DragonFly On-Line Manual Pages
FOPEN(3) DragonFly Library Functions Manual FOPEN(3)
fopen, fdopen, freopen -- stream open functions
Standard C Library (libc, -lc)
fopen(const char * restrict path, const char * restrict mode);
fdopen(int fildes, const char *mode);
freopen(const char * restrict path, const char * restrict mode,
FILE * restrict 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 file for reading. The stream is positioned at the beginning
of the file.
``w'' Open file for writing. Creates the file if it does not already
exist, truncates the file if it does.
``a'' Open file for append. Creates the file if it does not already
exist. Seeks to end of the file. Subsequent writes to the file
will always end up at the then current end of file, irrespective
of any intervening fseek(3) or similar operation.
Additional flags may modify the primary mode as follows:
``+'' The file will be opened for both reading and writing. For exam-
ple 'r+' or 'w+'.
``x'' The open will fail if the file already exists (C11). For example
``e'' Close-on-exec. The underlying descriptor will automatically be
closed on any exec() (glibc compatibility).
``b'' Binary mode. On UNIX systems file reading and writing is always
in binary mode so this flag has no effect.
The mode string can also include the letter ``b'' either as a third char-
acter or as a character between the characters in any of the two-charac-
ter strings described above. This is strictly for compatibility with
ISO/IEC 9899:2011 (``ISO C11'') and has no effect; the ``b'' is ignored.
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
Reads and writes may be intermixed on read/write streams in any order,
and do not require an intermediate seek as in previous versions of stdio.
This is not portable to other systems, however; ANSI C requires that a
file positioning function intervene between output and input, unless an
input operation encounters end-of-file.
The fdopen() function associates a stream with the existing file descrip-
tor, fildes. The mode of the stream must be compatible with the mode of
the file descriptor. When the stream is closed via fclose(3), fildes is
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.
If the path argument is NULL, freopen() attempts to re-open the file
associated with stream with a new mode. The new mode must be compatible
with the mode that the stream was originally opened with:
* Streams originally opened with mode ``r'' can only be reopened
with that same mode.
* Streams originally opened with mode ``a'' can be reopened with
the same mode, or mode ``w''.
* Streams originally opened with mode ``w'' can be reopened with
the same mode, or mode ``a''.
* Streams originally opened with mode ``r+'', ``w+'', or ``a+''
can be reopened with any mode.
The primary use of the freopen() function is to change the file associ-
ated 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 argument to fopen(), fdopen(), or freopen()
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).
open(2), fclose(3), fileno(3), fseek(3), funopen(3)
The fopen() and freopen() functions conform to ISO/IEC 9899:2011
(``ISO C11''). The fdopen() function conforms to IEEE Std 1003.1-1988
DragonFly 5.3 February 22, 2018 DragonFly 5.3