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FOPEN(3) Linux 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 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 cre-
ated 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 ANSI X3.159-1989 (‘‘ANSI C’’) 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.)
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 subse-
quent 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, 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 descriptor. 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 created by
fdopen is closed. The result of applying fdopen to a shared memory object is unde-
fined.
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 stan-
dard text stream (stderr, stdin, or stdout).
RETURN VALUE
Upon successful completion fopen, fdopen and freopen return a FILE pointer. Other-
wise, 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).
CONFORMING TO
The fopen and freopen functions conform to ANSI X3.159-1989 (‘‘ANSI C’’). The
fdopen function conforms to IEEE Std1003.1-1988 (‘‘POSIX.1’’).
SEE ALSO
open(2), fclose(3), fileno(3)
BSD MANPAGE 2002-01-03 FOPEN(3)
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