SETBUF

Section: Standard I/O Functions (3S)
Updated: October 16, 1987
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NAME

setbuf, setbuffer, setlinebuf, setvbuf - assign buffering to a stream

SYNOPSIS

#include <stdio.h>

setbuf(stream, buf)
FILE *stream;
char *buf;

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

setlinebuf(stream)
FILE *stream;

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

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 encountered or input is read from stdin. Fflush (see fclose(3S)) may be used to force the block out early. Normally all files are block buffered. A buffer is obtained from malloc(3) upon the first getc or putc(3S) on the file. If the standard stream stdout refers to a terminal it is line buffered. The standard stream stderr is always unbuffered.

Setbuf is used after a stream has been opened but before it is read or written. The character array buf is used instead of an automatically allocated buffer. If buf is the constant pointer NULL, input/output will be completely unbuffered. A manifest constant BUFSIZ tells how big an array is needed:

: char buf[BUFSIZ];

Setbuffer, an alternate form of setbuf, is used after a stream has been opened but before it is read or written. The character array buf whose size is determined by the size argument is used instead of an automatically allocated buffer. If buf is the constant pointer NULL, input/output will be completely unbuffered.

Setlinebuf is used to change stdout or stderr from block buffered or unbuffered to line buffered. Unlike setbuf and setbuffer it can be used at any time that the file descriptor is active.

Setvbuf is the final form of setbuf; as with setbuf, it must be invoked after a stream has been opened but before it is read or written. The argument mode determines how stream will be buffered: _IOFBF causes input/output to be fully buffered, _IOLBF causes output to be line buffered, and _IONBF causes input/output to be unbuffered. The argument size specifies how large a buffer to use. If buf is NULL , then setvbuf will allocate the buffer space; otherwise, it will use the space at buf (which must contain at least size bytes). Setvbuf returns zero on success, or nonzero if an invalid value is given for mode or if the request cannot be honored.

A file can be changed from unbuffered or line buffered to block buffered by using freopen (see fopen(3S)). A file can be changed from block buffered or line buffered to unbuffered by using freopen followed by setbuf with a buffer argument of NULL.

BUGS

The setbuffer and setlinebuf functions are not portable to non-4.2BSD versions of UNIX. On 4.2BSD and 4.3BSD systems, setbuf always uses a suboptimal buffer size and should be avoided. Setbuffer is not usually needed as the default file I/O buffer sizes are optimal.