mirror of https://github.com/mkerrisk/man-pages
353 lines
8.7 KiB
Groff
353 lines
8.7 KiB
Groff
.\" Copyright 2005, 2012, 2016 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
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.\" Distributed under the GPL.
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.\" %%%LICENSE_END
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.\"
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.TH FMEMOPEN 3 2020-04-11 "GNU" "Linux Programmer's Manual"
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.SH NAME
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fmemopen \- open memory as stream
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.SH SYNOPSIS
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.nf
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.B #include <stdio.h>
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.PP
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.BI "FILE *fmemopen(void *"buf ", size_t "size ", const char *" mode ");"
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.fi
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.PP
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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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.PP
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.BR fmemopen ():
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.PD 0
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.ad l
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.RS 4
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.TP 4
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Since glibc 2.10:
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_POSIX_C_SOURCE\ >=\ 200809L
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.TP
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Before glibc 2.10:
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_GNU_SOURCE
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.RE
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.ad
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.PD
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.SH DESCRIPTION
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The
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.BR fmemopen ()
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function opens a stream that permits the access specified by
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.IR mode .
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The stream allows I/O to be performed on the string or memory buffer
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pointed to by
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.IR buf .
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.PP
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The
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.I mode
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argument specifies the semantics of I/O on the stream,
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and is one of the following:
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.TP
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.I r
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The stream is opened for reading.
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.TP
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.I w
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The stream is opened for writing.
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.TP
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.I a
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Append; open the stream for writing,
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with the initial buffer position set to the first null byte.
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.TP
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.I r+
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Open the stream for reading and writing.
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.TP
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.I w+
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Open the stream for reading and writing.
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The buffer contents are truncated
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(i.e., \(aq\e0\(aq is placed in the first byte of the buffer).
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.TP
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.I a+
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Append; open the stream for reading and writing,
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with the initial buffer position set to the first null byte.
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.PP
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The stream maintains the notion of a current position,
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the location where the next I/O operation will be performed.
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The current position is implicitly updated by I/O operations.
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It can be explicitly updated using
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.BR fseek (3),
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and determined using
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.BR ftell (3).
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In all modes other than append,
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the initial position is set to the start of the buffer.
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In append mode, if no null byte is found within the buffer,
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then the initial position is
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.IR size+1 .
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.PP
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If
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.I buf
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is specified as NULL, then
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.BR fmemopen ()
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allocates a buffer of
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.I size
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bytes.
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This is useful for an application that wants to write data to
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a temporary buffer and then read it back again.
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The initial position is set to the start of the buffer.
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The buffer is automatically freed when the stream is closed.
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Note that the caller has no way to obtain a pointer to the
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temporary buffer allocated by this call (but see
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.BR open_memstream (3)).
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.PP
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If
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.I buf
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is not NULL, then it should point to a buffer of at least
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.I len
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bytes allocated by the caller.
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.PP
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When a stream that has been opened for writing is flushed
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.RB ( fflush (3))
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or closed
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.RB ( fclose (3)),
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a null byte is written at the end of the buffer if there is space.
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The caller should ensure that an extra byte is available in the
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buffer
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(and that
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.I size
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counts that byte)
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to allow for this.
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.PP
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In a stream opened for reading,
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null bytes (\(aq\e0\(aq) in the buffer do not cause read
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operations to return an end-of-file indication.
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A read from the buffer will indicate end-of-file
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only when the current buffer position advances
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.I size
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bytes past the start of the buffer.
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.PP
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Write operations take place either at the current position
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(for modes other than append), or at the current size of the stream
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(for append modes).
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.PP
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Attempts to write more than
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.I size
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bytes to the buffer result in an error.
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By default, such errors will be visible
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(by the absence of data) only when the
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.I stdio
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buffer is flushed.
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Disabling buffering with the following call
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may be useful to detect errors at the time of an output operation:
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.PP
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setbuf(stream, NULL);
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.SH RETURN VALUE
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Upon successful completion,
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.BR fmemopen ()
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returns a
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.I FILE
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pointer.
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Otherwise, NULL is returned and
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.I errno
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is set to indicate the error.
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.SH VERSIONS
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.BR fmemopen ()
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was already available in glibc 1.0.x.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lb lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR fmemopen (),
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T} Thread safety MT-Safe
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.TE
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.sp 1
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.SH CONFORMING TO
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POSIX.1-2008.
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This function is not specified in POSIX.1-2001,
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and is not widely available on other systems.
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.PP
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POSIX.1-2008 specifies that \(aqb\(aq in
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.IR mode
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shall be ignored.
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However, Technical Corrigendum 1
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.\" http://austingroupbugs.net/view.php?id=396
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adjusts the standard to allow implementation-specific treatment for this case,
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thus permitting the glibc treatment of \(aqb\(aq.
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.SH NOTES
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There is no file descriptor associated with the file stream
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returned by this function
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(i.e.,
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.BR fileno (3)
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will return an error if called on the returned stream).
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.PP
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With version 2.22, binary mode (see below) was removed,
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many longstanding bugs in the implementation of
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.BR fmemopen ()
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were fixed, and a new versioned symbol was created for this interface.
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.\"
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.SS Binary mode
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From version 2.9 to 2.21, the glibc implementation of
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.BR fmemopen ()
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supported a "binary" mode,
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enabled by specifying the letter \(aqb\(aq as the second character in
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.IR mode .
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In this mode,
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writes don't implicitly add a terminating null byte, and
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.BR fseek (3)
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.B SEEK_END
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is relative to the end of the buffer (i.e., the value specified by the
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.I size
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argument), rather than the current string length.
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.PP
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An API bug afflicted the implementation of binary mode:
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to specify binary mode, the \(aqb\(aq must be the
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.I second
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character in
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.IR mode .
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Thus, for example, "wb+" has the desired effect, but "w+b" does not.
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This is inconsistent with the treatment of
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.\" http://sourceware.org/bugzilla/show_bug.cgi?id=12836
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.IR mode
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by
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.BR fopen (3).
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.PP
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Binary mode was removed in glibc 2.22; a \(aqb\(aq specified in
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.I mode
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has no effect.
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.SH BUGS
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In versions of glibc before 2.22, if
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.I size
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is specified as zero,
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.BR fmemopen ()
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fails with the error
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.BR EINVAL .
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.\" http://sourceware.org/bugzilla/show_bug.cgi?id=11216
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It would be more consistent if this case successfully created
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a stream that then returned end-of-file on the first attempt at reading;
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since version 2.22, the glibc implementation provides that behavior.
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.PP
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In versions of glibc before 2.22,
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specifying append mode ("a" or "a+") for
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.BR fmemopen ()
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sets the initial buffer position to the first null byte, but
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.\" http://sourceware.org/bugzilla/show_bug.cgi?id=13152
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(if the current position is reset to a location other than
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the end of the stream)
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does not force subsequent writes to append at the end of the stream.
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This bug is fixed in glibc 2.22.
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.PP
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In versions of glibc before 2.22, if the
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.I mode
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argument to
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.BR fmemopen ()
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specifies append ("a" or "a+"), and the
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.I size
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argument does not cover a null byte in
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.IR buf ,
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then, according to POSIX.1-2008,
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the initial buffer position should be set to
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the next byte after the end of the buffer.
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However, in this case the glibc
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.\" http://sourceware.org/bugzilla/show_bug.cgi?id=13151
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.BR fmemopen ()
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sets the buffer position to \-1.
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This bug is fixed in glibc 2.22.
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.PP
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In versions of glibc before 2.22,
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.\" https://sourceware.org/bugzilla/show_bug.cgi?id=14292
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when a call to
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.BR fseek (3)
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with a
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.I whence
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value of
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.B SEEK_END
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was performed on a stream created by
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.BR fmemopen (),
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the
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.I offset
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was
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.IR subtracted
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from the end-of-stream position, instead of being added.
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This bug is fixed in glibc 2.22.
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.PP
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The glibc 2.9 addition of "binary" mode for
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.BR fmemopen ()
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.\" http://sourceware.org/bugzilla/show_bug.cgi?id=6544
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silently changed the ABI: previously,
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.BR fmemopen ()
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ignored \(aqb\(aq in
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.IR mode .
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.SH EXAMPLE
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The program below uses
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.BR fmemopen ()
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to open an input buffer, and
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.BR open_memstream (3)
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to open a dynamically sized output buffer.
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The program scans its input string (taken from the program's
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first command-line argument) reading integers,
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and writes the squares of these integers to the output buffer.
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An example of the output produced by this program is the following:
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.PP
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.in +4n
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.EX
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.RB "$" " ./a.out \(aq1 23 43\(aq"
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size=11; ptr=1 529 1849
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.EE
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.in
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.SS Program source
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\&
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.EX
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#define _GNU_SOURCE
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#include <string.h>
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#include <stdio.h>
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#include <stdlib.h>
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#define handle_error(msg) \e
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do { perror(msg); exit(EXIT_FAILURE); } while (0)
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int
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main(int argc, char *argv[])
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{
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FILE *out, *in;
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int v, s;
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size_t size;
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char *ptr;
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if (argc != 2) {
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fprintf(stderr, "Usage: %s \(aq<num>...\(aq\en", argv[0]);
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exit(EXIT_FAILURE);
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}
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in = fmemopen(argv[1], strlen(argv[1]), "r");
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if (in == NULL)
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handle_error("fmemopen");
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out = open_memstream(&ptr, &size);
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if (out == NULL)
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handle_error("open_memstream");
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for (;;) {
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s = fscanf(in, "%d", &v);
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if (s <= 0)
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break;
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s = fprintf(out, "%d ", v * v);
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if (s == \-1)
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handle_error("fprintf");
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}
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fclose(in);
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fclose(out);
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printf("size=%zu; ptr=%s\en", size, ptr);
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free(ptr);
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exit(EXIT_SUCCESS);
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}
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.EE
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.SH SEE ALSO
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.BR fopen (3),
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.BR fopencookie (3),
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.BR open_memstream (3)
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