FOPEN(3) Linux Programmer's Manual FOPEN(3)
NAME
fopen, fdopen, freopen - stream open functions
SYNOPSIS
#include <stdio.h>
FILE *fopen( char *path, char *mode);
FILE *fdopen( int fildes, char *mode);
FILE *freopen( char *path, 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 posi-
tioned at the beginning of the file.
r+ Open for reading and writing. The stream is posi-
tioned 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 writing. The file is created if it does
not exist. The stream is positioned at the end of
the file.
a+ Open for reading and writing. 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 third character or as a character between the charac-
ters in any of the two-character strings described above.
This is strictly for compatibility with ANSI C3.159-1989
(``ANSI C'') and has no effect; the ``b'' is ignored.
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, and do not require an intermediate seek as
in previous versions of stdio. This is not portable to
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FOPEN(3) Linux Programmer's Manual FOPEN(3)
other systems, but does work under Linux. 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 file
descriptor is not dup'ed.
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 func-
tion 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 ALSO
open(2), fclose(3)
STANDARDS
The fopen and freopen functions conform to ANSI
C3.159-1989 (``ANSI C''). The fdopen function conforms to
IEEE Std1003.1-1988 (``POSIX'').
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