setbuf.3s (2010 09)

s
setbuf(3S) setbuf(3S)
NAME
setbuf(), setvbuf(), setlinebuf() - assign buffering to a stream file
SYNOPSIS
#include <stdio.h>
void setbuf(FILE *__restrict stream, char *__restrict buf);
int setvbuf(FILE *__restrict stream, char *__restrict buf, int type,
size_t size);
void setlinebuf(FILE *stream);
Obsolescent Interface
int setvbuf_unlocked(FILE *stream, char *buf, int type, size_t size);
DESCRIPTION
setbuf() can 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];
setvbuf() can be used after a stream has been opened but before it is read or written. type determines
how stream is to 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 newline is written,
the buffer is full, or input is requested.
_IONBF causes input/output to be completely unbuffered.
When an output stream is unbuffered, information is queued for writing on the destination file or termi-
nal as soon as written; when it is buffered, many characters are saved up and written as a block. When
the output stream is line-buffered, each line of output is queued for writing on the destination terminal as
soon as the line is completed (that is, as soon as a new-line character is written or terminal input is
requested).
fflush() can also be used to explicitly write the buffer.
If buf is not the NULL pointer, the array it points to is used for buffering instead of an automatically allo-
cated buffer (from
malloc()). 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.
setlinebuf() is used to change stream from block-buffered or unbuffered to line-buffered.
setline-
buf() can be used any time the file descriptor is active.
Obsolescent Interface
setvbuf_unlocked() assigns buffering to a stream file.
DIAGNOSTICS
If an illegal value for type or size is provided,
setvbuf() and setvbuf_unlocked() return a non-
zero value. Otherwise, the value returned will be zero.
Note
A common source of error is allocating buffer space as an "automatic" variable in a code block, then failing
to close the stream in the same block.
Allocating a buffer of size or BUFSIZ bytes does not necessarily imply that all of size or BUFSIZ bytes
are used for the buffer area.
AUTHOR
setbuf, setvbuf, and setlinebuf() were developed by HP.
SEE ALSO
flockfile(3S), fopen(3S), getc(3S), malloc(3C), putc(3S), stdio(3S), thread_safety(5), glossary(9).
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1