[linux-cifs-client] fcntl(): file unlocking failure

[Loris Boillet]

Hello,

I need to open a file and to lock it for both read and write access.

The file is in a network directory on a Windows server. To access it
from Linux (Debian), I use CIFS with mandatory locks activated:

mount -t cifs -o mand \\\\server\\share MountFolder

I test it with a C program using fcntl(). It behaves well if I mount the
share with SMBFS. But with CIFS, unlocking operation fails: I get Error
5: Input / Output Error.

Test program:

    fd = open("MountFolder/b.txt", O_RDWR | O_EXCL | O_APPEND);

    if (fd == -1) 
    {
         // erreur ...
    }

    fl.l_type = F_WRLCK;
    fl.l_whence = SEEK_SET;
    fl.l_start = 0;
    fl.l_len = 0;

    if (fcntl(fd, F_SETLK, &fl) == -1)
    {
	// Error...
    } 
    
    else 
    {
        printf("Lock granted.\n");

        // Unlock the locked bytes 
        fl.l_type = F_UNLCK;

        if (fcntl(fd, F_SETLK, &fl) == -1) 
        {
           printf("Unlocking operation failed.\nError: %i \n", errno);
           perror(s);
        }
        else
        {
            printf("File unlocked.\n");
        }
    }

Why does unlock fail?

Thanks