setbuf(3s)
NAME
setbuf, setvbuf − assign buffering to a stream
SYNTAX
#include <stdio.h>
void setbuf(stream, buf)
FILE *stream;
char *buf;
int setvbuf(stream, buf, type, size)
FILE *stream;
char *buf;
int type, size;
DESCRIPTION
The setbuf subroutine may be used after a stream has been opened but before it is read or written. It causes the array pointed to by buf to be used instead of an automatically allocated buffer. If buf is the NULL pointer input/output will be completely unbuffered.
A constant BUFSIZ, defined in the <stdio.h> header file, tells how big an array is needed:
char buf[BUFSIZ];
The setvbuf subroutine may be used after a stream has been opened but before it is read or written. The 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 either when a newline is written, when 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 I/O is unbuffered, buf and size are ignored.
By default, output to a terminal is line buffered and all other input/output is fully buffered.
NOTE
A common source of error is allocating buffer space as an “automatic” variable in a code block, and then failing to close the stream in the same block.
DIAGNOSTICS
If an illegal value for type or size is provided, setvbuf returns a non-zero value. Otherwise, the value returned will be zero.