NAME
setvbuf —
stream buffering operations
SYNOPSIS
#include
<stdio.h>
int
setvbuf(FILE
*stream, char *buf,
int mode,
size_t size);
DESCRIPTION
The three types of stream 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 line buffered, characters are saved up until a newline
(‘\n’) is output or input is read from
any stream attached to a terminal device (typically
stdin).
The fflush(3) function may be used to force the block out early.
Normally, all files are block buffered. When the first I/O operation occurs on a file, malloc(3) is called, and an optimally sized buffer is obtained. If a stream refers to a terminal (as stdout normally does), it is line buffered.
The standard error stream stderr is initially unbuffered.
The
setvbuf()
function may be used to alter the buffering behavior of a stream. The
mode parameter must be one of the following three
macros:
The size parameter may be given as zero to
obtain deferred optimal-size buffer allocation as usual. If it is not zero,
then except for unbuffered files, the buf argument
should point to a buffer at least size bytes long;
this buffer will be used instead of the current buffer. (If the
size argument is not zero but
buf is NULL, a buffer of the
given size will be allocated immediately, and released on close. This is an
extension to ANSI C; portable code should use a size of 0 with any
NULL buffer.)
The
setvbuf()
function may be used at any time, but may have peculiar side effects (such
as discarding input or flushing output) if the stream is
“active”. Portable applications should call it only once on
any given stream, and before any I/O is performed.
RETURN VALUES
Upon successful completion, a value of 0 is returned. If mode is invalid or if the request cannot be honored, a non-zero value is returned, possibly setting errno to indicate the error. The stream is not modified in the error case.
ERRORS
The setvbuf() function will fail if:
- [
EBADF] - The stream specified is not associated with a valid file descriptor.
SEE ALSO
fclose(3), fopen(3), fread(3), malloc(3), printf(3), puts(3), setbuf(3)
STANDARDS
The setvbuf() function conforms to
ISO/IEC 9899:1999
(“ISO C99”).
HISTORY
The setvbuf() function first appeared in
4.4BSD.