1.\" $NetBSD: setbuf.3,v 1.20 2018/12/14 03:43:22 uwe 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 66.Va stdin ) . 67.Pp 68The default buffer settings can be overwritten per descriptor 69.Ev ( STDBUF Ns Ar n , 70where 71.Ar n 72is the numeric value of the file descriptor represented by the stream), or 73for all descriptors 74.Ev ( STDBUF ) . 75The environment variable value is a letter followed by an optional numeric 76value indicating the size of the buffer. 77Valid sizes range from 0B to 1MB. 78Valid letters are: 79.Bl -tag -offset indent -width X 80.It Li U 81unbuffered 82.It Li L 83line buffered 84.It Li F 85fully buffered 86.El 87.Pp 88The function 89.Xr fflush 3 90may be used to force the block out early. 91(See 92.Xr fclose 3 . ) 93.Pp 94Normally all files are block buffered. 95When the first I/O operation occurs on a file, 96.Xr malloc 3 97is called, 98and an optimally-sized buffer is obtained. 99If a stream refers to a terminal 100(as 101.Va stdout 102normally does) it is line buffered. 103The standard error stream 104.Va stderr 105is initially unbuffered. 106.Pp 107The 108.Fn setvbuf 109function 110may be used to alter the buffering behavior of a stream. 111The 112.Fa mode 113parameter must be one of the following three macros: 114.Bl -tag -width _IOFBF -offset indent 115.It Dv _IONBF 116unbuffered 117.It Dv _IOLBF 118line buffered 119.It Dv _IOFBF 120fully buffered 121.El 122.Pp 123The 124.Fa size 125parameter may be given as zero 126to obtain deferred optimal-size buffer allocation as usual. 127If it is not zero, 128then except for unbuffered files, the 129.Fa buf 130argument should point to a buffer at least 131.Fa size 132bytes long; 133this buffer will be used instead of the current buffer. 134(If the 135.Fa size 136argument 137is not zero but 138.Fa buf 139is 140.Dv NULL , 141a buffer of the given size will be allocated immediately, 142and released on close. 143This is an extension to ANSI C; 144portable code should use a size of 0 with any 145.Dv NULL 146buffer.) 147.Pp 148The 149.Fn setvbuf 150function may be used at any time, 151but may have peculiar side effects 152(such as discarding input or flushing output) 153if the stream is ``active''. 154Portable applications should call it only once on any given stream, 155and before any I/O is performed. 156.Pp 157The other three calls are, in effect, simply aliases for calls to 158.Fn setvbuf . 159Except for the lack of a return value, the 160.Fn setbuf 161function is exactly equivalent to the call 162.Pp 163.Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);" 164.Pp 165The 166.Fn setbuffer 167function 168is the same, except that the size of the buffer is up to the caller, 169rather than being determined by the default 170.Dv BUFSIZ . 171The 172.Fn setlinebuf 173function 174is exactly equivalent to the call: 175.Pp 176.Dl "setvbuf(stream, (char *)NULL, _IOLBF, 0);" 177.Sh RETURN VALUES 178The 179.Fn setvbuf 180function returns 0 on success, or 181.Dv EOF 182if the request cannot be honored 183(note that the stream is still functional in this case). 184.Pp 185The 186.Fn setlinebuf 187function returns what the equivalent 188.Fn setvbuf 189would have returned. 190.Sh SEE ALSO 191.Xr fclose 3 , 192.Xr fopen 3 , 193.Xr fread 3 , 194.Xr malloc 3 , 195.Xr printf 3 , 196.Xr puts 3 197.Sh STANDARDS 198The 199.Fn setbuf 200and 201.Fn setvbuf 202functions 203conform to 204.St -ansiC . 205.Sh HISTORY 206The 207.Fn setbuf 208function first appeared in 209.At v7 . 210The 211.Fn setbuffer 212function first appeared in 213.Bx 4.1c . 214The 215.Fn setlinebuf 216function first appeared in 217.Bx 4.2 . 218The 219.Fn setvbuf 220function first appeared in 221.Bx 4.4 . 222.Sh BUGS 223The 224.Fn setbuf 225function usually uses a suboptimal buffer size and should be avoided. 226