setvbuf(3) - NetBSD Manual Pages

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SETBUF(3)                 NetBSD 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);

     int
     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 ap-
     pears 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 oc-
     curs on a file, malloc(3) is called, and an optimally-sized buffer is ob-
     tained.  If a stream refers to a terminal (as stdout normally does) it is
     line buffered.  The standard error stream stderr is always 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:

           _IONBF  unbuffered

           _IOLBF  line buffered

           _IOFBF  fully buffered

     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 un-
     buffered 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 exten-
     sion 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

     fopen(3),  fclose(3),  fread(3),  malloc(3),  puts(3),  printf(3)


STANDARDS

     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. On 4.2BSD and 4.3BSD systems, setbuf() always uses
     a suboptimal buffer size and should be avoided.

4th Berkeley Distribution        June 4, 1993                                2