NAME
fopen, fdopen, freopen - stream open functions
SYNOPSIS
#include <stdio.h>
I FILE *fopen(const char * path , const char * mode );
I FILE *fdopen(int fildes , const char * mode );
I FILE *freopen(const char * path , const char * mode , FILE * stream );
DESCRIPTION
The
R 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 C89
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
R S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH
(0666), as modified by the process's 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(3)
or
fgetpos(3)
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
R 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
R fdopen ()
function associates a stream with the existing file descriptor,
R 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
R 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
R fdopen ()
is closed.
The result of applying
R fdopen ()
to a shared memory object is undefined.
The
R 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
R fopen ()
function.
The primary use of the
R freopen ()
function is to change the file associated with a standard text stream
R ( stderr , stdin , or stdout ).
RETURN VALUE
Upon successful completion
R fopen (),
R fdopen ()
and
R 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
R fopen (),
R fdopen (),
or
R freopen ()
was invalid.
The
R fopen (),
R fdopen ()
and
R freopen ()
functions may also fail and set
errno
for any of the errors specified for the routine
malloc(3).
The
R fopen ()
function may also fail and set
errno
for any of the errors specified for the routine
open(2).
The
R fdopen ()
function may also fail and set
errno
for any of the errors specified for the routine
fcntl(2).
The
R 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
R fopen ()
and
R freopen ()
functions conform to C89.
The
R fdopen ()
function conforms to POSIX.1-1990.
NOTES
Glibc Notes
The GNU C library allows the following extensions for the string specified in
R mode :
R c (since glibc 2.3.3)
Do not make the open operation,
or subsequent read and write operations,
thread cancellation points.
R m (since glibc 2.3)
Attempt to access the file using
mmap(2),
rather than I/O system calls
(read(2),
write(2)).
Currently,
use of
mmap(2)
is only attempted for a file opened for reading.
x
Open the file exclusively
(like the
O_EXCL
flag of
open(2)).
If the file already exists,
R fopen ()
fails, and sets
errno
to
R EEXIST .
This flag is ignored for
R fdopen ().
SEE ALSO