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.\" $FreeBSD$ 34.\" 35.Dd May 1, 2020 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.Fn setbuf "FILE * restrict stream" "char * restrict buf" 50.Ft void 51.Fn setbuffer "FILE *stream" "char *buf" "int size" 52.Ft int 53.Fn setlinebuf "FILE *stream" 54.Ft int 55.Fn setvbuf "FILE * restrict stream" "char * restrict buf" "int mode" "size_t size" 56.Sh DESCRIPTION 57The three types of buffering available are unbuffered, block buffered, 58and line buffered. 59When an output stream is unbuffered, information appears on the 60destination file or terminal as soon as written; 61when it is block buffered many characters are saved up and written as a block; 62when it is line buffered characters are saved up until a newline is 63output or input is read from any stream attached to a terminal device 64(typically 65.Dv stdin ) . 66The function 67.Xr fflush 3 68may be used to force the block out early. 69(See 70.Xr fclose 3 . ) 71.Pp 72Normally all files are block buffered. 73When the first 74.Tn I/O 75operation occurs on a file, 76.Xr malloc 3 77is called, 78and an optimally-sized buffer is obtained. 79If a stream refers to a terminal 80(as 81.Dv stdout 82normally does) it is line buffered. 83The standard error stream 84.Dv stderr 85is always unbuffered. 86Note that these defaults may be altered using the 87.Xr stdbuf 1 88utility. 89.Pp 90The 91.Fn setvbuf 92function 93may be used to alter the buffering behavior of a stream. 94The 95.Fa mode 96argument must be one of the following three macros: 97.Bl -tag -width _IOFBF -offset indent 98.It Dv _IONBF 99unbuffered 100.It Dv _IOLBF 101line buffered 102.It Dv _IOFBF 103fully buffered 104.El 105.Pp 106The 107.Fa size 108argument may be given as zero 109to obtain deferred optimal-size buffer allocation as usual. 110If it is not zero, 111then except for unbuffered files, the 112.Fa buf 113argument should point to a buffer at least 114.Fa size 115bytes long; 116this buffer will be used instead of the current buffer. 117If 118.Fa buf 119is not 120.Dv NULL , 121it is the caller's responsibility to 122.Xr free 3 123this buffer after closing the stream. 124(If the 125.Fa size 126argument 127is not zero but 128.Fa buf 129is 130.Dv NULL , 131a buffer of the given size will be allocated immediately, 132and released on close. 133This is an extension to ANSI C; 134portable code should use a size of 0 with any 135.Dv NULL 136buffer.) 137.Pp 138The 139.Fn setvbuf 140function may be used at any time, 141but may have peculiar side effects 142(such as discarding input or flushing output) 143if the stream is ``active''. 144Portable applications should call it only once on any given stream, 145and before any 146.Tn I/O 147is performed. 148.Pp 149The other three calls are, in effect, simply aliases for calls to 150.Fn setvbuf . 151Except for the lack of a return value, the 152.Fn setbuf 153function is exactly equivalent to the call 154.Pp 155.Dl "setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);" 156.Pp 157The 158.Fn setbuffer 159function 160is the same, except that the size of the buffer is up to the caller, 161rather than being determined by the default 162.Dv BUFSIZ . 163The 164.Fn setlinebuf 165function 166is exactly equivalent to the call: 167.Pp 168.Dl "setvbuf(stream, (char *)NULL, _IOLBF, 0);" 169.Sh RETURN VALUES 170The 171.Fn setvbuf 172function returns 0 on success, or 173.Dv EOF 174if the request cannot be honored 175(note that the stream is still functional in this case). 176.Pp 177The 178.Fn setlinebuf 179function returns what the equivalent 180.Fn setvbuf 181would have returned. 182.Sh SEE ALSO 183.Xr stdbuf 1 , 184.Xr fclose 3 , 185.Xr fopen 3 , 186.Xr fread 3 , 187.Xr malloc 3 , 188.Xr printf 3 , 189.Xr puts 3 190.Sh STANDARDS 191The 192.Fn setbuf 193and 194.Fn setvbuf 195functions 196conform to 197.St -isoC . 198.Sh HISTORY 199The 200.Fn setbuf 201function first appeared in 202.At v7 . 203The 204.Fn setbuffer 205function first appeared in 206.Bx 4.1c . 207The 208.Fn setlinebuf 209function first appeared in 210.Bx 4.2 . 211The 212.Fn setvbuf 213function first appeared in 214.Bx 4.4 . 215.Sh BUGS 216.Fn setbuf 217usually uses a suboptimal buffer size and should be avoided. 218