`fopen_s`, `_wfopen_s`

Article
05/01/2023

Opens a file. These versions of fopen, _wfopen have security enhancements, as described in Security features in the CRT.

Syntax

errno_t fopen_s(
   FILE** pFile,
   const char *filename,
   const char *mode
);
errno_t _wfopen_s(
   FILE** pFile,
   const wchar_t *filename,
   const wchar_t *mode
);

Parameters

pFile
A pointer to the file pointer that receives the pointer to the opened file.

filename
The name of the file to open.

mode
Type of access permitted.

Return value

Zero if successful; an error code on failure. For more information about these error codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.

Error conditions

`pFile`	`filename`	`mode`	Return Value	Contents of `pFile`
`NULL`	any	any	`EINVAL`	unchanged
any	`NULL`	any	`EINVAL`	unchanged
any	any	`NULL`	`EINVAL`	unchanged

Remarks

The fopen_s and _wfopen_s functions can't open a file for sharing. If you need to share the file, use _fsopen or _wfsopen with the appropriate sharing mode constant—for example, use _SH_DENYNO for read/write sharing.

The fopen_s function opens the file specified by filename. _wfopen_s is a wide-character version of fopen_s and the arguments to _wfopen_s are wide-character strings. _wfopen_s and fopen_s behave identically, otherwise.

fopen_s accepts paths that are valid on the file system at the point of execution; UNC paths and paths that involve mapped network drives are accepted by fopen_s as long as the system that's executing the code has access to the share or mapped network drive at the time of execution. When you construct paths for fopen_s, don't make assumptions about the availability of drives, paths, or network shares in the execution environment. You can use either forward slashes (/) or backslashes (\) as the directory separators in a path.

These functions validate their parameters. If pFile, filename, or mode is a null pointer, these functions generate an invalid parameter exception, as described in Parameter validation.

Always check the return value to see if the function succeeded before you do any further operations on the file. If an error occurs, the error code is returned, and the global variable errno is set. For more information, see errno, _doserrno, _sys_errlist, and _sys_nerr.

By default, this function's global state is scoped to the application. To change it, see Global state in the CRT.

Unicode support

fopen_s supports Unicode file streams. To open a new or existing Unicode file, pass a ccs flag that specifies the desired encoding to fopen_s, for example:

fopen_s(&fp, "newfile.txt", "w+, ccs=UNICODE");

Allowed values of the ccs flag are UNICODE, UTF-8, and UTF-16LE. If no value is specified for ccs, fopen_s uses ANSI encoding.

If the file already exists and is opened for reading or appending, the byte order mark (BOM), if present in the file, determines the encoding. The BOM encoding takes precedence over the encoding that's specified by the ccs flag. The ccs encoding is only used when no BOM is present or if the file is a new file.

Note

BOM-detection only applies to files that are opened in Unicode mode; that is, by passing the ccs flag.

The following table summarizes the modes for various ccs flag values that are given to fopen_s and for BOMs in the file.

Encodings used based on `ccs` flag and BOM

`ccs` flag	No BOM (or new file)	BOM: UTF-8	BOM: UTF-16
`UNICODE`	`UTF-8`	`UTF-8`	`UTF-16LE`
`UTF-8`	`UTF-8`	`UTF-8`	`UTF-16LE`
`UTF-16LE`	`UTF-16LE`	`UTF-8`	`UTF-16LE`

Files that are opened for writing in Unicode mode have a BOM written to them automatically.

If mode is "a, ccs=UNICODE", "a, ccs=UTF-8", or "a, ccs=UTF-16LE", fopen_s first tries to open the file with both read access and write access. If successful, the function reads the BOM to determine the encoding for the file; if unsuccessful, the function uses the default encoding for the file. In either case, fopen_s then reopens the file with write-only access. (This behavior applies to a mode only, not a+.)

The character string mode specifies the kind of access that's requested for the file, as follows.

`mode`	Access
`"r"`	Opens for reading. If the file doesn't exist or can't be found, the `fopen_s` call fails.
`"w"`	Opens an empty file for writing. If the given file exists, its contents are destroyed.
`"a"`	Opens for writing at the end of the file (appending) without removing the end-of-file (EOF) marker before new data is written to the file. Creates the file if it doesn't exist.
`"r+"`	Opens for both reading and writing. The file must exist.
`"w+"`	Opens an empty file for both reading and writing. If the file exists, its contents are destroyed.
`"a+"`	Opens for reading and appending. The appending operation includes the removal of the EOF marker before new data is written to the file. The EOF marker isn't restored after writing is completed. Creates the file if it doesn't exist.

When a file is opened by using the "a" or "a+" access type, all write operations occur at the end of the file. The file pointer can be repositioned by using fseek or rewind, but it's always moved back to the end of the file before any write operation is carried out so that existing data can't be overwritten.

The "a" mode doesn't remove the EOF marker before appending to the file. After appending has occurred, the MS-DOS TYPE command only shows data up to the original EOF marker and not any data that's appended to the file. The "a+" mode does remove the EOF marker before appending to the file. After appending, the MS-DOS TYPE command shows all data in the file. The "a+" mode is required for appending to a stream file that is terminated with the CTRL+Z EOF marker.

When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed. (The file is said to be open for "update".) However, when you switch from reading to writing, the input operation must come across an EOF marker. If there's no EOF marker, you must use an intervening call to a file-positioning function. The file-positioning functions are fsetpos, fseek, and rewind. When you switch from writing to reading, you must use an intervening call to either fflush or to a file-positioning function.

Starting in C11, you can append "x" to "w" or "w+" to cause the function fail if the file exists, instead of overwriting it.

In addition to the previous values, the following characters can be included in mode to specify the translation mode for newline characters:

`mode` modifier	Translation mode
`t`	Open in text (translated) mode. Carriage return-line feed (CR-LF) combinations are translated into single line feeds (LF) on input and LF characters are translated to CR-LF combinations on output. CTRL+Z is interpreted as an end-of-file character on input.
`b`	Open in binary (untranslated) mode; translations involving carriage-return and line feed characters are suppressed.

In text (translated) mode, CTRL+Z is interpreted as an end-of-file character on input. For files opened for reading/writing with "a+", fopen_s checks for a CTRL+Z at the end of the file and removes it, if possible. It's removed because using fseek and ftell to move within a file that ends with a CTRL+Z, may cause fseek to behave improperly near the end of the file.

Also, in text mode, carriage return/line feed (CRLF) combinations are translated into single line feed (LF) characters on input, and LF characters are translated to CRLF combinations on output. When a Unicode stream-I/O function operates in text mode (the default), the source or destination stream is assumed to be a sequence of multibyte characters. The Unicode stream-input functions convert multibyte characters to wide characters (as if by a call to the mbtowc function). For the same reason, the Unicode stream-output functions convert wide characters to multibyte characters (as if by a call to the wctomb function).

If t or b isn't given in mode, the default translation mode is defined by the global variable _fmode. If t or b is prefixed to the argument, the function fails and returns NULL.

For more information about using text and binary modes in Unicode and multibyte stream-I/O, see Text and binary mode file I/O and Unicode stream I/O in text and binary modes.

`mode` modifier	Behavior
`c`	Enable the commit flag for the associated `filename` so that the contents of the file buffer are written directly to disk if either `fflush` or `_flushall` is called.
`n`	Reset the commit flag for the associated `filename` to "no-commit." This flag is the default. It also overrides the global commit flag if you link your program with `COMMODE.OBJ`. The global commit flag default is "no-commit" unless you explicitly link your program with `COMMODE.OBJ` (see Link options).
`N`	Specifies that the file isn't inherited by child processes.
`S`	Specifies that caching is optimized for, but not restricted to, sequential access from disk.
`R`	Specifies that caching is optimized for, but not restricted to, random access from disk.
`T`	Specifies a file that isn't written to disk unless memory pressure requires it.
`D`	Specifies a temporary file that is deleted when the last file pointer to it is closed.
`ccs=UNICODE`	Specifies UNICODE as the encoded character set to use for this file. Leave unspecified if you want ANSI encoding.
`ccs=UTF-8`	Specifies UTF-8 as the encoded character set to use for this file. Leave unspecified if you want ANSI encoding.
`ccs=UTF-16LE`	Specifies UTF-16LE as the encoded character set to use for this file. Leave unspecified if you want ANSI encoding.

Valid characters for the mode string used in fopen_s and _fdopen correspond to oflag arguments used in _open and _sopen, as follows.

Characters in `mode` string	Equivalent `oflag` value for `_open`/`_sopen`
`a`	`_O_WRONLY \| _O_APPEND` (usually `_O_WRONLY \| _O_CREAT \| _O_APPEND`)
`a+`	`_O_RDWR \| _O_APPEND` (usually `_O_RDWR \| _O_APPEND \| _O_CREAT`)
`R`	`_O_RDONLY`
`r+`	`_O_RDWR`
`w`	`_O_WRONLY` (usually `_O_WRONLY \| _O_CREAT \| _O_TRUNC`)
`w+`	`_O_RDWR` (usually **`_O_RDWR \| _O_CREAT \| _O_TRUNC`)
`b`	`_O_BINARY`
`t`	`_O_TEXT` (translated)
`c`	None
`n`	None
`D`	`_O_TEMPORARY`
`R`	`_O_RANDOM`
`S`	`_O_SEQUENTIAL`
`T`	`_O_SHORTLIVED`
`ccs=UNICODE`	`_O_WTEXT`
`ccs=UTF-8`	`_O_UTF8`
`ccs=UTF-16LE`	`_O_UTF16`

The c, n, R, S, t, T, and D mode options are Microsoft extensions for fopen_s and _wfopen_s and shouldn't be used when you want ANSI portability.

If you're using rb mode, memory mapped Win32 files might also be an option if you don't need to port your code, you expect to read much of the file, or you don't care about network performance.

Regarding T and D:

T avoids writing the file to disk as long as memory pressure doesn't require it. For more information, see FILE_ATTRIBUTE_TEMPORARY in File attribute constants, and also this blog post It's only temporary.
D specifies a regular file that is written to disk. The difference is that it's automatically deleted when it's closed. You can combine TD to get both semantics.

Requirements

Function	Required header	C++ header
`fopen_s`	`<stdio.h>`	`<cstdio>`
`_wfopen_s`	`<stdio.h>` or `<wchar.h>`	`<cstdio>`

For more information on standards conformance and naming conventions in the C runtime library, see Compatibility.

Generic-text routine mappings

`<tchar.h>` routine	`_UNICODE` and `_MBCS` not defined	`_MBCS` defined	`_UNICODE` defined
`_tfopen_s`	`fopen_s`	`fopen_s`	`_wfopen_s`

Libraries

All versions of the C run-time libraries.

Example

// crt_fopen_s.c
// This program opens two files. It uses
// fclose to close the first file and
// _fcloseall to close all remaining files.

#include <stdio.h>

FILE *stream, *stream2;

int main( void )
{
   errno_t err;

   // Open for read (will fail if file "crt_fopen_s.c" doesn't exist)
   err  = fopen_s( &stream, "crt_fopen_s.c", "r" );
   if( err == 0 )
   {
      printf( "The file 'crt_fopen_s.c' was opened\n" );
   }
   else
   {
      printf( "The file 'crt_fopen_s.c' was not opened\n" );
   }

   // Open for write
   err = fopen_s( &stream2, "data2", "w+, ccs=UTF-8" );
   if( err == 0 )
   {
      printf( "The file 'data2' was opened\n" );
   }
   else
   {
      printf( "The file 'data2' was not opened\n" );
   }

   // Close stream if it isn't NULL
   if( stream )
   {
      err = fclose( stream );
      if ( err == 0 )
      {
         printf( "The file 'crt_fopen_s.c' was closed\n" );
      }
      else
      {
         printf( "The file 'crt_fopen_s.c' was not closed\n" );
      }
   }

   // All other files are closed:
   int numclosed = _fcloseall( );
   printf( "Number of files closed by _fcloseall: %u\n", numclosed );
}

The file 'crt_fopen_s.c' was opened
The file 'data2' was opened
Number of files closed by _fcloseall: 1

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`fopen_s`, `_wfopen_s`

Syntax

Parameters

Return value

Error conditions

Remarks

Unicode support

Encodings used based on `ccs` flag and BOM

Requirements

Generic-text routine mappings

Libraries

Example

See also

Feedback

Additional resources

Share via

fopen_s, _wfopen_s

Syntax

Parameters

Return value

Error conditions

Remarks

Unicode support

Encodings used based on ccs flag and BOM

Requirements

Generic-text routine mappings

Libraries

Example

See also

Feedback

Additional resources

`fopen_s`, `_wfopen_s`

Encodings used based on `ccs` flag and BOM