setvbuf(3) - NetBSD Manual Pages

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SETBUF(3)               NetBSD 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, size_t 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 out-
     put or input is read from any stream attached to a terminal device (typi-
     cally stdin).

     The default buffer settings can be overwritten per descriptor (STDBUFn,
     where n is the numeric value of the file descriptor represented by the
     stream), or for all descriptors (STDBUF).  The environment variable value
     is a letter followed by an optional numeric value indicating the size of
     the buffer.  Valid sizes range from 0B to 1MB.  Valid letters are:

           U  unbuffered

           L  line buffered

           F  fully buffered

     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 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:

           _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
     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 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

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


STANDARDS

     The setbuf() and setvbuf() functions conform to ANSI X3.159-1989
     (``ANSI C89'').


HISTORY

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


BUGS

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

NetBSD 9.4_STABLE                June 4, 1993                NetBSD 9.4_STABLE