FOPEN(3) FreeBSD Library Functions Manual FOPEN(3)
NAME
fopen, fdopen, freopen, fmemopen - stream open functions
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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);
DESCRIPTION
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" Open for reading. The stream is positioned at the beginning of
the file. Fail if the file does not exist.
"w" Open for writing. The stream is positioned at the beginning of
the file. Truncate the file to zero length if it exists or
create the file if it does not exist.
"a" Open for writing. The stream is positioned at the 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. Create the file if it does not exist.
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; 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
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. 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.
RETURN VALUES
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.
ERRORS
[EINVAL] The mode argument to 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.
SEE ALSO
open(2), fclose(3), fileno(3), fseek(3), funopen(3)
STANDARDS
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.
HISTORY
An fopen() function appeared in Version 1 AT&T UNIX.
FreeBSD 13.1-RELEASE-p6 May 1, 2020 FreeBSD 13.1-RELEASE-p6
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