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:
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 terminal
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 allocated 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.
setlinebuf()
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.
STANDARDS CONFORMANCE
setbuf(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1, ANSI C
setvbuf(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, ANSI C
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