FOPEN(3) | Library Functions Manual | FOPEN(3) |
fopen
, fdopen
,
freopen
, fmemopen
—
stream open functions
Standard C Library (libc, -lc)
#include
<stdio.h>
FILE *
fopen
(const
char * restrict path,
const char * restrict
mode);
FILE *
fdopen
(int
fildes, const char
*mode);
FILE *
freopen
(const
char *path, const char
*mode, FILE
*stream);
FILE *
fmemopen
(void
* restrict buf, size_t
size, const char *
restrict mode);
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 letters:
r
”w
”a
”An optional “+
”
following “r
”,
“w
”, or
“a
” opens the file for both reading
and writing. An optional “x
” following
“w
” or
“w+
” causes the
fopen
()
call to fail if the file already exists. An optional
“e
” following the above causes the
fopen
() call to set the
FD_CLOEXEC
flag on the underlying file
descriptor.
The mode string can also
include the letter “b
” after either
the “+
” or the first letter. This is
strictly for compatibility with ISO/IEC 9899:1990
(“ISO C90”) and has effect only for
fmemopen
();
otherwise “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
umask(2)).
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; ISO/IEC 9899:1990 (“ISO C90”) and IEEE Std 1003.1 (“POSIX.1”) both require 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 descriptor,
fildes. The mode of the stream must be compatible with
the mode of the file descriptor. The
“x
” mode option is ignored. If the
“e
” mode option is present, the
FD_CLOEXEC
flag is set, otherwise it remains
unchanged. When the stream is closed via
fclose(3), fildes
is closed also.
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 open for reading can only be
re-opened for reading, streams open for writing can only be re-opened for
writing, and streams open for reading and writing can be re-opened in any
mode. The “x
” mode option is not
meaningful in this context.
The primary use of the
freopen
()
function is to change the file associated with a standard text stream
(stderr
, stdin
, or
stdout
).
The
fmemopen
()
function associates the buffer given by the buf and
size arguments with a stream. The
buf argument is either a null pointer or point to a
buffer that is at least size bytes long. If a null
pointer is specified as the buf argument,
fmemopen
() allocates size
bytes of memory. This buffer is automatically freed when the stream is
closed. If a non-null pointer is specified, the caller retains ownership of
the buffer and is responsible for disposing of it after the stream has been
closed. Buffers can be opened in text-mode (default) or binary-mode (if
“b
” is present in the second or third
position of the mode argument). Buffers opened in
text-mode make sure that writes are terminated with a
NULL
byte, if the last write hasn't filled up the
whole buffer. Buffers opened in binary-mode never append a
NULL
byte.
Input and output against the opened stream will be fully buffered, unless it refers to an interactive terminal device, or a different kind of buffering is specified in the environment. See setvbuf(3) for additional details.
Upon successful completion fopen
(),
fdopen
(), freopen
() and
fmemopen
() return a FILE pointer. Otherwise,
NULL
is returned and the global variable
errno is set to indicate the error.
EINVAL
]fopen
(), fdopen
(),
freopen
(), or fmemopen
()
was invalid.The fopen
(),
fdopen
(), freopen
() and
fmemopen
() 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).
The fmemopen
() function may also fail and
set errno if the size argument
is 0.
The fopen
() and
freopen
() functions conform to
ISO/IEC 9899:1990 (“ISO C90”),
with the exception of the “x
” mode
option which conforms to ISO/IEC 9899:2011
(“ISO C11”). The
fdopen
() function conforms to IEEE
Std 1003.1-1988 (“POSIX.1”). The
“e
” mode option does not conform to
any standard but is also supported by glibc. The
fmemopen
() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”). The
“b
” mode does not conform to any
standard but is also supported by glibc.
An fopen
() function appeared in
Version 1 AT&T UNIX.
September 1, 2023 | macOS 14.6 |