setvbuf

SETBUF(3)		  Linux Programmer’s Manual		    SETBUF(3)



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

SYNOPSIS
       #include <stdio.h>

       void setbuf(FILE *stream, char *buf);
       void setbuffer(FILE *stream, char *buf, size_t size);
       void setlinebuf(FILE *stream);
       int setvbuf(FILE *stream, char *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 new-
       line 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 a 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 by default.

       The setvbuf function may be used on any	open  stream  to  change  its
       buffer.	The mode parameter must be one of the following three macros:

	      _IONBF unbuffered

	      _IOLBF line buffered

	      _IOFBF fully buffered

       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 cur-
       rent buffer.  If the argument buf is NULL, only the mode is  affected;
       a  new  buffer  will be allocated on the next read or write operation.
       The setvbuf function may only be	 used  after  opening  a  stream  and
       before any other operations have been performed on it.

       The  other  three  calls	 are,  in effect, simply aliases for calls to
       setvbuf.	 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 BUF-
       SIZ.  The setlinebuf function is exactly equivalent to the call:

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

RETURN VALUE
       The function setvbuf returns 0 on success.  It can return any value on
       failure,	 but returns nonzero when mode is invalid or the request can-
       not be honoured. It may set errno on failure.  The other functions are
       void.

CONFORMING TO
       The  setbuf  and setvbuf functions conform to ANSI X3.159-1989 (‘‘ANSI
       C’’).

BUGS
       The setbuffer and setlinebuf functions are not portable to versions of
       BSD  before  4.2BSD,  and are available under Linux since libc 4.5.21.
       On 4.2BSD and 4.3BSD systems, setbuf always uses a  suboptimal  buffer
       size and should be avoided.

       You  must  make	sure  that  both buf and the space it points to still
       exist by the time stream is closed, which also happens at program ter-
       mination.

       For example, the following is illegal:

       #include <stdio.h>
       int main()
       {
	   char buf[BUFSIZ];
	   setbuf(stdin, buf);
	   printf("Hello, world!\n");
	   return 0;
       }


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



Linux				  2001-06-09			    SETBUF(3)