FOPEN(3) Linux Programmer's Manual FOPEN(3)
fopen, fdopen, freopen - stream open functions
FILE *fopen(const char *path, const char *mode);
FILE *fdopen(int fildes, const char *mode);
FILE *freopen(const char *path, const char *mode, FILE *stream);
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 follow-
ing sequences (Additional characters may follow these sequences.):
r Open text file for reading. The stream is positioned at the
beginning of the file.
r+ Open for reading and writing. The stream is positioned at the
beginning of the file.
w Truncate file to zero length or create text file for writing.
The stream is positioned at the beginning of the file.
w+ Open for reading and writing. The file is created if it does
not exist, otherwise it is truncated. The stream is positioned
at the beginning of the file.
a Open for appending (writing at end of file). The file is cre-
ated if it does not exist. The stream is positioned at the end
of the file.
a+ Open for reading and appending (writing at end of file). The
file is created if it does not exist. The stream is positioned
at the end of the file.
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 ANSI X3.159-1989 (``ANSI C'') and has no effect; the ``b'' is
ignored on all POSIX conforming systems, including Linux. (Other sys-
tems 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.)
Any created files will have mode
S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH (0666), as modified by
the process' 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
or fgetpos 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 by an
The fdopen function associates a stream with the existing file descrip-
tor, fildes. The mode of the stream (one of the values "r", "r+", "w",
"w+", "a", "a+") must be compatible with the mode of the file descrip-
tor. The file position indicator of the new stream is set to that
belonging to fildes, and the error and end-of-file indicators are
cleared. Modes "w" or "w+" do not cause truncation of the file. The
file descriptor is not dup'ed, and will be closed when the stream cre-
ated by fdopen is closed. The result of applying fdopen to a shared
memory object is undefined.
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. The primary use of the freopen function
is to change the file associated with a standard text stream (stderr,
stdin, or stdout).
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.
EINVAL The mode provided 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).
The fopen and freopen functions conform to ANSI X3.159-1989 (``ANSI
C''). The fdopen function conforms to IEEE Std1003.1-1988
open(2), fclose(3), fileno(3)
BSD MANPAGE 2002-01-03 FOPEN(3)