setbuf(3) — Linux manual page

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | BUGS | SEE ALSO | COLOPHON

setbuf(3)                Library Functions Manual               setbuf(3)

NAME         top

       setbuf, setbuffer, setlinebuf, setvbuf - stream buffering
       operations

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <stdio.h>

       int setvbuf(FILE *restrict stream, char buf[restrict .size],
                   int mode, size_t size);

       void setbuf(FILE *restrict stream, char *restrict buf);
       void setbuffer(FILE *restrict stream, char buf[restrict .size],
                   size_t size);
       void setlinebuf(FILE *stream);

   Feature Test Macro Requirements for glibc (see
   feature_test_macros(7)):

       setbuffer(), setlinebuf():
           Since glibc 2.19:
               _DEFAULT_SOURCE
           glibc 2.19 and earlier:
               _BSD_SOURCE

DESCRIPTION         top

       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.  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 argument 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 current 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 be used only 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 BUFSIZ.  The setlinebuf() function is exactly equivalent
       to the call:

           setvbuf(stream, NULL, _IOLBF, 0);

RETURN VALUE         top

       The function setvbuf() returns 0 on success.  It returns nonzero
       on failure (mode is invalid or the request cannot be honored).  It
       may set errno on failure.

       The other functions do not return a value.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).
       ┌──────────────────────────────────────┬───────────────┬─────────┐
       │ Interface                            Attribute     Value   │
       ├──────────────────────────────────────┼───────────────┼─────────┤
       │ setbuf(), setbuffer(), setlinebuf(), │ Thread safety │ MT-Safe │
       │ setvbuf()                            │               │         │
       └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS         top

       setbuf()
       setvbuf()
              C11, POSIX.1-2008.

HISTORY         top

       setbuf()
       setvbuf()
              C89, POSIX.1-2001.

CAVEATS         top

       POSIX notes that the value of errno is unspecified after a call to
       setbuf() and further notes that, since the value of errno is not
       required to be unchanged after a successful call to setbuf(),
       applications should instead use setvbuf() in order to detect
       errors.

BUGS         top

       You must make sure that the space that buf points to still exists
       by the time stream is closed, which also happens at program
       termination.  For example, the following is invalid:

       #include <stdio.h>

       int
       main(void)
       {
           char buf[BUFSIZ];

           setbuf(stdout, buf);
           printf("Hello, world!\n");
           return 0;
       }

SEE ALSO         top

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

COLOPHON         top

       This page is part of the man-pages (Linux kernel and C library
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Linux man-pages 6.10            2024-07-23                      setbuf(3)

Pages that refer to this page: fclose(3)fcloseall(3)fflush(3)fpurge(3)open_memstream(3)procio(3)stdin(3)stdio(3)