Perl Cookbook

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.

The struct_flock function in the lockarea program packs and unpacks in the proper format for the current architectures by consulting the $^O variable, which contains your current operating system name. There is no struct_flock function declaration. It’s just aliased to the architecture-specific version. Function aliasing is discussed in Section 10.14.

The lockarea program opens a temporary file, clobbering any existing contents and writing a screenful (80 by 23) of blanks. Each line is the same length.

The program then forks one or more times and lets all the child processes try to update the file at the same time. The first argument, N