setbuf man page


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.

setbuf — assign buffering to a stream


#include <stdio.h>

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


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.1‐2008 defers to the ISO C standard.

Except that it returns no value, the function call:

setbuf(stream, buf)

shall be equivalent to:

setvbuf(stream, buf, _IOFBF, BUFSIZ)

if buf is not a null pointer, or to:

setvbuf(stream, buf, _IONBF, BUFSIZ)

if buf is a null pointer.

Return Value

The setbuf() function shall not return a value.


Although the setvbuf() interface may set errno in defined ways, the value of errno after a call to setbuf() is unspecified.

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

Since errno is not required to be unchanged on success, in order to correctly detect and possibly recover from errors, applications should use setvbuf() instead of setbuf().



Future Directions


See Also

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

The Base Definitions volume of POSIX.1‐2008, <stdio.h>


2013 IEEE/The Open Group POSIX Programmer's Manual