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.\" @(#)setbuf.3 8.1 (Berkeley) 6/4/93 33.\" 34.Dd May 1, 2020 35.Dt SETBUF 3 36.Os 37.Sh NAME 38.Nm setbuf , 39.Nm setbuffer , 40.Nm setlinebuf , 41.Nm setvbuf 42.Nd stream buffering operations 43.Sh LIBRARY 44.Lb libc 45.Sh SYNOPSIS 46.In stdio.h 47.Ft void 48.Fn setbuf "FILE * restrict stream" "char * restrict buf" 49.Ft void 50.Fn setbuffer "FILE *stream" "char *buf" "int size" 51.Ft int 52.Fn setlinebuf "FILE *stream" 53.Ft int 54.Fn setvbuf "FILE * restrict stream" "char * restrict buf" "int mode" "size_t size" 55.Sh DESCRIPTION 56The three types of buffering available are unbuffered, block buffered, 57and line buffered. 58When an output stream is unbuffered, information appears on the 59destination file or terminal as soon as written; 60when it is block buffered many characters are saved up and written as a block; 61when it is line buffered characters are saved up until a newline is 62output or input is read from any stream attached to a terminal device 63(typically 64.Dv stdin ) . 65The function 66.Xr fflush 3 67may be used to force the block out early. 68(See 69.Xr fclose 3 . ) 70.Pp 71Normally all files are block buffered. 72When the first 73.Tn I/O 74operation occurs on a file, 75.Xr malloc 3 76is called, 77and an optimally-sized buffer is obtained. 78If a stream refers to a terminal 79(as 80.Dv stdout 81normally does) it is line buffered. 82The standard error stream 83.Dv stderr 84is always unbuffered. 85Note that these defaults may be altered using the 86.Xr stdbuf 1 87utility. 88.Pp 89The 90.Fn setvbuf 91function 92may be used to alter the buffering behavior of a stream. 93The 94.Fa mode 95argument must be one of the following three macros: 96.Bl -tag -width _IOFBF -offset indent 97.It Dv _IONBF 98unbuffered 99.It Dv _IOLBF 100line buffered 101.It Dv _IOFBF 102fully buffered 103.El 104.Pp 105The 106.Fa size 107argument may be given as zero 108to obtain deferred optimal-size buffer allocation as usual. 109If it is not zero, 110then except for unbuffered files, the 111.Fa buf 112argument should point to a buffer at least 113.Fa size 114bytes long; 115this buffer will be used instead of the current buffer. 116If 117.Fa buf 118is not 119.Dv NULL , 120it is the caller's responsibility to 121.Xr free 3 122this buffer after closing the stream. 123(If the 124.Fa size 125argument 126is not zero but 127.Fa buf 128is 129.Dv NULL , 130a buffer of the given size will be allocated immediately, 131and released on close. 132This is an extension to ANSI C; 133portable code should use a size of 0 with any 134.Dv NULL 135buffer.) 136.Pp 137The 138.Fn setvbuf 139function may be used at any time, 140but may have peculiar side effects 141(such as discarding input or flushing output) 142if the stream is ``active''. 143Portable applications should call it only once on any given stream, 144and before any 145.Tn I/O 146is performed. 147.Pp 148The other three calls are, in effect, simply aliases for calls to 149.Fn setvbuf . 150Except for the lack of a return value, the 151.Fn setbuf 152function is exactly equivalent to the call 153.Pp 154.Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);" 155.Pp 156The 157.Fn setbuffer 158function 159is the same, except that the size of the buffer is up to the caller, 160rather than being determined by the default 161.Dv BUFSIZ . 162The 163.Fn setlinebuf 164function 165is exactly equivalent to the call: 166.Pp 167.Dl "setvbuf(stream, (char *)NULL, _IOLBF, 0);" 168.Sh RETURN VALUES 169The 170.Fn setvbuf 171function returns 0 on success, or 172.Dv EOF 173if the request cannot be honored 174(note that the stream is still functional in this case). 175.Pp 176The 177.Fn setlinebuf 178function returns what the equivalent 179.Fn setvbuf 180would have returned. 181.Sh SEE ALSO 182.Xr stdbuf 1 , 183.Xr fclose 3 , 184.Xr fopen 3 , 185.Xr fread 3 , 186.Xr malloc 3 , 187.Xr printf 3 , 188.Xr puts 3 189.Sh STANDARDS 190The 191.Fn setbuf 192and 193.Fn setvbuf 194functions 195conform to 196.St -isoC . 197.Sh HISTORY 198The 199.Fn setbuf 200function first appeared in 201.At v7 . 202The 203.Fn setbuffer 204function first appeared in 205.Bx 4.1c . 206The 207.Fn setlinebuf 208function first appeared in 209.Bx 4.2 . 210The 211.Fn setvbuf 212function first appeared in 213.Bx 4.4 . 214.Sh BUGS 215.Fn setbuf 216usually uses a suboptimal buffer size and should be avoided. 217