buffer, 0, bytes);
+        ensure("LLFile:read() did not read correct number of bytes", bytes == testdata1.length() + testdata2.length());
+        ensure_memory_matches("LLFile:read() did not read correct testdata1", testdata1.c_str(), (U32)testdata1.length(), buffer, (U32)testdata1.length());
+        ensure_memory_matches("LLFile:read() did not read correct testdata2", testdata2.c_str(), (U32)testdata2.length(), buffer + testdata1.length(), (U32)testdata2.length());
+    }
+
+    template<> template<>
+    void llfile_test_object_t::test<3>()
+    {
+        const size_t numints = 1024;
+
+        // Testing the LLFile class implementation
+        std::filesystem::path testfile = testdir;
+        testfile.append("llfile_test.bin");
+
+        int data[numints];
+        for (int &t : data)
+        {
+            t = rand();
+        }
+
+        std::error_code ec;
+        LLFile fileout(testfile.string(), LLFile::out, ec);
+        ensure("LLFile constructor did not open correctly", (bool)fileout);
+        ensure("error_code from LLFile constructor should not indicate an error", !ec);
+        if (fileout)
+        {
+            S64 length = fileout.size(ec);
+            ensure("freshly created file should be empty", length == 0);
+            ensure("error_code from LLFile::size() should not indicate an error", !ec);
+            S64 bytes = fileout.write(data, sizeof(data), ec);
+            ensure("LLFile::write() did not write correctly", bytes == sizeof(data));
+            ensure("error_code from LLFile::write() should not indicate an error", !ec);
+            bytes = fileout.write(data, sizeof(data), ec);
+            ensure("LLFile::write() did not write correctly", bytes == sizeof(data));
+            ensure("error_code from LLFile::write() should not indicate an error", !ec);
+            bytes = fileout.size(ec);
+            ensure("LLFile::size() returned wrong size", bytes == 2 * sizeof(data));
+            ensure("error_code from LLFile::size() should not indicate an error", !ec);
+            fileout.close();
+        }
+
+        LLFile filein(testfile.string(), LLFile::in, ec);
+        ensure("LLFile constructor did not open correctly", (bool)filein);
+        ensure("error_code from LLFile constructor should not indicate an error", !ec);
+        if (filein)
+        {
+            S64 length = filein.size(ec);
+            ensure("LLFile::size() returned wrong size", length == 2 * sizeof(data));
+            ensure("error_code from LLFile::size() should not indicate an error", !ec);
+            char* buffer = (char*)malloc(length);
+            S64 bytes  = filein.read(buffer, length, ec);
+            ensure("LLFile::read() did not read correctly", bytes == length);
+            ensure("error_code from LLFile::read() should not indicate an error", !ec);
+            ensure_memory_matches("LLFile:read() did not read correct data1", data, (U32)sizeof(data), buffer, (U32)sizeof(data));
+            ensure_memory_matches("LLFile:read() did not read correct data2", data, (U32)sizeof(data), buffer + sizeof(data), (U32)sizeof(data));
+            S64 offset = filein.tell(ec);
+            ensure("LLFile::tell() returned a bad offset", offset == length);
+            ensure("error_code from LLFile::read() should not indicate an error", !ec);
+            offset = sizeof(data) / 2;
+            int rc = filein.seek(offset, ec);
+            ensure("LLFile::seek() indicated an error", rc == 0);
+            ensure("error_code from LLFile::seek() should not indicate an error", !ec);
+            bytes = filein.read(buffer, 2 * sizeof(data), ec);
+            ensure("LLFile::read() did not read correctly", bytes == sizeof(data) + offset);
+            ensure("error_code from LLFile::read() should not indicate an error", !ec);
+            ensure_memory_matches("LLFile:read() did not read correct data3", (char*)data + offset, (U32)offset, buffer, (U32)offset);
+            ensure_memory_matches("LLFile:read() did not read correct data4", (char*)data, (U32)sizeof(data), buffer + offset, (U32)sizeof(data));
+            filein.close();
+
+            free(buffer);
+        }
+    }
+
+    template<> template<>
+    void llfile_test_object_t::test<4>()
+    {
+        // Testing the LLFile class implementation with wrong paths and parameters
+        std::filesystem::path testfile = testdir;
+        testfile.append("llfile_test.bin");
+
+        std::error_code ec;
+        LLFile file(testfile.string(), LLFile::out | LLFile::noreplace, ec);
+        ensure("LLFile constructor should not have opened the already existing file", !file);
+        ensure("error_code from LLFile constructor should indicate an error", (bool)ec);
+
+        LLFile::remove(testfile.string());
+        file = LLFile(testfile.string(), LLFile::out | LLFile::app | LLFile::trunc, ec);
+        ensure("LLFile constructor should not have opened the file with conflicting flags", !file);
+        ensure("error_code from LLFile constructor should indicate an error", (bool)ec);
+
+        file = LLFile(testfile.string(), LLFile::out | LLFile::app | LLFile::noreplace, ec);
+        ensure("LLFile constructor should not have opened the file with conflicting flags", !file);
+        ensure("error_code from LLFile constructor should indicate an error", (bool)ec);
+
+        testfile = testdir;
+        testfile.append("llfile_test");
+        testfile.append("llfile_test.bin");
+
+        file = LLFile(testfile.string(), LLFile::in, ec);
+        ensure("LLFile constructor should not have been able to open the file in the non-existing directory", !file);
+        ensure("error_code from LLFile constructor should indicate an error", (bool)ec);
+    }
+} // namespace tut