.\" Copyright (c) 1980, 1991 Regents of the University of California. .\" All rights reserved. .\" .\" %sccs.include.redist.man% .\" .\" @(#)setbuf.3 6.9 (Berkeley) 04/19/91 .\" .Dd .Dt SETBUF 3 .Os BSD 4 .Sh NAME .Nm setbuf , .Nm setbuffer , .Nm setlinebuf , .Nm setvbuf .Nd stream buffering operations .Sh SYNOPSIS .Fd #include .Ft int .Fn setbuf "FILE *stream" "char *buf" .Ft int .Fn setbuffer "FILE *stream" "char *buf" "size_t size" .Ft int .Fn setlinebuf "FILE *stream" .Ft int .Fn setvbuf "FILE *stream" "char *buf" "int mode" "size_t size" .Sh DESCRIPTION The three types of buffering available are unbuffered, block buffered, and line buffered. When an output stream is unbuffered, information appears on the destination file or terminal as soon as written; when it is block buffered many characters are saved up and written as a block; when it is line buffered characters are saved up until a newline is output or input is read from any stream attached to a terminal device (typically stdin). The function .Xr fflush 3 may be used to force the block out early. (See .Xr fclose 3 . ) Normally all files are block buffered. When the first .Tn I/O operation occurs on a file, .Xr malloc 3 is called, and a buffer is obtained. If a stream refers to a terminal (as .Em stdout normally does) it is line buffered. The standard error stream .Em stderr is always unbuffered. .Pp The .Fn setvbuf function may be used at any time on any open stream to change its buffer. The .Fa mode parameter must be one of the following three macros: .Bl -tag -width _IOFBF -offset indent .It Dv _IONBF unbuffered .It Dv _IOLBF line buffered .It Dv _IOFBF fully buffered .El .Pp Except for unbuffered files, the .Fa buf argument should point to a buffer at least .Fa size bytes long; this buffer will be used instead of the current buffer. If the argument .Fa buf is NULL, only the mode is affected; a new buffer will be allocated on the next read or write operation. The .Fn setvbuf function may be used at any time, but can only change the mode of a stream when it is not ``active'': that is, before any .Tn I/O , or immediately after a call to .Xr fflush . .Pp The other three calls are, in effect, simply aliases for calls to .Fn setvbuf . The .Fn setbuf function is exactly equivalent to the call .Pp .Dl setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ); .Pp The .Fn setbuffer function is the same, except that the size of the buffer is up to the caller, rather than being determined by the default .Dv BUFSIZ . The .Fn setlinebuf function is exactly equivalent to the call: .Pp .Dl setvbuf(stream, (char *)NULL, _IOLBF, 0); .Sh SEE ALSO .Xr fopen 3 , .Xr fclose 3 , .Xr fread 3 , .Xr malloc 3 , .Xr puts 3 , .Xr printf 3 .Sh STANDARDS The .Fn setbuf and .Fn setvbuf functions conform to .St -ansiC . .Sh BUGS The .Fn setbuffer and .Fn setlinebuf functions are not portable to versions of .Bx before .Bx 4.2 . On .Bx 4.2 and .Bx 4.3 systems, .Fn setbuf always uses a suboptimal buffer size and should be avoided.