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NAME
fopen, fdopen, freopen - stream open functions
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <stdio.h>
FILE *fopen(const char *restrict pathname, const char *restrict mode);
FILE *fdopen(int fd, const char *mode);
FILE *freopen(const char *restrict pathname, const char *restrict mode,
FILE *restrict stream);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fdopen():
_POSIX_C_SOURCE
DESCRIPTION
The fopen() function opens the file whose name is the string pointed to by pathname and associates a
stream with it.
The argument mode points to a string beginning with one of the following sequences (possibly followed by
additional characters, as described below):
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 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.
a+ Open for reading and appending (writing at end of file). The file is created if it does not
exist. Output is always appended to the end of the file. POSIX is silent on what the initial
read position is when using this mode. For glibc, the initial file position for reading is at the
beginning of the file, but for Android/BSD/MacOS, the initial file position for reading is 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
ISO 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.)
See NOTES below for details of glibc extensions for mode.
Any created file will have the mode S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH (0666), as
modified by the process's 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(3) or fsetpos(3) 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).
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 the call:
fseek(stream, 0, SEEK_END);
The file descriptor associated with the stream is opened as if by a call to open(2) with the following
flags:
┌──────────────┬───────────────────────────────┐
│ fopen() mode │ open() flags │
├──────────────┼───────────────────────────────┤
│ r │ O_RDONLY │
├──────────────┼───────────────────────────────┤
│ w │ O_WRONLY | O_CREAT | O_TRUNC │
├──────────────┼───────────────────────────────┤
│ a │ O_WRONLY | O_CREAT | O_APPEND │
├──────────────┼───────────────────────────────┤
│ r+ │ O_RDWR │
├──────────────┼───────────────────────────────┤
│ w+ │ O_RDWR | O_CREAT | O_TRUNC │
├──────────────┼───────────────────────────────┤
│ a+ │ O_RDWR | O_CREAT | O_APPEND │
└──────────────┴───────────────────────────────┘
fdopen()
The fdopen() function associates a stream with the existing file descriptor, fd. 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 fd, 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.
freopen()
The freopen() function opens the file whose name is the string pointed to by pathname 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 pathname argument is a null pointer, freopen() changes the mode of the stream to that specified in
mode; that is, freopen() reopens the pathname that is associated with the stream. The specification for
this behavior was added in the C99 standard, which says:
In this case, the file descriptor associated with the stream need not be closed if the call to
freopen() succeeds. It is implementation-defined which changes of mode are permitted (if any),
and under what circumstances.
The primary use of the freopen() function is to change the file associated with a standard text stream
(stderr, stdin, or stdout).
RETURN VALUE
Upon successful completion fopen(), fdopen(), and freopen() return a FILE pointer. Otherwise, NULL is
returned and errno is set to indicate the error.
ERRORS
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).
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌─────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│ fopen(), fdopen(), freopen() │ Thread safety │ MT-Safe │
└─────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
fopen()
freopen()
C11, POSIX.1-2008.
fdopen()
POSIX.1-2008.
HISTORY
fopen()
freopen()
POSIX.1-2001, C89.
fdopen()
POSIX.1-2001.
NOTES
glibc notes
The GNU C library allows the following extensions for the string specified in mode:
c (since glibc 2.3.3)
Do not make the open operation, or subsequent read and write operations, thread cancelation
points. This flag is ignored for fdopen().
e (since glibc 2.7)
Open the file with the O_CLOEXEC flag. See open(2) for more information. This flag is ignored
for fdopen().
m (since glibc 2.3)
Attempt to access the file using mmap(2), rather than I/O system calls (read(2), write(2)).
Currently, use of mmap(2) is attempted only for a file opened for reading.
x Open the file exclusively (like the O_EXCL flag of open(2)). If the file already exists, fopen()
fails, and sets errno to EEXIST. This flag is ignored for fdopen().
In addition to the above characters, fopen() and freopen() support the following syntax in mode:
,ccs=string
The given string is taken as the name of a coded character set and the stream is marked as wide-oriented.
Thereafter, internal conversion functions convert I/O to and from the character set string. If the
,ccs=string syntax is not specified, then the wide-orientation of the stream is determined by the first
file operation. If that operation is a wide-character operation, the stream is marked wide-oriented, and
functions to convert to the coded character set are loaded.
BUGS
When parsing for individual flag characters in mode (i.e., the characters preceding the "ccs"
specification), the glibc implementation of fopen() and freopen() limits the number of characters
examined in mode to 7 (or, before glibc 2.14, to 6, which was not enough to include possible
specifications such as "rb+cmxe"). The current implementation of fdopen() parses at most 5 characters in
mode.
SEE ALSO
open(2), fclose(3), fileno(3), fmemopen(3), fopencookie(3), open_memstream(3)
Linux man-pages 6.7 2023-10-31 fopen(3)