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 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 ).
RETURN VALUE
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) .
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'').
