49
What is the difference between lockf and fcntl:
On many systems, the lockf() library routine is just a wrapper around fcntl(). That is to say lockf offers a subset of the functionality that fcntl does.
But on some systems, fcntl and lockf locks are completely independent.
Since it is implementation dependent, make sure to always use the same convention. So either always use lockf from both your processes or always use fcntl. There is a good chance that they will be interchangeable, but it's safer to use the same one.
Which one you chose doesn't matter.
Some notes on mandatory vs advisory locks:
Locking in unix/linux is by default advisory, meaning other processes don't need to follow the locking rules that are set. So it doesn't matter which way you lock, as long as your co-operating processes also use the same convention.
Linux does support mandatory locking, but only if your file system is mounted with the option on and the file special attributes set. You can use mount -o mand to mount the file system and set the file attributes g-x,g+s to enable mandatory locks, then use fcntl or lockf. For more information on how mandatory locks work see here.
Note that locks are applied not to the individual file, but to the inode. This means that 2 filenames that point to the same file data will share the same lock status.
In Windows on the other hand, you can actively exclusively open a file, and th