fcntl() bug? if called second times!

fcntl() bug? if called second times!

[Heiko Elger]

Hello,

I try to comiple an run the distcc package - but I get errors while running.
I figured out, tha the problems are the fcntl() calls - perhaps
ther is a bug in cygwin?
The following code runs differently in cygwin and linux.
If I want ro lock the whole file twice, but still the same process, I will
get an "permission denied" error on cygwin.
The same code works on linux!

===> Cygwin console
    heiko@HEIKO ~/src/fslock
    $ ./fslocktest
    Lock 1
    Lock 2
    lock failed: fslocktest.txt: Permission denied

===> linux console
    heiko@linux:~/src/fslock> ./fslocktest
    Lock 1
    Lock 2

Below is the code to reproduce the error.

I'm using the following software:
- Windows XP Professional Ver 5.1 Build 2600 Service Pack 1
  CYGWIN=ntsec
- Cygwin DLL version info:
     DLL version: 1.3.18
     DLL epoch: 19
     DLL bad signal mask: 19005
     DLL old termios: 5
     DLL malloc env: 28
     API major: 0
     API minor: 69
     Shared data: 3
     DLL identifier: cygwin1
     Mount registry: 2
     Cygnus registry name: Cygnus Solutions
     Cygwin registry name: Cygwin
     Program options name: Program Options
     Cygwin mount registry name: mounts v2
     Cygdrive flags: cygdrive flags
     Cygdrive prefix: cygdrive prefix
     Cygdrive default prefix:
     Build date: Wed Dec 25 15:37:50 EST 2002
     Shared id: cygwin1S3
- Linux: Suse 8.1

Perhaps someone can give me a hint ...

Best regards
  Heiko Elger


------------ snip - snip - snip --------------------
/*
   make CFLAGS="-Wall -g -O0" testfcntl.c
*/
int main(int argc, char** argv, char** envp)
{
    int fd ;
    char* fname = "fslocktest.txt";
    int rc;
    struct flock lockparam;

    lockparam.l_type = F_WRLCK;
    lockparam.l_whence = SEEK_SET;
    lockparam.l_start = 0;
    lockparam.l_len = 0;        /* whole file */
    
    fd = open(fname, O_WRONLY|O_CREAT, 0600);
    if (fd == -1 && errno != EEXIST) 
    {
        printf("failed to creat %s: %s", fname, strerror(errno));
        return errno;
    }

    printf("Lock 1\n");
    rc  = fcntl(fd, F_SETLK, &lockparam);
    if (rc==-1)
    {
        printf("lock failed: %s: %s", fname, strerror(errno));
        return errno;
    }
    printf("Lock 2\n");
    rc  = fcntl(fd, F_SETLK, &lockparam);
    if (rc==-1)
    {
        printf("lock failed: %s: %s", fname, strerror(errno));
        return errno;
    }

    close(fd);
    return 0;
}
------------ snip - snip - snip --------------------




Heiko Elger

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Re: fcntl() bug? if called second times!

[Pavel Tsekov]

Hello,

Cygwin doesn't support advisory file locking. It implements file locking 
via the Win32 api LockFile*. Find out more here:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/fileio/base/lockfile.asp

And more specifically this part:
[...]
Locking a portion of a file for shared access denies all processes write 
access to the specified region of the file, including the process that 
first locks the region. All processes can read the locked region.
[...]

In fact I think the Win32 error code returned in this case is 
ERROR_LOCK_VIOLATION and Cygwin translates it to EACCESS.

Some time before I started porting sendmail to Cygwin and met this 
problem. I decided to try to implement something like advisory file 
locking for Cygwin back then but lack of time prevented me from doing so 
:(

On Mon, 13 Jan 2003, Heiko Elger wrote:

> Hello,
> 
> I try to comiple an run the distcc package - but I get errors while running.
> I figured out, tha the problems are the fcntl() calls - perhaps
> ther is a bug in cygwin?
> The following code runs differently in cygwin and linux.
> If I want ro lock the whole file twice, but still the same process, I will
> get an "permission denied" error on cygwin.
> The same code works on linux!
> 
> ===> Cygwin console
>     heiko@HEIKO ~/src/fslock
>     $ ./fslocktest
>     Lock 1
>     Lock 2
>     lock failed: fslocktest.txt: Permission denied
> 
> ===> linux console
>     heiko@linux:~/src/fslock> ./fslocktest
>     Lock 1
>     Lock 2



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