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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(): POSIX.1-2001, POSIX.1-2008, C99.
fdopen(): POSIX.1-2001, POSIX.1-2008.
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)