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
 | 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)