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.):
Open text file for reading. The stream is positioned at the
beginning of the file.
Open for reading and writing. The stream is positioned at
the beginning of the file.
Truncate file to zero length or create text file for
writing. The stream is positioned at the beginning of the file.
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.
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
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
Any created files will have mode
(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
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
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).
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
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