stdio/FreeBSD/setbuf.3

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  32 .\"     @(#)setbuf.3    8.1 (Berkeley) 6/4/93
  33 .\" $FreeBSD: src/lib/libc/stdio/setbuf.3,v 1.17 2007/01/09 00:28:07 imp Exp $
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  35 .Dd June 4, 1993
  36 .Dt SETBUF 3
  37 .Os
  38 .Sh NAME
  39 .Nm setbuf ,
  40 .Nm setbuffer ,
  41 .Nm setlinebuf ,
  42 .Nm setvbuf
  43 .Nd stream buffering operations
  44 .Sh LIBRARY
  45 .Lb libc
  46 .Sh SYNOPSIS
  47 .In stdio.h
  48 .Ft void
  49 .Fo setbuf
  50 .Fa "FILE *restrict stream"
  51 .Fa "char *restrict buf"
  52 .Fc
  53 .Ft void
  54 .Fo setbuffer
  55 .Fa "FILE *stream"
  56 .Fa "char *buf"
  57 .Fa "int size"
  58 .Fc
  59 .Ft int
  60 .Fo setlinebuf
  61 .Fa "FILE *stream"
  62 .Fc
  63 .Ft int
  64 .Fo setvbuf
  65 .Fa "FILE *restrict stream"
  66 .Fa "char *restrict buf"
  67 .Fa "int type"
  68 .Fa "size_t size"
  69 .Fc
  70 .Sh DESCRIPTION
  71 Three types of buffering are available:
  72 unbuffered, block buffered, and line buffered.
  73 When an output stream is unbuffered, information appears on the
  74 destination file or terminal as soon as written;
  75 when it is block buffered,
  76 many characters are saved up and written as a block;
  77 when it is line buffered,
  78 characters are saved up until a newline is output
  79 or input is read from any stream attached to a terminal device
  80 (typically
  81 .Dv stdin ) .
  82 The function
  83 .Xr fflush 3
  84 may be used to force the block out early.
  85 (See
  86 .Xr fclose 3 . )
  87 .Pp
  88 Normally, all files are block buffered.
  89 When the first
  90 .Tn I/O
  91 operation occurs on a file,
  92 .Xr malloc 3
  93 is called and an optimally-sized buffer is obtained.
  94 If a stream refers to a terminal
  95 (as
  96 .Dv stdout
  97 normally does), it is line buffered.
  98 The standard error stream
  99 .Dv stderr
 100 is always unbuffered.
 101 .Pp
 102 The
 103 .Fn setvbuf
 104 function
 105 may be used to alter the buffering behavior of a stream.
 106 The
 107 .Fa type
 108 argument must be one of the following three macros:
 109 .Bl -tag -width _IOFBF -offset indent
 110 .It Dv _IONBF
 111 unbuffered
 112 .It Dv _IOLBF
 113 line buffered
 114 .It Dv _IOFBF
 115 fully buffered
 116 .El
 117 .Pp
 118 The
 119 .Fa size
 120 argument may be given as zero
 121 to obtain deferred optimal-size buffer allocation as usual.
 122 If it is not zero,
 123 then except for unbuffered files, the
 124 .Fa buf
 125 argument should point to a buffer at least
 126 .Fa size
 127 bytes long;
 128 this buffer will be used instead of the current buffer.
 129 If
 130 .Fa buf
 131 is not
 132 .Dv NULL ,
 133 it is the caller's responsibility to
 134 .Xr free 3
 135 this buffer after closing the stream.
 136 (If the
 137 .Fa size
 138 argument
 139 is not zero but
 140 .Fa buf
 141 is
 142 .Dv NULL ,
 143 a buffer of the given size will be allocated immediately,
 144 and released on close.
 145 This is an extension to ANSI C;
 146 portable code should use a size of 0 with any
 147 .Dv NULL
 148 buffer.)
 149 .Pp
 150 The
 151 .Fn setvbuf
 152 function may be used at any time,
 153 but may have peculiar side effects
 154 (such as discarding input or flushing output)
 155 if the stream is ``active''.
 156 Portable applications should call it only once on any given stream,
 157 and before any
 158 .Tn I/O
 159 is performed.
 160 .Pp
 161 The other three calls are, in effect, simply aliases for calls to
 162 .Fn setvbuf .
 163 Except for the lack of a return value, the
 164 .Fn setbuf
 165 function is exactly equivalent to the call
 166 .Pp
 167 .Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);"
 168 .Pp
 169 The
 170 .Fn setbuffer
 171 function
 172 is the same, except that the size of the buffer is up to the caller,
 173 rather than being determined by the default
 174 .Dv BUFSIZ .
 175 The
 176 .Fn setlinebuf
 177 function
 178 is exactly equivalent to the call:
 179 .Pp
 180 .Dl "setvbuf(stream, (char *)NULL, _IOLBF, 0);"
 181 .Sh RETURN VALUES
 182 The
 183 .Fn setvbuf
 184 function returns 0 on success, or
 185 .Dv EOF
 186 if the request cannot be honored
 187 (note that the stream is still functional in this case).
 188 .Pp
 189 The
 190 .Fn setlinebuf
 191 function returns what the equivalent
 192 .Fn setvbuf
 193 would have returned.
 194 .Sh SEE ALSO
 195 .Xr fclose 3 ,
 196 .Xr fopen 3 ,
 197 .Xr fread 3 ,
 198 .Xr malloc 3 ,
 199 .Xr printf 3 ,
 200 .Xr puts 3
 201 .Sh STANDARDS
 202 The
 203 .Fn setbuf
 204 and
 205 .Fn setvbuf
 206 functions
 207 conform to
 208 .St -isoC .
 209 .Sh BUGS
 210 The
 211 .Fn setbuffer
 212 and
 213 .Fn setlinebuf
 214 functions are not portable to versions of
 215 .Bx
 216 before
 217 .Bx 4.2 .
 218 On
 219 .Bx 4.2
 220 and
 221 .Bx 4.3
 222 systems,
 223 .Fn setbuf
 224 always uses a suboptimal buffer size and should be avoided.