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Date: Wed, 12 Oct 2005 23:15:31 +0200
From: Alex Riesen <raa.lkml@gmail.com>
To: "linux-os (Dick Johnson)" <linux-os@analogic.com>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>, boi@boi.at,
       Linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: blocking file lock functions (lockf,flock,fcntl) do not return after timer signal
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linux-os (Dick Johnson), Wed, Oct 12, 2005 17:20:26 +0200:
> >>>> flock, lockf, fcntl do not return even after the signal SIGALRM  has
> >>>> been raised and the signal handler function has been executed
> >>>> the functions should return with a return value EWOULDBLOCK as described
> >>>> in the man pages
> >>
> >> Works for me on a local filesystem.
> >>
> >> Desktop$ ./gnurr gnarg
> >> locking...
> >> timeout
> >
> > Doesn't look so. I'd expect "flock: EWOULDBLOCK" and "sleeping" after
> > the first timeout.

It's EINTR, btw.

linux-os (Dick Johnson), Wed, Oct 12, 2005 17:20:26 +0200:
> As I told you, you use sigaction(). Also flock() will not block
> unless there is another open on the file. The code will run to
> your blocking read(), wait 10 seconds, get your "timeout" from
> the signal handler, then read() will return with -1 and ERESTARTSYS
> in errno as required.

Ahh yes, of course. signal(2) places a syscall-restarting handler in glibc.
My bad, sorry.

For the last time:

// everything works as expected, flock returns with EINTR in the
// second instance of the program.
#include <unistd.h>
#include <sys/time.h>
#include <sys/file.h>
#include <stdio.h>
#include <signal.h>
#include <errno.h>

void alrm(int sig)
{
     write(2, "timeout\n", 8);
}

int main(int argc, char* argv[])
{
     struct itimerval tv = {
         .it_interval = {.tv_sec = 10, .tv_usec = 0},
         .it_value    = {.tv_sec = 10, .tv_usec = 0},
     };
     struct sigaction sa = { .sa_handler = alrm, .sa_flags = 0 };
     sigaction(SIGALRM, &sa, NULL);
     setitimer(ITIMER_REAL, &tv, NULL);
     int fd = open(argv[1], O_RDWR);
     if ( fd < 0 ) {
         perror(argv[1]);
         return 1;
     }
     printf("locking...\n");
     if ( flock(fd, LOCK_EX) < 0 ) {
         perror("flock");
         return 1;
     }
     printf("sleeping...\n");
     int ch;
     while ( read(0, &ch, 1) < 0 && EINTR == errno )
	 ;
     close(fd);
     return 0;
}