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SETBUF(3) Library Functions Manual SETBUF(3)

setbuf, setbuffer, setlinebuf, setvbufstream buffering operations

#include <stdio.h>

setbuf(FILE *stream, char *buf);

setbuffer(FILE *stream, char *buf, size_t size);

setlinebuf(FILE *stream);

setvbuf(FILE *stream, char *buf, int mode, size_t size);

The three types of stream 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 line buffered, characters are saved up until a newline (‘\n’) is output or input is read from any stream attached to a terminal device (typically ).

The fflush(3) function may be used to force the block out early.

Normally, all files are block buffered. When the first I/O operation occurs on a file, malloc(3) is called, and an optimally sized buffer is obtained. If a stream refers to a terminal (as normally does), it is line buffered.

The standard error stream is initially unbuffered.

The () function may be used to alter the buffering behavior of a stream. The mode parameter must be one of the following three macros:

line buffered
fully buffered

The size parameter may be given as zero to obtain deferred optimal-size buffer allocation as usual. If it is not zero, then except for unbuffered files, the buf argument should point to a buffer at least size bytes long; this buffer will be used instead of the current buffer. (If the size argument is not zero but buf is NULL, a buffer of the given size will be allocated immediately, and released on close. This is an extension to ANSI C; portable code should use a size of 0 with any NULL buffer.)

The () function may be used at any time, but may have peculiar side effects (such as discarding input or flushing output) if the stream is “active”. Portable applications should call it only once on any given stream, and before any I/O is performed.

The other three calls are, in effect, simply aliases for calls to (). Except for the lack of a return value, the () function is exactly equivalent to the call

setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);

The () function is the same, except that the size of the buffer is up to the caller, rather than being determined by the default BUFSIZ. The () function is exactly equivalent to the call:

setvbuf(stream, NULL, _IOLBF, 0);

The setvbuf() function returns 0 on success, or EOF if the request cannot be honored (note that the stream is still functional in this case).

The setlinebuf() function returns what the equivalent setvbuf() would have returned.

fclose(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)

The setbuf() and setvbuf() functions conform to ANSI X3.159-1989 (“ANSI C89”).

The function setbuf() first appeared in Version 7 AT&T UNIX.

The setbuffer() and setlinebuf() functions are not portable to versions of BSD before 4.2BSD. On 4.2BSD and 4.3BSD systems, setbuf() always uses a suboptimal buffer size and should be avoided.

July 17, 2013 OpenBSD-5.5