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 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 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 C3.159-1989
(``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.
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.
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 ).
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).