fopen, fdopen, freopen - stream open functions
FILE *fopen (const char *path, const char *mode);
FILE *fdopen (int fildes, const char *mode);
FILE *freopen (const char *path, const char *mode, FILE
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
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
a Open for writing. The file is created if it does
not exist. The stream is positioned at the end of
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 last 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 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
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 posi
tioning 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 neces
sary 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.
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 com
patible 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 indica
tors are cleared. Modes "w" or "w+" do not cause trunca
tion 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 func
tion 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
the global variable errno is set to indicate the error.
EINVAL The mode provided to fopen, fdopen, or freopen was
The fopen, fdopen and freopen functions may also fail and
set errno for any of the errors specified for the routine
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).
of the errors specified for the routines open(2),
fclose(3) and fflush(3).
The fopen and freopen functions conform to ANSI
X3.159-1989 (``ANSI C''). The fdopen function conforms to
IEEE Std1003.1-1988 (``POSIX.1'').
open(2), fclose(3), fileno(3)