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HP-UX 11i Version 3: February 2007

Technical documentation

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setbuf(), setvbuf(), setlinebuf() — assign buffering to a stream file


#include <stdio.h>

void setbuf(FILE *__restrict stream, char *__restrict buf);

int setvbuf(FILE *__restrict stream, char *__restrict buf, int type, size_t size);

void setlinebuf(FILE *stream);

Obsolescent Interface

int setvbuf_unlocked(FILE *stream, char *buf, int type, size_t size);


setbuf() can be used after a stream has been opened but before it is read or written. It causes the array pointed to by buf to be used instead of an automatically allocated buffer. If buf is the NULL pointer input/output will be completely unbuffered.

A constant BUFSIZ, defined in the <stdio.h> header file, tells how big an array is needed:

char buf[BUFSIZ];

setvbuf() can be used after a stream has been opened but before it is read or written. type determines how stream is to be buffered. Legal values for type (defined in <stdio.h>) are:


causes input/output to be fully buffered.


causes output to be line buffered; the buffer will be flushed when a newline is written, the buffer is full, or input is requested.


causes input/output to be completely unbuffered.

When an output stream is unbuffered, information is queued for writing on the destination file or terminal as soon as written; when it is buffered, many characters are saved up and written as a block. When the output stream is line-buffered, each line of output is queued for writing on the destination terminal as soon as the line is completed (that is, as soon as a new-line character is written or terminal input is requested). fflush() can also be used to explicitly write the buffer.

If buf is not the NULL pointer, the array it points to is used for buffering instead of an automatically allocated buffer (from malloc()). size specifies the size of the buffer to be used. The constant BUFSIZ in <stdio.h> is suggested as a good buffer size. If input/output is unbuffered, buf and size are ignored.

By default, output to a terminal is line buffered and all other input/output is fully buffered.

setlinebuf() is used to change stream from block-buffered or unbuffered to line-buffered. setlinebuf() can be used any time the file descriptor is active.

Obsolescent Interface

setvbuf_unlocked() assigns buffering to a stream file.


If an illegal value for type or size is provided, setvbuf() and setvbuf_unlocked() return a non-zero value. Otherwise, the value returned will be zero.


A common source of error is allocating buffer space as an "automatic" variable in a code block, then failing to close the stream in the same block.

Allocating a buffer of size or BUFSIZ bytes does not necessarily imply that all of size or BUFSIZ bytes are used for the buffer area.


setbuf, setvbuf, and setlinebuf() were developed by HP.


setbuf(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1, ANSI C

setvbuf(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, ANSI C

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