Originální popis anglicky:
fopen, fdopen, freopen - stream open functions
Návod, kniha: Linux Programmer's Manual
#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);
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 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 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 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,
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 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 the global
variable
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 ANSI X3.159-1989
(``ANSI C''). The
fdopen function conforms to IEEE Std1003.1-1988
(``POSIX.1'').
open(2),
fclose(3),
fileno(3)
