setvbuf - Man Page

assign buffering to a stream


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <stdio.h>

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


The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

The setvbuf() function may be used after the stream pointed to by stream is associated with an open file but before any other operation (other than an unsuccessful call to setvbuf()) is performed on the stream. The argument type determines how stream shall be buffered, as follows:


{_IOFBF} shall cause input/output to be fully buffered.


{_IOLBF} shall cause input/output to be line buffered.


{_IONBF} shall cause 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() and the argument size specifies the size of the array; otherwise, size may determine the size of a buffer allocated by the setvbuf() function. The contents of the array at any time are unspecified.

For information about streams, see Section 2.5, Standard I/O Streams.

Return Value

Upon successful completion, setvbuf() shall return 0. Otherwise, it shall return a non-zero value if an invalid value is given for type or if the request cannot be honored, and may set errno to indicate the error.


The setvbuf() function may fail if:


The file descriptor underlying stream is not valid.

The following sections are informative.



Application Usage

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.



Future Directions


See Also

Section 2.5, Standard I/O Streams, fopen(), setbuf()

The Base Definitions volume of POSIX.1-2017, <stdio.h>

Referenced By

cat(1p), setbuf(3p), stdin(3p), stdio.h(0p).

2017 IEEE/The Open Group POSIX Programmer's Manual