SETBUF(3) FreeBSD Library Functions Manual SETBUF(3)


setbuf, setbuffer, setlinebuf, setvbufstream buffering operations


Standard C Library (libc, -lc)


#include < stdio.h>

setbuf( FILE * restrict stream, char * restrict buf);

setbuffer( FILE *stream, char *buf, int size);

setlinebuf( FILE *stream);

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


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. 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 always unbuffered. Note that these defaults may be altered using the stdbuf(1) utility.

The setvbuf() function may be used to alter the buffering behavior of a stream. The mode argument must be one of the following three macros:

line buffered
fully buffered

The size argument 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 buf is not NULL, it is the caller's responsibility to free(3) this buffer after closing the stream. (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 extension 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);


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.


The setbuf() and setvbuf() functions conform to ISO/IEC 9899:1990 (“ISO C90”).


setbuf() usually uses a suboptimal buffer size and should be avoided.
February 18, 2013 FreeBSD