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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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fopen(3) Library Functions Manual fopen(3)
fopen, fdopen, freopen - stream open functions
Standard C library (libc, -lc)
#include <stdio.h>
FILE *fopen(const char *restrict path, const char *restrict mode);
FILE *fdopen(int fd, const char *mode);
FILE *freopen(const char *restrict path, const char *restrict mode,
FILE *restrict stream);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
fdopen():
_POSIX_C_SOURCE
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 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 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 path 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).
Upon successful completion fopen(), fdopen(), and freopen() return
a FILE pointer. Otherwise, NULL is returned and errno is set to
indicate the error.
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).
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ fopen(), fdopen(), freopen() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
fopen()
freopen()
C11, POSIX.1-2008.
fdopen()
POSIX.1-2008.
fopen()
freopen()
POSIX.1-2001, C89.
fdopen()
POSIX.1-2001.
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.
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.
open(2), fclose(3), fileno(3), fmemopen(3), fopencookie(3),
open_memstream(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
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Linux man-pages 6.15 2025-05-17 fopen(3)
Pages that refer to this page: open(2), fclose(3), fcloseall(3), ferror(3), fflush(3), fgetc(3), fgetgrent(3), fgetpwent(3), fgetwc(3), fgetws(3), FILE(3type), fileno(3), fmemopen(3), fopencookie(3), fputwc(3), fputws(3), getline(3), getmntent(3), gets(3), libexpect(3), open_memstream(3), pmopenlog(3), popen(3), procio(3), pthread_getattr_np(3), puts(3), setbuf(3), stdin(3), stdio(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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