Atomic file truncation

[William Kucharski]

This was posted before in a discussion about providing a work-alike of BSD 
ftruncate(2) in SYSV, but here goes again:

The routine depends on the undocumented fcntl F_FREESP, which is present in
SVR3.1 and above.  The fcntl frees file space starting at the location
indicated by fl.l_start.

#include <sys/types.h>
#include <fcntl.h>

int
truncate(fd, length)
int	fd;
off_t	length;
{
	struct flock	fl;

	/* truncate file to length <length> */

	fl.l_whence = 0;
	fl.l_len = 0;
	fl.l_start = length;	/* make offset "start" the new EOF */
	fl.l_type = F_WRLCK;	/* why not write lock it? */

	if (fcntl(fd, F_FREESP, &fl) < 0)
		return(-1);

	return(0);
}


-- 
					William Kucharski

ARPA: kucharsk at uts.amdahl.com
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Disclaimer:  The opinions expressed above are my own, and may not agree with
	     those of any other sentient being, not to mention those of my 
	     employer.  So there.