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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
FOPEN(3) Linux Programmer's Manual FOPEN(3)
fopen, fdopen, freopen - stream open functions
#include <stdio.h>
FILE *fopen(const char *pathname, const char *mode);
FILE *fdopen(int fd, const char *mode);
FILE *freopen(const char *pathname, const char *mode, FILE *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 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. The initial file
position for reading is at the beginning of the file, but
output is always appended to 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 C89 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 fgetpos(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 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).
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(): POSIX.1-2001, POSIX.1-2008, C89, C99.
fdopen(): POSIX.1-2001, POSIX.1-2008.
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 cancellation 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, in glibc versions before 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)
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description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2017-03-13 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), fmemopen(3), fopencookie(3), fputwc(3), fputws(3), getline(3), getmntent(3), gets(3), open_memstream(3), pmopenlog(3), popen(3), pthread_getattr_np(3), puts(3), setbuf(3), stdin(3)