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Setvbuf: Difference between revisions

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#REDIRECT ]
{{lowercase|setvbuf}}
In computing, <code>'''setvbuf'''</code> is a ] in ] which lets the programmer control the buffering of a file stream. It is declared in <]>. <code>setvbuf</code> and its function prototype is:


{{Redirect category shell|
<source lang="c">int setvbuf(FILE *stream, char *buf, int mode, size_t size);</source>
{{R with history}}

{{R to anchor}}
Where ''stream'' is a pointer to the file stream for which the relevant buffering operations will be performed; ''buf'' is a char array of ''size'' length or a ]; and ''mode'' is the kind of buffering desired: <code>_IOFBF</code>, for fully buffered, <code>_IOLBF</code> for line buffered and <code>_IONBF</code> for unbuffered. These three macros are defined in <stdio.h>. <code>setvbuf</code> returns zero on success or nonzero on failure.
⚫ }}

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

<source lang="c">void setbuf(FILE *stream, char *buf);</source>

<code>setbuf</code>'s behavior is equivalent to:

<source lang="c">(void)setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);</source>

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

== Example ==

The output of this program should be ''Hello world'' followed by a newline.
<source lang="c">
#include <stdio.h>
#include <stdlib.h>

int main(void) {
char buf;

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('\n'); /* stdout is line buffered; everything in the buffer is now written to stdout along with the newline */

return EXIT_SUCCESS;
⚫ }
</source>

== See also ==
* <code>]</code>
* ]

]

Latest revision as of 23:40, 27 April 2023

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