1.\" $NetBSD: setbuf.3,v 1.14 2015/07/15 19:08:43 christos Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" This code is derived from software contributed to Berkeley by 7.\" the American National Standards Committee X3, on Information 8.\" Processing Systems. 9.\" 10.\" Redistribution and use in source and binary forms, with or without 11.\" modification, are permitted provided that the following conditions 12.\" are met: 13.\" 1. Redistributions of source code must retain the above copyright 14.\" notice, this list of conditions and the following disclaimer. 15.\" 2. Redistributions in binary form must reproduce the above copyright 16.\" notice, this list of conditions and the following disclaimer in the 17.\" documentation and/or other materials provided with the distribution. 18.\" 3. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)setbuf.3 8.1 (Berkeley) 6/4/93 35.\" 36.Dd June 4, 1993 37.Dt SETBUF 3 38.Os 39.Sh NAME 40.Nm setbuf , 41.Nm setbuffer , 42.Nm setlinebuf , 43.Nm setvbuf 44.Nd stream buffering operations 45.Sh LIBRARY 46.Lb libc 47.Sh SYNOPSIS 48.In stdio.h 49.Ft void 50.Fn setbuf "FILE * restrict stream" "char * restrict buf" 51.Ft void 52.Fn setbuffer "FILE *stream" "char *buf" "size_t size" 53.Ft int 54.Fn setlinebuf "FILE *stream" 55.Ft int 56.Fn setvbuf "FILE * restrict stream" "char * restrict buf" "int mode" "size_t size" 57.Sh DESCRIPTION 58The three types of buffering available are unbuffered, block buffered, 59and line buffered. 60When an output stream is unbuffered, information appears on the 61destination file or terminal as soon as written; 62when it is block buffered many characters are saved up and written as a block; 63when it is line buffered characters are saved up until a newline is 64output or input is read from any stream attached to a terminal device 65(typically stdin). 66.Pp 67The default buffer settings can be overwritten per descriptor 68.Dv ( STDBUFn ) 69where 70.Dv n 71is the numeric value of the file descriptor represented by the stream, or 72for all descriptors 73.Dv ( STDBUF ) . 74The environment variable value is a letter followed by an optional numeric 75value indicating the size of the buffer. 76Valid sizes range from 0B to 1MB. 77Valid letters are: 78.Bl -tag -width X -indent 79.It Dv Li U 80Unbuffered. 81.It Dv Li L 82Line-buffered. 83.It Dv Li F 84Fully-buffered. 85.El 86.Pp 87The function 88.Xr fflush 3 89may be used to force the block out early. 90(See 91.Xr fclose 3 . ) 92.Pp 93Normally all files are block buffered. 94When the first 95.Tn I/O 96operation occurs on a file, 97.Xr malloc 3 98is called, 99and an optimally-sized buffer is obtained. 100If a stream refers to a terminal 101(as 102.Em stdout 103normally does) it is line buffered. 104The standard error stream 105.Em stderr 106is initially unbuffered. 107.Pp 108The 109.Fn setvbuf 110function 111may be used to alter the buffering behavior of a stream. 112The 113.Fa mode 114parameter must be one of the following three macros: 115.Bl -tag -width _IOFBF -offset indent 116.It Dv _IONBF 117unbuffered 118.It Dv _IOLBF 119line buffered 120.It Dv _IOFBF 121fully buffered 122.El 123.Pp 124The 125.Fa size 126parameter may be given as zero 127to obtain deferred optimal-size buffer allocation as usual. 128If it is not zero, 129then except for unbuffered files, the 130.Fa buf 131argument should point to a buffer at least 132.Fa size 133bytes long; 134this buffer will be used instead of the current buffer. 135(If the 136.Fa size 137argument 138is not zero but 139.Fa buf 140is 141.Dv NULL , 142a buffer of the given size will be allocated immediately, 143and released on close. 144This is an extension to ANSI C; 145portable code should use a size of 0 with any 146.Dv NULL 147buffer.) 148.Pp 149The 150.Fn setvbuf 151function may be used at any time, 152but may have peculiar side effects 153(such as discarding input or flushing output) 154if the stream is ``active''. 155Portable applications should call it only once on any given stream, 156and before any 157.Tn I/O 158is performed. 159.Pp 160The other three calls are, in effect, simply aliases for calls to 161.Fn setvbuf . 162Except for the lack of a return value, the 163.Fn setbuf 164function is exactly equivalent to the call 165.Pp 166.Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);" 167.Pp 168The 169.Fn setbuffer 170function 171is the same, except that the size of the buffer is up to the caller, 172rather than being determined by the default 173.Dv BUFSIZ . 174The 175.Fn setlinebuf 176function 177is exactly equivalent to the call: 178.Pp 179.Dl "setvbuf(stream, (char *)NULL, _IOLBF, 0);" 180.Sh RETURN VALUES 181The 182.Fn setvbuf 183function returns 0 on success, or 184.Dv EOF 185if the request cannot be honored 186(note that the stream is still functional in this case). 187.Pp 188The 189.Fn setlinebuf 190function returns what the equivalent 191.Fn setvbuf 192would have returned. 193.Sh SEE ALSO 194.Xr fclose 3 , 195.Xr fopen 3 , 196.Xr fread 3 , 197.Xr malloc 3 , 198.Xr printf 3 , 199.Xr puts 3 200.Sh STANDARDS 201The 202.Fn setbuf 203and 204.Fn setvbuf 205functions 206conform to 207.St -ansiC . 208.Sh BUGS 209The 210.Fn setbuffer 211and 212.Fn setlinebuf 213functions are not portable to versions of 214.Bx 215before 216.Bx 4.2 . 217On 218.Bx 4.2 219and 220.Bx 4.3 221systems, 222.Fn setbuf 223always uses a suboptimal buffer size and should be avoided. 224