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FOPEN(3) Linux Programmer's Manual FOPEN(3)
fopen, fdopen, freopen - stream open functions
#include <stdio.h>
FILE *fopen(const char *path, const char *mode);
FILE *fdopen(int fd, const char *mode);
FILE *freopen(const char *path, const char *mode, FILE *stream);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fdopen(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
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 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 created 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 initial file position for reading is
at the beginning of the file, but output is always appended to 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 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 files will have 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 by an
fseek(stream,0,SEEK_END);
call.
The fdopen() function associates a stream with the existing file descriptor,
fd. The mode of the stream (one of the values "r", "r+", "w", "w+", "a",
"a+") must be compatible with the mode of the file descriptor. The file
position indicator of the new stream is set to that belonging to fd, 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 created 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 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 C89. The fdopen() function
conforms to POSIX.1-1990.
The GNU C library allows the following extensions for the string specified in
mode:
c (since glibc 2.3.3)
Do not make the open operation, or subsequent read and write
operations, thread cancellation points.
e (since glibc 2.7)
Open the file with the O_CLOEXEC flag. See open(2) for more
information.
m (since glibc 2.3)
Attempt to access the file using mmap(2), rather than I/O system calls
(read(2), write(2)). Currently, use of mmap(2) is only attempted for a
file opened for reading.
x Open the file exclusively (like the O_EXCL flag of open(2)). If the
file already exists, fopen() fails, and sets errno to EEXIST. This
flag is ignored for fdopen().
open(2), fclose(3), fileno(3), fmemopen(3), fopencookie(3)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
GNU 2009-02-23 FOPEN(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface