Section (3) fopen
fopen, fdopen, freopen — stream open functions
||const char *pathname,|
|const char *mode
|const char *mode
||const char *pathname,|
|const char *mode,|
fopen() function opens
the file whose name is the string pointed to by
pathname and associates a
stream with it.
points to a string beginning with one of the following
sequences (possibly followed by additional characters, as
Open text file for reading. The stream is positioned at the beginning of the file.
Open for reading and writing. The stream is positioned at the beginning of the file.
Truncate file to zero length or create text file for writing. The stream is positioned at the beginning of the file.
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.
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.
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.
can also include the letter _zsingle_quotesz_b_zsingle_quotesz_ 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 _zsingle_quotesz_b_zsingle_quotesz_ is ignored
on all POSIX conforming systems, including Linux. (Other
systems may treat text files and binary files differently,
and adding the _zsingle_quotesz_b_zsingle_quotesz_ may be a good idea if you do I/O to a
binary file and expect that your program may be ported to
See NOTES below for details of glibc extensions for
Any created file will have the mode
S_IWOTH (0666), as modified by the
process_zsingle_quotesz_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
O_WRONLY O_CREAT O_TRUNC
O_WRONLY O_CREAT O_APPEND
O_RDWR O_CREAT O_TRUNC
O_RDWR O_CREAT O_APPEND
associates a stream with the existing file descriptor,
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
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_zsingle_quotesz_ed, and will be closed when the
stream created by
closed. The result of applying
fdopen() to a shared memory object is
opens the file whose name is the string pointed to by
associates the stream pointed to by
stream with it. The original
stream (if it exists) is closed. The
mode argument is used just as
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 (
Upon successful completion
freopen() return a FILE pointer. Otherwise, NULL is returned and
errno is set to indicate the
freopen() was invalid.
freopen() functions may also fail and set
errno for any of the errors
specified for the routine malloc(3).
fopen() function may
also fail and set
errno for any
of the errors specified for the routine open(2).
fdopen() function may
also fail and set
errno for any
of the errors specified for the routine fcntl(2).
For an explanation of the terms used in this section, see attributes(7).
freopen(): POSIX.1-2001, POSIX.1-2008, C89,
The GNU C library allows the following extensions for
the string specified in
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
e(since glibc 2.7)
Open the file with the
O_CLOEXECflag. See open(2) for more information. This flag is ignored for
m(since glibc 2.3)
Open the file exclusively (like the
O_EXCLflag of open(2)). If the file already exists,
fopen() fails, and sets
errnoto EEXIST. This flag is ignored for
In addition to the above characters,
freopen() support the following syntax in
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
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
When parsing for individual flag characters in
mode (i.e., the characters
preceding the ccs specification), the glibc implementation
freopen() limits the number of characters
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
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Copyright (c) 1990, 1991 The Regents of the University of California.
All rights reserved.
This code is derived from software contributed to Berkeley by
Chris Torek and the American National Standards Committee X3,
on Information Processing Systems.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
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THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS_zsingle_quotesz__zsingle_quotesz_ AND
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(#)fopen.3 6.8 (Berkeley) 6/29/91
Converted for Linux, Mon Nov 29 15:22:01 1993, faithcs.unc.edu
Modified, aeb, 960421, 970806
Modified, joey, aeb, 2002-01-03