stdio/setbuf.3

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  36 .\"     @(#)setbuf.3    8.1 (Berkeley) 6/4/93
  37 .\" $FreeBSD: src/lib/libc/stdio/setbuf.3,v 1.10 2001/10/01 16:08:59 ru Exp $
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  39 .Dd June 4, 1993
  40 .Dt SETBUF 3
  41 .Os
  42 .Sh NAME
  43 .Nm setbuf ,
  44 .Nm setbuffer ,
  45 .Nm setlinebuf ,
  46 .Nm setvbuf
  47 .Nd stream buffering operations
  48 .Sh LIBRARY
  49 .Lb libc
  50 .Sh SYNOPSIS
  51 .In stdio.h
  52 .Ft void
  53 .Fn setbuf "FILE *stream" "char *buf"
  54 .Ft void
  55 .Fn setbuffer "FILE *stream" "char *buf" "int size"
  56 .Ft int
  57 .Fn setlinebuf "FILE *stream"
  58 .Ft int
  59 .Fn setvbuf "FILE *stream" "char *buf" "int mode" "size_t size"
  60 .Sh DESCRIPTION
  61 The three types of buffering available are unbuffered, block buffered,
  62 and line buffered.
  63 When an output stream is unbuffered, information appears on the
  64 destination file or terminal as soon as written;
  65 when it is block buffered many characters are saved up and written as a block;
  66 when it is line buffered characters are saved up until a newline is
  67 output or input is read from any stream attached to a terminal device
  68 (typically stdin).
  69 The function
  70 .Xr fflush 3
  71 may be used to force the block out early.
  72 (See
  73 .Xr fclose 3 . )
  74 .Pp
  75 Normally all files are block buffered.
  76 When the first
  77 .Tn I/O
  78 operation occurs on a file,
  79 .Xr malloc 3
  80 is called,
  81 and an optimally-sized buffer is obtained.
  82 If a stream refers to a terminal
  83 (as
  84 .Em stdout
  85 normally does) it is line buffered.
  86 The standard error stream
  87 .Em stderr
  88 is always unbuffered.
  89 .Pp
  90 The
  91 .Fn setvbuf
  92 function
  93 may be used to alter the buffering behavior of a stream.
  94 The
  95 .Fa mode
  96 parameter must be one of the following three macros:
  97 .Bl -tag -width _IOFBF -offset indent
  98 .It Dv _IONBF
  99 unbuffered
 100 .It Dv _IOLBF
 101 line buffered
 102 .It Dv _IOFBF
 103 fully buffered
 104 .El
 105 .Pp
 106 The
 107 .Fa size
 108 parameter may be given as zero
 109 to obtain deferred optimal-size buffer allocation as usual.
 110 If it is not zero,
 111 then except for unbuffered files, the
 112 .Fa buf
 113 argument should point to a buffer at least
 114 .Fa size
 115 bytes long;
 116 this buffer will be used instead of the current buffer.
 117 (If the
 118 .Fa size
 119 argument
 120 is not zero but
 121 .Fa buf
 122 is
 123 .Dv NULL ,
 124 a buffer of the given size will be allocated immediately,
 125 and released on close.
 126 This is an extension to ANSI C;
 127 portable code should use a size of 0 with any
 128 .Dv NULL
 129 buffer.)
 130 .Pp
 131 The
 132 .Fn setvbuf
 133 function may be used at any time,
 134 but may have peculiar side effects
 135 (such as discarding input or flushing output)
 136 if the stream is ``active''.
 137 Portable applications should call it only once on any given stream,
 138 and before any
 139 .Tn I/O
 140 is performed.
 141 .Pp
 142 The other three calls are, in effect, simply aliases for calls to
 143 .Fn setvbuf .
 144 Except for the lack of a return value, the
 145 .Fn setbuf
 146 function is exactly equivalent to the call
 147 .Pp
 148 .Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);"
 149 .Pp
 150 The
 151 .Fn setbuffer
 152 function
 153 is the same, except that the size of the buffer is up to the caller,
 154 rather than being determined by the default
 155 .Dv BUFSIZ .
 156 The
 157 .Fn setlinebuf
 158 function
 159 is exactly equivalent to the call:
 160 .Pp
 161 .Dl "setvbuf(stream, (char *)NULL, _IOLBF, 0);"
 162 .Sh RETURN VALUES
 163 The
 164 .Fn setvbuf
 165 function returns 0 on success, or
 166 .Dv EOF
 167 if the request cannot be honored
 168 (note that the stream is still functional in this case).
 169 .Pp
 170 The
 171 .Fn setlinebuf
 172 function returns what the equivalent
 173 .Fn setvbuf
 174 would have returned.
 175 .Sh SEE ALSO
 176 .Xr fclose 3 ,
 177 .Xr fopen 3 ,
 178 .Xr fread 3 ,
 179 .Xr malloc 3 ,
 180 .Xr printf 3 ,
 181 .Xr puts 3
 182 .Sh STANDARDS
 183 The
 184 .Fn setbuf
 185 and
 186 .Fn setvbuf
 187 functions
 188 conform to
 189 .St -isoC .
 190 .Sh BUGS
 191 The
 192 .Fn setbuffer
 193 and
 194 .Fn setlinebuf
 195 functions are not portable to versions of
 196 .Bx
 197 before
 198 .Bx 4.2 .
 199 On
 200 .Bx 4.2
 201 and
 202 .Bx 4.3
 203 systems,
 204 .Fn setbuf
 205 always uses a suboptimal buffer size and should be avoided.