SETBUF(3)

NAME
setbuf, setvbuf - assign buffering to a stream

SYNOPSIS
#include <stdio.h>
int setbuf(FILE *stream, char *buf)
int setvbuf(FILE *stream, char *buf, int type, 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 encountered or input is read from stdin. Fflush (see fclose(3)) 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(3) 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];

Setvbuf, an alternate form of setbuf, is used after a stream has been opened but before it is read or written. It has three uses, depending on the value of the type argument:

setvbuf(stream, buf, _IOFBF, size)
    Causes input/output to be fully buffered using the character array buf whose size is determined by the size argument. If buf is the constant pointer NULL, then an automatically allocated buffer will be used.

setvbuf(stream, buf, _IOLBF, size)
    Like above, except that output will be line buffered, i.e. the buffer will be flushed when a newline is written, the buffer is full, or input is requested.

setvbuf(stream, buf, _IONBF, size)
    Causes input/output to be completely unbuffered. Buf and size are ignored.

A file can be changed between unbuffered, line buffered, or block buffered by using freopen (see fopen(3)) followed by the appropriate setvbuf call.

SEE ALSO
fopen(3), getc(3), putc(3), malloc(3), fclose(3), puts(3), printf(3), fread(3).