Name

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

Syntax

#include <stdio.h>

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

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

setlinebuf(stream)
FILE *stream;

int setvbuf(stream, buf, type, size)
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 new line is encountered or input is read from stdin.  The routine fflush, may be used to force the block out early. Normally all files are block buffered. For further information, see fclose(.). A buffer is obtained from malloc() upon the first getc or putc on the file. If the standard stream stdout refers to a terminal it is line buffered.  The standard stream stderr is always unbuffered. 

The setbuf routine 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];

The setbuffer routine, 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. 

The setlinebuf routine 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.

The setvbuf routine may be used after a stream has been opened but before it is read or written. Type determines how stream will be buffered.  Legal values for type, defined in stdio.h are:

 _IOFBF causes input/output to be fully buffered. 

 _IOLBF causes output to be line buffered; the buffer will be flushed when a new line is written, the buffer is full, or input is requested. 

 _IONBF causes input/output to be completely unbuffered. 

If buf is not the NULL pointer, the array it points to will be used for buffering, instead of an automatically allocated buffer.  The size specifies the size of the buffer to be used.  The constant BUFSIZ in <stdio.h> is suggested as a good buffer size.  If input/output is unbuffered, buf and size are ignored. 

By default, output to a terminal is line buffered and all other input/output is fully buffered. 

A file can be changed from unbuffered or line buffered to block buffered by using freopen. For further information, see fopen(.). 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.

Restrictions

The standard error stream should be line buffered by default. 

The setbuffer and setlinebuf functions are not portable to non 4.2 BSD versions of UNIX.

See Also

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