SETBUF(3)              FreeBSD Library Functions Manual              SETBUF(3)

NAME
     setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

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

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

     int
     setlinebuf(FILE *stream);

     int
     setvbuf(FILE * restrict stream, char * restrict buf, int mode,
         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 newline is
     output or input is read from any stream attached to a terminal device
     (typically stdin).  The function fflush(3) may be used to force the block
     out early.  (See fclose(3).)

     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 always unbuffered.
     Note that these defaults may be altered using the stdbuf(1) utility.

     The setvbuf() function may be used to alter the buffering behavior of a
     stream.  The mode argument must be one of the following three macros:

           _IONBF  unbuffered

           _IOLBF  line buffered

           _IOFBF  fully buffered

     The size argument 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
     buf is not NULL, it is the caller's responsibility to free(3) this buffer
     after closing the stream.  (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.

     The other three calls are, in effect, simply aliases for calls to
     setvbuf().  Except for the lack of a return value, the setbuf() function
     is exactly equivalent to the call

           setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);

     The setbuffer() function is the same, except that the size of the buffer
     is up to the caller, rather than being determined by the default BUFSIZ.
     The setlinebuf() function is exactly equivalent to the call:

           setvbuf(stream, (char *)NULL, _IOLBF, 0);

RETURN VALUES
     The setvbuf() function returns 0 on success, or EOF if the request cannot
     be honored (note that the stream is still functional in this case).

     The setlinebuf() function returns what the equivalent setvbuf() would
     have returned.

SEE ALSO
     stdbuf(1), fclose(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)

STANDARDS
     The setbuf() and setvbuf() functions conform to ISO/IEC 9899:1990
     ("ISO C90").

HISTORY
     The setbuf() function first appeared in Version 7 AT&T UNIX.  The
     setbuffer() function first appeared in 4.1cBSD.  The setlinebuf()
     function first appeared in 4.2BSD.  The setvbuf() function first appeared
     in 4.4BSD.

BUGS
     setbuf() usually uses a suboptimal buffer size and should be avoided.

FreeBSD 13.1-RELEASE-p6           May 1, 2020          FreeBSD 13.1-RELEASE-p6