On Fri, Jul 13, 2018 at 03:09:08PM +0200, Marc-André Lureau wrote:
> According to Daniel Berrange, fcntl() locks have better portable
> semantics than lockf().
Specifically I was referring to this from 'man lockf':
On Linux, lockf() is just an interface on top of fcntl(2) locking.
Many other systems implement lockf() in this way, but note that POSIX.1
leaves the relationship between lockf() and fcntl(2) locks unspecified.
A portable application should probably avoid mixing calls to these
interfaces.
IOW, if its just a shim around fcntl() on many systems, it is clearer
if we just use fcntl() directly, as we then know how fcntl() locks will
behave if they're on a network filesystem like NFS.
> Use an exclusive lock on the first byte with fcntl().
>
> Signed-off-by: Marc-André Lureau <marcandre.lur...@redhat.com>
> ---
> util/oslib-posix.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> index da1d4a3201..26b11490b9 100644
> --- a/util/oslib-posix.c
> +++ b/util/oslib-posix.c
> @@ -92,6 +92,11 @@ bool qemu_write_pidfile(const char *pidfile, Error **errp)
> {
> int pidfd;
> char pidstr[32];
> + struct flock lock = {
> + .l_type = F_WRLCK,
> + .l_whence = SEEK_SET,
> + .l_len = 1,
> + };
For the same semantics as lockf we should use len == 0 (ie infinity)
Regards,
Daniel
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