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
” or “rb
” - Open file for reading.
- “
r+
” or “rb+
” or “r+b
” - Open for reading and writing.
- “
w
” or “wb
” - Open for writing. The file is created if it does not exist, otherwise it is truncated.
- “
w+
” or “wb+
” or “w+b
” - Open for reading and writing. The file is created if it does not exist, otherwise it is truncated.
- “
a
” or “ab
” - Open for writing. The file is created if it does not exist.
- “
a+
” or “ab+
” or “a+b
” - Open for reading and writing. The file is created if it does not exist.
The letter “b” in the mode strings above is strictly for compatibility with ANSI X3.159-1989 (“ANSI C89”) and has no effect; the “b” is ignored.
After any of the above prefixes, the mode string can also include zero or more of the following:
- “
e
” - The close-on-exec flag is set on the underlying file descriptor of the new FILE.
- “
x
” - If the mode string starts with “w” or
“a” then the function shall fail if the file specified by
path already exists, as if the
O_EXCL
flag was passed to the open(2) function. It has no effect if used withfdopen
() or the mode string begins with “r”.
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
(), orfreopen
() 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
STANDARDS
The fopen
() and
freopen
() functions conform to ANSI
X3.159-1989 (“ANSI C89”). The
fdopen
() function conforms to IEEE
Std 1003.1-1988 (“POSIX.1”).
HISTORY
The fopen
() function first appeared in
Version 1 AT&T UNIX. The
fdopen
() and freopen
()
functions first appeared in Version 7 AT&T
UNIX.
Support for the “e” and “x” mode letters appeared in OpenBSD 5.7.
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); }