man3/open_memstream.3

   1 .\" Copyright 2005, 2012, 2016 Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\"
   3 .\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
   4 .\" Distributed under the GPL.
   5 .\" %%%LICENSE_END
   6 .\"
   7 .\" 2008-12-04, Petr Baudis <pasky@suse.cz>: Document open_wmemstream()
   8 .\"
   9 .TH OPEN_MEMSTREAM 3 2017-09-15 "GNU" "Linux Programmer's Manual"
  10 .SH NAME
  11 open_memstream, open_wmemstream \-  open a dynamic memory buffer stream
  12 .SH SYNOPSIS
  13 .nf
  14 .B #include <stdio.h>
  15 .PP
  16 .BI "FILE *open_memstream(char **" ptr ", size_t *" sizeloc );
  17
  18 .B #include <wchar.h>
  19 .PP
  20 .BI "FILE *open_wmemstream(wchar_t **" ptr ", size_t *" sizeloc );
  21 .fi
  22 .PP
  23 .in -4n
  24 Feature Test Macro Requirements for glibc (see
  25 .BR feature_test_macros (7)):
  26 .in
  27 .PP
  28 .BR open_memstream (),
  29 .BR open_wmemstream ():
  30 .PD 0
  31 .ad l
  32 .RS 4
  33 .TP 4
  34 Since glibc 2.10:
  35 _POSIX_C_SOURCE\ >=\ 200809L
  36 .TP
  37 Before glibc 2.10:
  38 _GNU_SOURCE
  39 .RE
  40 .ad
  41 .PD
  42 .SH DESCRIPTION
  43 The
  44 .BR open_memstream ()
  45 function opens a stream for writing to a memory buffer.
  46 The function dynamically allocates the buffer,
  47 and the buffer automatically grows as needed.
  48 Initially, the buffer has a size of zero.
  49 After closing the stream, the caller should
  50 .BR free (3)
  51 this buffer.
  52 .PP
  53 The locations pointed to by
  54 .IR ptr
  55 and
  56 .I sizeloc
  57 are used to report, respectively,
  58 the current location and the size of the buffer.
  59 The locations referred to by these pointers are updated
  60 each time the stream is flushed
  61 .RB ( fflush (3))
  62 and when the stream is closed
  63 .RB ( fclose (3)).
  64 These values remain valid only as long as the caller
  65 performs no further output on the stream.
  66 If further output is performed, then the stream
  67 must again be flushed before trying to access these values.
  68 .PP
  69 A null byte is maintained at the end of the buffer.
  70 This byte is
  71 .I not
  72 included in the size value stored at
  73 .IR sizeloc .
  74 .PP
  75 The stream maintains the notion of a current position,
  76 which is initially zero (the start of the buffer).
  77 Each write operation implicitly adjusts the buffer position.
  78 The stream's buffer position can be explicitly changed with
  79 .BR fseek (3)
  80 or
  81 .BR fseeko (3).
  82 Moving the buffer position past the end
  83 of the data already written fills the intervening space with
  84 null characters.
  85 .PP
  86 The
  87 .BR open_wmemstream ()
  88 is similar to
  89 .BR open_memstream (),
  90 but operates on wide characters instead of bytes.
  91 .SH RETURN VALUE
  92 Upon successful completion,
  93 .BR open_memstream ()
  94 and
  95 .BR open_wmemstream ()
  96 return a
  97 .I FILE
  98 pointer.
  99 Otherwise, NULL is returned and
 100 .I errno
 101 is set to indicate the error.
 102 .SH VERSIONS
 103 .BR open_memstream ()
 104 was already available in glibc 1.0.x.
 105 .BR open_wmemstream ()
 106 is available since glibc 2.4.
 107 .SH ATTRIBUTES
 108 For an explanation of the terms used in this section, see
 109 .BR attributes (7).
 110 .TS
 111 allbox;
 112 lb lb lb
 113 l l l.
 114 Interface       Attribute       Value
 115 T{
 116 .BR open_memstream (),
 117 .br
 118 .BR open_wmemstream
 119 T}      Thread safety   MT-Safe
 120 .TE
 121 .sp 1
 122 .SH CONFORMING TO
 123 POSIX.1-2008.
 124 These functions are not specified in POSIX.1-2001,
 125 and are not widely available on other systems.
 126 .SH NOTES
 127 There is no file descriptor associated with the file stream
 128 returned by these functions
 129 (i.e.,
 130 .BR fileno (3)
 131 will return an error if called on the returned stream).
 132 .SH BUGS
 133 In glibc before version 2.7, seeking past the end of a stream created by
 134 .BR open_memstream ()
 135 does not enlarge the buffer; instead the
 136 .BR fseek (3)
 137 call fails, returning \-1.
 138 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=1996
 139 .SH EXAMPLE
 140 See
 141 .BR fmemopen (3).
 142 .SH SEE ALSO
 143 .BR fmemopen (3),
 144 .BR fopen (3),
 145 .BR setbuf (3)