mirror of https://github.com/mkerrisk/man-pages
302 lines
8.2 KiB
Groff
302 lines
8.2 KiB
Groff
.\" Copyright (c) 1990, 1991 The Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" Chris Torek and the American National Standards Committee X3,
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.\" on Information Processing Systems.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)fopen.3 6.8 (Berkeley) 6/29/91
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.\"
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.\" Converted for Linux, Mon Nov 29 15:22:01 1993, faith@cs.unc.edu
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.\" Modified, aeb, 960421, 970806
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.\" Modified, joey, aeb, 2002-01-03
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.\"
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.TH FOPEN 3 2007-11-26 "GNU" "Linux Programmer's Manual"
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.SH NAME
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fopen, fdopen, freopen \- stream open functions
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.SH SYNOPSIS
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.nf
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.B #include <stdio.h>
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.sp
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.BI "FILE *fopen(const char *" path ", const char *" mode );
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.BI "FILE *fdopen(int " fd ", const char *" mode );
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.BI "FILE *freopen(const char *" path ", const char *" mode ", FILE *" stream );
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.fi
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.sp
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.in -4n
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.in
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.sp
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.BR fdopen ():
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_POSIX_C_SOURCE || _XOPEN_SOURCE
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.SH DESCRIPTION
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The
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.BR fopen ()
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function opens the file whose name is the string pointed to by
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.I path
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and associates a stream with it.
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.PP
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The argument
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.I mode
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points to a string beginning with one of the following sequences
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(Additional characters may follow these sequences.):
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.TP
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.B r
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Open text file for reading.
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The stream is positioned at the beginning of the file.
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.TP
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.B r+
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Open for reading and writing.
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The stream is positioned at the beginning of the file.
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.TP
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.B w
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Truncate file to zero length or create text file for writing.
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The stream is positioned at the beginning of the file.
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.TP
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.B w+
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Open for reading and writing.
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The file is created if it does not exist, otherwise it is truncated.
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The stream is positioned at the beginning of
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the file.
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.TP
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.B a
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Open for appending (writing at end of file).
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The file is created if it does not exist.
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The stream is positioned at the end of the file.
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.TP
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.B a+
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Open for reading and appending (writing at end of file).
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The file is created if it does not exist.
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The initial file position for reading is at the beginning of the file,
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but output is always appended to the end of the file.
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.PP
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The
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.I mode
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string can also include the letter \(aqb\(aq either as a last character or as
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a character between the characters in any of the two-character strings
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described above.
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This is strictly for compatibility with C89
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and has no effect; the \(aqb\(aq is ignored on all POSIX
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conforming systems, including Linux.
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(Other systems may treat text files and binary files differently,
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and adding the \(aqb\(aq may be a good idea if you do I/O to a binary
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file and expect that your program may be ported to non-Unix
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environments.)
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.PP
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Any created files will have mode
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.BR S_IRUSR " | " S_IWUSR " | " S_IRGRP " | " S_IWGRP " | " S_IROTH " | " S_IWOTH
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(0666), as modified by the process's umask value (see
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.BR umask (2)).
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.PP
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Reads and writes may be intermixed on read/write streams in any order.
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Note that ANSI C requires that a file positioning function intervene
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between output and input, unless an input operation encounters end-of-file.
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(If this condition is not met, then a read is allowed to return the
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result of writes other than the most recent.)
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Therefore it is good practice (and indeed sometimes necessary
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under Linux) to put an
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.BR fseek (3)
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or
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.BR fgetpos (3)
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operation between write and read operations on such a stream.
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This operation may be an apparent no-op
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(as in \fIfseek(..., 0L, SEEK_CUR)\fP
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called for its synchronizing side effect.
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.PP
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Opening a file in append mode (\fBa\fP as the first character of
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.IR mode )
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causes all subsequent write operations to this stream to occur
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at end-of-file, as if preceded by an
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.nf
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fseek(stream,0,SEEK_END);
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.fi
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.PP
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call.
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.PP
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The
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.BR fdopen ()
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function associates a stream with the existing file descriptor,
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.IR fd .
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The
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.I mode
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of the stream (one of the values "r", "r+", "w", "w+", "a", "a+")
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must be compatible with the mode of the file descriptor.
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The file position indicator of the new stream is set to that
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belonging to
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.IR fd ,
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and the error and end-of-file indicators are cleared.
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Modes "w" or "w+" do not cause truncation of the file.
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The file descriptor is not dup'ed, and will be closed when
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the stream created by
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.BR fdopen ()
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is closed.
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The result of applying
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.BR fdopen ()
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to a shared memory object is undefined.
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.PP
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The
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.BR freopen ()
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function opens the file whose name is the string pointed to by
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.I path
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and associates the stream pointed to by
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.I stream
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with it.
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The original stream (if it exists) is closed.
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The
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.I mode
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argument is used just as in the
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.BR fopen ()
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function.
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The primary use of the
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.BR freopen ()
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function is to change the file associated with a standard text stream
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.RI ( stderr ", " stdin ", or " stdout ).
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.SH "RETURN VALUE"
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Upon successful completion
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.BR fopen (),
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.BR fdopen ()
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and
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.BR freopen ()
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return a
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.I FILE
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pointer.
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Otherwise, NULL is returned and the global variable
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EINVAL
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The
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.I mode
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provided to
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.BR fopen (),
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.BR fdopen (),
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or
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.BR freopen ()
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was invalid.
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.PP
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The
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.BR fopen (),
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.BR fdopen ()
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and
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.BR freopen ()
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functions may also fail and set
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.I errno
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for any of the errors specified for the routine
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.BR malloc (3).
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.PP
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The
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.BR fopen ()
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function may also fail and set
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.I errno
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for any of the errors specified for the routine
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.BR open (2).
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.PP
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The
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.BR fdopen ()
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function may also fail and set
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.I errno
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for any of the errors specified for the routine
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.BR fcntl (2).
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.PP
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The
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.BR freopen ()
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function may also fail and set
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.I errno
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for any of the errors specified for the routines
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.BR open (2),
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.BR fclose (3)
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and
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.BR fflush (3).
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.SH "CONFORMING TO"
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The
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.BR fopen ()
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and
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.BR freopen ()
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functions conform to C89.
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The
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.BR fdopen ()
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function conforms to POSIX.1-1990.
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.SH NOTES
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.SS Glibc Notes
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The GNU C library allows the following extensions for the string specified in
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.IR mode :
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.TP
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.BR c " (since glibc 2.3.3)"
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Do not make the open operation,
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or subsequent read and write operations,
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thread cancellation points.
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.TP
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.BR e " (since glibc 2.7)"
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Open the file with the
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.B O_CLOEXEC
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flag.
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See
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.BR open (2)
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for more information.
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.TP
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.BR m " (since glibc 2.3)"
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Attempt to access the file using
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.BR mmap (2),
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rather than I/O system calls
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.RB ( read (2),
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.BR write (2)).
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Currently,
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.\" As at glibc 2.4:
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use of
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.BR mmap (2)
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is only attempted for a file opened for reading.
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.TP
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.B x
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.\" Since glibc 2.0?
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Open the file exclusively
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(like the
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.B O_EXCL
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flag of
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.BR open (2)).
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If the file already exists,
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.BR fopen ()
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fails, and sets
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.I errno
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to
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.BR EEXIST .
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This flag is ignored for
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.BR fdopen ().
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.\" FIXME document /,ccs= charset/
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.SH "SEE ALSO"
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.BR open (2),
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.BR fclose (3),
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.BR fileno (3)
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