#include <stdio.h> void setbuf(FILE *stream, char *buf); void setbuffer(FILE *stream, char *buf, size_t size); void setlinebuf(FILE *stream); int setvbuf(FILE *stream, char *buf, int mode, size_t size);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Normally all files are block buffered. If a stream refers to a terminal (as stdout normally does), it is line buffered. The standard error stream stderr is always unbuffered by default.
The setvbuf() function may be used on any open stream to change its buffer. The mode argument must be one of the following three macros:
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 argument buf is NULL, only the mode is affected; a new buffer will be allocated on the next read or write operation. The setvbuf() function may be used only after opening a stream and before any other operations have been performed on it.
The other three calls are, in effect, simply aliases for calls to setvbuf(). The setbuf() function is exactly equivalent to the call
setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);
The setbuffer() function is the same, except that the size of the buffer is up to the caller, rather than being determined by the default BUFSIZ. The setlinebuf() function is exactly equivalent to the call:
You must make sure that the space that buf points to still exists by the time stream is closed, which also happens at program termination. For example, the following is invalid: