setvbuf

Changes the buffering mode of the given file stream stream as indicated by the argument mode. In addition,

• If buffer is a null pointer, resizes the internal buffer to size.
• If buffer is not a null pointer, instructs the stream to use the user-provided buffer of size size beginning at buffer. The stream must be closed (with fclose ) before the lifetime of the array pointed to by buffer ends. The contents of the array after a successful call to setvbuf are indeterminate and any attempt to use it is undefined behavior.

[edit] Parameters

[edit] Return value

​0​ on success or nonzero on failure.

[edit] Notes

This function may only be used after stream has been associated with an open file, but before any other operation (other than a failed call to setbuf /setvbuf).

Not all size bytes will necessarily be used for buffering: the actual buffer size is usually rounded down to a multiple of 2, a multiple of page size, etc.

On many implementations, line buffering is only available for terminal input streams.

A common error is setting the buffer of stdin or stdout to an array whose lifetime ends before the program terminates:

int main(void) {
 char buf[BUFSIZ ];
 setbuf (stdin, buf);
} // lifetime of buf ends, undefined behavior

The default buffer size BUFSIZ is expected to be the most efficient buffer size for file I/O on the implementation, but POSIX fstat often provides a better estimate.

[edit] Example

// Make some POSIX functions, such as `int fileno(FILE*)`, visible:
#define _POSIX_SOURCE
 
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
 
int main(void)
{
 FILE * fp = fopen ("/tmp/test.txt", "w+");
 if (fp == NULL )
 {
 perror ("fopen");
 return EXIT_FAILURE ;
 }
 
 struct stat stats;
 if (fstat(fileno(fp), &stats) == -1) // POSIX only
 {
 perror ("fstat");
 return EXIT_FAILURE ;
 }
 
 printf ("BUFSIZ is %d, but optimal block size is %ld\n", BUFSIZ, stats.st_blksize);
 if (setvbuf(fp, NULL, _IOFBF, stats.st_blksize) != 0)
 {
 perror ("setvbuf failed"); // POSIX version sets errno
 return EXIT_FAILURE ;
 }
 
 int ch;
 while((ch=fgetc (fp)) != EOF ); // read entire file: use truss/strace to
 // observe the read(2) syscalls used
 
 fclose (fp);
 return EXIT_SUCCESS ;
}

BUFSIZ is 8192, but optimal block size is 65536

[edit] References

[edit] See also

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