Setvbuf

setvbuf is a function in standard C which lets the programmer control the buffering of a file stream. It is declared in <stdio.h>; its function prototype is:

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

"stream" is a pointer to the file stream for which the relevant buffering operations will be performed; "buf" is a char array of "size" in length, or a null pointer; and "mode" is the kind of buffering desired: _IOFBF, for fully buffered, _IOLBF for line buffered and _IONBF for unbuffered. These three macros are defined in . setvbuf returns zero on success or nonzero on failure.

A related function, setbuf also controls the buffering of a file stream. Unlike setvbuf, setbuf takes only two arguments. The prototype is:

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

setbuf's behavior is equivalent to:

(void)setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);

That is, if "buf" is not NULL, set the stream to fully buffered using the given buffer; otherwise, set the stream to unbuffered. If a buffer is provided to setbuf, it must be at least BUFSIZ bytes long. The function always succeeds.

Example

The output of this program should be "Hello world" followed by a newline.
#include
#include

int main(void) { char buf [42] ;

if(setvbuf(stdout, buf, _IOFBF, sizeof buf)) { perror("failed to change the buffer of stdout"); return EXIT_FAILURE; } printf("He"); /* The buffer contains "He"; nothing is written yet to stdout */ fflush(stdout); /* "He" is actually written to stdout */

if(setvbuf(stdout, NULL, _IONBF, 0)) { perror("failed to change the buffer of stdout"); return EXIT_FAILURE; } printf("llo w"); /* "llo w" is written to stdout, there is no buffering */

if(setvbuf(stdout, buf, _IOLBF, sizeof buf)) { perror("failed to change the buffer of stdout"); return EXIT_FAILURE; } printf("orld"); /* The buffer now contains "orld"; nothing is written yet to stdout */ putchar(' '); /* stdout is line buffered; everything in the buffer is now written to stdout along with the newline */

return EXIT_SUCCESS;}

See also

* fflush
* Data buffer