DESCRIPTION

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

The argument mode points to a string beginning with one of the following sequences (possibly followed by additional characters, as described below):

The mode string can also include the letter 'b' either as a last character or as a character between the characters in any of the two-character strings described above. This is strictly for compatibility with C89 and has no effect; the 'b' is ignored on all POSIX conforming systems, including Linux. (Other systems may treat text files and binary files differently, and adding the 'b' may be a good idea if you do I/O to a binary file and expect that your program may be ported to non-UNIX environments.)

See NOTES below for details of glibc extensions for mode.

Any created file will have the mode S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH (0666), as modified by the process's umask value (see umask(2)).

Reads and writes may be intermixed on read/write streams in any order. Note that ANSI C requires that a file positioning function intervene between output and input, unless an input operation encounters end-of-file. (If this condition is not met, then a read is allowed to return the result of writes other than the most recent.) Therefore it is good practice (and indeed sometimes necessary under Linux) to put an fseek(3) or fgetpos(3) operation between write and read operations on such a stream. This operation may be an apparent no-op (as in fseek(..., 0L, SEEK_CUR) called for its synchronizing side effect).

Opening a file in append mode (a as the first character of mode) causes all subsequent write operations to this stream to occur at end-of-file, as if preceded the call:

The file descriptor associated with the stream is opened as if by a call to open(2) with the following flags:

RETURN VALUE

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

ERRORS

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).

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

CONFORMING TO

fopen(), freopen(): POSIX.1-2001, POSIX.1-2008, C89, C99.

fdopen(): POSIX.1-2001, POSIX.1-2008.

NOTES

BUGS

When parsing for individual flag characters in mode (i.e., the characters preceding the "ccs" specification), the glibc implementation of fopen() and freopen() limits the number of characters examined in mode to 7 (or, in glibc versions before 2.14, to 6, which was not enough to include possible specifications such as "rb+cmxe"). The current implementation of fdopen() parses at most 5 characters in mode.

SEE ALSO

open(2), fclose(3), fileno(3), fmemopen(3), fopencookie(3), open_memstream(3)