The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


setvbuf - assign buffering to a stream


#include <stdio.h>

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


The setvbuf() function may be used after the stream pointed to by stream is associated with an open file but before any other operation is performed on the stream. The argument type determines how stream will be buffered, as follows: _IOFBF causes input/output to be fully buffered; _IOLBF causes input/output to be line buffered; _IONBF causes input/output to be unbuffered. If buf is not a null pointer, the array it points to may be used instead of a buffer allocated by setvbuf(). The argument size specifies the size of the array. The contents of the array at any time are indeterminate.

For information about streams, see Standard I/O Streans.


Upon successful completion, setvbuf() returns 0. Otherwise, it returns a non-zero value if an invalid value is given for type or if the request cannot be honoured.


The setvbuf() function may fail if:
The file descriptor underlying stream is not valid.




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

With setvbuf(), allocating a buffer of size bytes does not necessarily imply that all of size bytes are used for the buffer area.

Applications should note that many implementations only provide line buffering on input from terminal devices.




fopen(), setbuf(), <stdio.h>.


Derived from Issue 1 of the SVID.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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