fopen(3) Library Functions Manual fopen(3)
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 follow-
ing sequences (possibly followed by additional characters, as described
below):
r Open text file for reading. The stream is positioned at the be-
ginning 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 cre-
ated 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 char-
acter or as a character between the characters in any of the two-char-
acter 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 bi-
nary 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_IW-
GRP | 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 de-
scriptor, 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 de-
scriptor. 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 de-
scriptor 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 im-
plementation-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 associ-
ated 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 in-
valid.
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 er-
rors specified for the routines open(2), fclose(3), and fflush(3).
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(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 speci-
fied in mode:
c (since glibc 2.3.3)
Do not make the open operation, or subsequent read and write op-
erations, 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 in-
formation. 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 at-
tempted 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 EEX-
IST. 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 charac-
ters 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 pos-
sible 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_mem-
stream(3)
Linux man-pages 6.7 2023-10-31 fopen(3)
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