1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" This code is derived from software contributed to Berkeley by 5.\" the American National Standards Committee X3, on Information 6.\" Processing Systems. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, this list of conditions and the following disclaimer. 13.\" 2. Redistributions in binary form must reproduce the above copyright 14.\" notice, this list of conditions and the following disclaimer in the 15.\" documentation and/or other materials provided with the distribution. 16.\" 3. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.Dd May 1, 2020 33.Dt SETBUF 3 34.Os 35.Sh NAME 36.Nm setbuf , 37.Nm setbuffer , 38.Nm setlinebuf , 39.Nm setvbuf 40.Nd stream buffering operations 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In stdio.h 45.Ft void 46.Fn setbuf "FILE * restrict stream" "char * restrict buf" 47.Ft void 48.Fn setbuffer "FILE *stream" "char *buf" "int size" 49.Ft int 50.Fn setlinebuf "FILE *stream" 51.Ft int 52.Fn setvbuf "FILE * restrict stream" "char * restrict buf" "int mode" "size_t size" 53.Sh DESCRIPTION 54The three types of buffering available are unbuffered, block buffered, 55and line buffered. 56When an output stream is unbuffered, information appears on the 57destination file or terminal as soon as written; 58when it is block buffered many characters are saved up and written as a block; 59when it is line buffered characters are saved up until a newline is 60output or input is read from any stream attached to a terminal device 61(typically 62.Dv stdin ) . 63The function 64.Xr fflush 3 65may be used to force the block out early. 66(See 67.Xr fclose 3 . ) 68.Pp 69Normally all files are block buffered. 70When the first 71.Tn I/O 72operation occurs on a file, 73.Xr malloc 3 74is called, 75and an optimally-sized buffer is obtained. 76If a stream refers to a terminal 77(as 78.Dv stdout 79normally does) it is line buffered. 80The standard error stream 81.Dv stderr 82is always unbuffered. 83Note that these defaults may be altered using the 84.Xr stdbuf 1 85utility. 86.Pp 87The 88.Fn setvbuf 89function 90may be used to alter the buffering behavior of a stream. 91The 92.Fa mode 93argument must be one of the following three macros: 94.Bl -tag -width _IOFBF -offset indent 95.It Dv _IONBF 96unbuffered 97.It Dv _IOLBF 98line buffered 99.It Dv _IOFBF 100fully buffered 101.El 102.Pp 103The 104.Fa size 105argument may be given as zero 106to obtain deferred optimal-size buffer allocation as usual. 107If it is not zero, 108then except for unbuffered files, the 109.Fa buf 110argument should point to a buffer at least 111.Fa size 112bytes long; 113this buffer will be used instead of the current buffer. 114If 115.Fa buf 116is not 117.Dv NULL , 118it is the caller's responsibility to 119.Xr free 3 120this buffer after closing the stream. 121(If the 122.Fa size 123argument 124is not zero but 125.Fa buf 126is 127.Dv NULL , 128a buffer of the given size will be allocated immediately, 129and released on close. 130This is an extension to ANSI C; 131portable code should use a size of 0 with any 132.Dv NULL 133buffer.) 134.Pp 135The 136.Fn setvbuf 137function may be used at any time, 138but may have peculiar side effects 139(such as discarding input or flushing output) 140if the stream is ``active''. 141Portable applications should call it only once on any given stream, 142and before any 143.Tn I/O 144is performed. 145.Pp 146The other three calls are, in effect, simply aliases for calls to 147.Fn setvbuf . 148Except for the lack of a return value, the 149.Fn setbuf 150function is exactly equivalent to the call 151.Pp 152.Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);" 153.Pp 154The 155.Fn setbuffer 156function 157is the same, except that the size of the buffer is up to the caller, 158rather than being determined by the default 159.Dv BUFSIZ . 160The 161.Fn setlinebuf 162function 163is exactly equivalent to the call: 164.Pp 165.Dl "setvbuf(stream, (char *)NULL, _IOLBF, 0);" 166.Sh RETURN VALUES 167The 168.Fn setvbuf 169function returns 0 on success, or 170.Dv EOF 171if the request cannot be honored 172(note that the stream is still functional in this case). 173.Pp 174The 175.Fn setlinebuf 176function returns what the equivalent 177.Fn setvbuf 178would have returned. 179.Sh SEE ALSO 180.Xr stdbuf 1 , 181.Xr fclose 3 , 182.Xr fopen 3 , 183.Xr fread 3 , 184.Xr malloc 3 , 185.Xr printf 3 , 186.Xr puts 3 187.Sh STANDARDS 188The 189.Fn setbuf 190and 191.Fn setvbuf 192functions 193conform to 194.St -isoC . 195.Sh HISTORY 196The 197.Fn setbuf 198function first appeared in 199.At v7 . 200The 201.Fn setbuffer 202function first appeared in 203.Bx 4.1c . 204The 205.Fn setlinebuf 206function first appeared in 207.Bx 4.2 . 208The 209.Fn setvbuf 210function first appeared in 211.Bx 4.4 . 212.Sh BUGS 213.Fn setbuf 214usually uses a suboptimal buffer size and should be avoided. 215