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SETVBUF(3) Library Functions Manual SETVBUF(3)

setvbufstream buffering operations

#include <stdio.h>


The three types of stream 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 line buffered, characters are saved up until a newline (‘\n’) is output or input is read from any stream attached to a terminal device (typically ).


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 normally does), it is line buffered.

The standard error stream is initially unbuffered.

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

line buffered
fully buffered

The size parameter 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 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 () 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.

Upon successful completion, a value of 0 is returned. If mode is invalid or if the request cannot be honored, a non-zero value is returned, possibly setting errno to indicate the error. The stream is not modified in the error case.

The setvbuf() function will fail if:

The stream specified is not associated with a valid file descriptor.

fclose(3), fopen(3), fread(3), malloc(3), printf(3), puts(3), setbuf(3)

The setvbuf() function conforms to ISO/IEC 9899:1999 (“ISO C99”).

The setvbuf() function first appeared in 4.4BSD.

November 26, 2014 OpenBSD-6.6