SETBUF(3) | Library Functions Manual | SETBUF(3)

# NAME

`setbuf`, `setbuffer`, `setlinebuf`, `setvbuf` —
stream buffering operations

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <stdio.h>`

void
`setbuf`(FILE * restrict stream, char * restrict buf);

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

int
`setlinebuf`(FILE *stream);

int
`setvbuf`(FILE * restrict stream, char * restrict buf, int mode, size_t 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 output or input is read from any stream attached to a terminal device (typically stdin).

The default buffer settings can be overwritten per descriptor (`STDBUF` n, where n is the numeric value of the file descriptor represented by the stream), or for all descriptors (`STDBUF`). The environment variable value is a letter followed by an optional numeric value indicating the size of the buffer. Valid sizes range from 0B to 1MB. Valid letters are:

`U`
  unbuffered
`L`
  line buffered
`F`
  fully buffered

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

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 stdout normally does) it is line buffered. The standard error stream stderr is initially unbuffered.

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

`_IONBF`
  unbuffered
`_IOLBF`
  line buffered
`_IOFBF`
  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 `setvbuf`() 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 `setvbuf`(). Except for the lack of a return value, the `setbuf`() function is exactly equivalent to the call

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

The `setbuffer`() 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 `setlinebuf`() function is exactly equivalent to the call:

`setvbuf(stream, (char *)NULL, _IOLBF, 0);`

# RETURN VALUES

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.

# SEE ALSO

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

# STANDARDS

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

# HISTORY

The `setbuf`() function first appeared in Version 7 AT&T UNIX. The `setbuffer`() function first appeared in 4.1cBSD. The `setlinebuf`() function first appeared in 4.2BSD. The `setvbuf`() function first appeared in 4.4BSD.

# BUGS

The `setbuf`() function usually uses a suboptimal buffer size and should be avoided.

June 4, 1993 | NetBSD 9.4