FOPEN(3) OpenBSD Programmer's Manual FOPEN(3)NAME
fopen, fdopen, freopen - stream open functions
SYNOPSIS
#include <stdio.h>
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);
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.
``r+'' Open for reading and writing.
``w'' Truncate file to zero length or create text file for writing.
``w+'' Open for reading and writing. The file is created if it does not
exist, otherwise it is truncated.
``a'' Open for writing. The file is created if it does not exist.
``a+'' Open for reading and writing. The file is created if it does not
exist.
The mode string can also include the letter ``b'' either as the 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.
The fopen() and freopen() functions initially position the stream at the
start of the file unless the file is opened in append mode (`a' or `a+'),
in which case the stream is initially positioned at the end of the file.
Opening a file in append mode causes all subsequent writes to it to be
forced to the current end-of-file, regardless of intervening
repositioning of the stream.
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 cannot be arbitrarily intermixed on read/write streams.
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. The stream is positioned at the file offset
of the file descriptor. If fdopen() fails, the file descriptor fildes is
not affected in any way.
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 always closed, even if freopen() fails.
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).
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 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).
SEE ALSOopen(2), fclose(3), fseek(3), funopen(3)STANDARDS
The fopen() and freopen() functions conform to ANSI X3.159-1989 (``ANSI
C''). The fdopen() function conforms to IEEE Std 1003.1-1988
(``POSIX'').
CAVEATS
Proper code using fdopen() with error checking should close(2) fildes in
case of failure, and fclose(3) the resulting FILE * in case of success.
FILE *file;
int fd;
if ((file = fdopen(fd, "r")) != NULL) {
/* perform operations on the FILE * */
fclose(file);
} else {
/* failure, report the error */
close(fd);
}
OpenBSD 4.9 March 26, 2010 OpenBSD 4.9