Recipe 7.25. Program: lockarea

Perl's flock function only locks complete files, not regions of the file. Although fcntl supports locking of a file's regions, this is difficult to access from Perl, largely because no one has written an XS module that portably packs up the necessary structure.

The program in Example 7-11 implements fcntl, but only for the three architectures it already knows about: SunOS, BSD, and Linux. If you're running something else, you'll have to figure out the layout of the flock structure. We did this by eyeballing the C-language sys/fcntl.h #include file—and running the c2ph program to figure out alignment and typing. This program, while included with Perl, only works on systems with a strong Berkeley heritage, like those listed above. As with Unix—or Perl itself—you don't have to use c2ph, but it sure makes life easier if you can.