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(), setlinebuf(), setvbuf()                              │ Thread safety │ MT-Safe │
       └─────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

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)

Linux man-pages 6.7                                2024-02-26                                          setbuf(3)