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From: bin@primate.wisc.edu (Brain in Neutral)
Newsgroups: comp.sys.mips
Subject: flock(2) doesn't return EWOULDBLOCK
Message-ID: <448@indri.primate.wisc.edu>
Date: 24 Jul 89 16:59:02 GMT
Organization: UW-Madison Primate Center
Lines: 66


Synopsis:  flock does not return EWOULDBLOCK as advertised.

Description:  flock is supposed to return EWOULDBLOCK when trying to acquire
an exclusive lock on an open file descriptor and the "don't block" flag is
specified.  It doesn't, it returns EACCES "Permission denied" instead.  This
can be seen by compiling the code below and running two instances of it
simultaneously.

# include	<sys/file.h>
# include	<errno.h>


main ()
{
extern int	errno;
int		f;

	f = open ("junk", O_WRONLY|O_CREAT, 0644);
	if (f < 0)
	{
		perror ("f open");
		exit (1);
	}
	printf ("f opened\n");
	if (flock (f, LOCK_EX|LOCK_NB) < 0)
	{
		if (errno == EWOULDBLOCK)
		{
			perror ("f flock1");
			exit (1);
		}
		perror ("f flock2");
		exit(1);
	}
	printf ("f locked\n");
	sleep (10);
}

The first instance prints:
	f opened
	f locked

The second instance prints:
	f opened
	f flock2: Permission denied

This is not only wrong, but EACCES is not one of the possible error returns
mentioned in the man page.

Package/Version:

OS: RISC/os 4.0
Configuration:  MIPS M/120, 16MB, 2 328MB disks, cartridge tape, etc.

Suggested workaround:

Do

	if (errno == EWOULDBLOCK || errno == EACCES)

instead of

	if (errno == EWOULDBLOCK)

Of course this is ugly and shouldn't have to be done.