setbuf(3) — Linux manual page

setbuf(3)               Library Functions Manual               setbuf(3)

NAME

       setbuf, setbuffer, setlinebuf, setvbuf - stream buffering
       operations

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

       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

       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

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

STANDARDS

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

HISTORY

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

CAVEATS

       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

       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

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

COLOPHON

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       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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       man-pages@man7.org

Linux man-pages 6.9.1          2024-06-15                      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)

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