FOPEN(3)

NAME
fopen, fdopen, freopen - stream open functions

LIBRARY
libc

SYNOPSIS
#include <stdio.h>
FILE *
fopen(const char * restrict path, const char * restrict mode);
FILE *
fdopen(int fildes, const char *mode);
FILE *
freopen(const char * restrict path, const char * restrict mode, FILE * restrict stream);

DESCRIPTION
The fopen() function opens the file whose name is the string pointed to by path and associates a stream with it.

The argument mode points to a string beginning with one of the following sequences (Additional characters may follow these sequences.):

“r” Open file for reading. The stream is positioned at the beginning of the file.

“w” Open file for writing. Creates the file if it does not already exist, truncates the file if it does.

“a” Open file for append. Creates the file if it does not already exist. Seeks to end of the file. Subsequent writes to the file will always end up at the then current end of file, irrespective of any intervening fseek(3) or similar operation.

Additional flags may modify the primary mode as follows:

“+” The file will be opened for both reading and writing. For example 'r+' or 'w+'.

“x” The open will fail if the file already exists (C11). For example 'wx'.

“e” Close-on-exec. The underlying descriptor will automatically be closed on any exec() (glibc compatibility).

“b” Binary mode. On UNIX systems file reading and writing is always in binary mode so this flag has no effect.

The mode string can also include the letter ``b'' either as a third character or as a character between the characters in any of the two-character strings described above. This is strictly for compatibility with isoC-2011 and has no effect; the ``b'' is ignored.

Any created files will have mode \*qS_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH\*q (0666), as modified by the process' umask value (see umask(2)).

Reads and writes may be intermixed on read/write streams in any order, and do not require an intermediate seek as in previous versions of stdio. This is not portable to other systems, however; ANSI C requires that a file positioning function intervene between output and input, unless an input operation encounters end-of-file.

The fdopen() function associates a stream with the existing file descriptor, fildes. The mode of the stream must be compatible with the mode of the file descriptor. When the stream is closed via fclose(3), fildes is closed also.

The freopen() function opens the file whose name is the string pointed to by path and associates the stream pointed to by stream with it. The original stream (if it exists) is closed. The mode argument is used just as in the fopen() function.

If the path argument is NULL, freopen() attempts to re-open the file associated with stream with a new mode. The new mode must be compatible with the mode that the stream was originally opened with:

•   Streams originally opened with mode “r” can only be reopened with that same mode.

•   Streams originally opened with mode “a” can be reopened with the same mode, or mode “w”.

•   Streams originally opened with mode “w” can be reopened with the same mode, or mode “a”.

•   Streams originally opened with mode “r+”, “w+”, or “a+” can be reopened with any mode.

The primary use of the freopen() function is to change the file associated with a standard text stream (stderr, stdin, or stdout).

RETURN VALUES
Upon successful completion fopen(), fdopen() and freopen() return a FILE pointer. Otherwise, NULL is returned and the global variable errno is set to indicate the error.

ERRORS

[EINVAL]
    The mode argument to fopen(), fdopen(), or freopen() was invalid.

The fopen(), fdopen() and freopen() functions may also fail and set errno for any of the errors specified for the routine malloc(3).

The fopen() function may also fail and set errno for any of the errors specified for the routine open(2).

The fdopen() function may also fail and set errno for any of the errors specified for the routine fcntl(2).

The freopen() function may also fail and set errno for any of the errors specified for the routines open(2), fclose(3) and fflush(3).

SEE ALSO
open(2), fclose(3), fileno(3), fseek(3), funopen(3)

STANDARDS
The fopen() and freopen() functions conform to isoC-2011. The fdopen() function conforms to p1003.1-88.