SETBUF(3) Linux Programmer's Manual SETBUF(3)
setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations
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)):
setbuffer(), setlinebuf(): _BSD_SOURCE
The three types of buffering available are unbuffered, block buffered, and line buffered.
When an output stream is unbuffered, information appears on the destination file or termi-
nal as soon as written; when it is block buffered many characters are saved up and written
as a block; when it is line buffered characters are saved up until a newline is output or
input is read from any stream attached to a terminal device (typically stdin). The func-
tion fflush(3) may be used to force the block out early. (See fclose(3).) Normally all
files are block buffered. When the first I/O operation occurs on a file, malloc(3) is
called, and a buffer is obtained. If a stream refers to a terminal (as stdout normally
does) it is line buffered. The standard error stream stderr is always unbuffered by
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:
_IOLBF line buffered
_IOFBF fully buffered
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:
setvbuf(stream, NULL, _IOLBF, 0);
The function setvbuf() returns 0 on success. It returns nonzero on failure (mode is
invalid or the request cannot be honored). It may set errno on failure.
The other functions do not return a value.
The setbuf() and setvbuf() functions conform to C89 and C99.
The setbuffer() and setlinebuf() functions are not portable to versions of BSD before
4.2BSD, and are available under Linux since libc 4.5.21. On 4.2BSD and 4.3BSD systems,
setbuf() always uses a suboptimal buffer size and should be avoided.
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:
fclose(3), fflush(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)
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Linux 2012-08-03 SETBUF(3)