Thank you very much. I'll try to explain something more about my application. The application is interfaced to a database containing a lot of exercises and solutions. An exercise consists in a description of the problem (a .ps file), an input file (exercise_code.in) and a output file (exercise_code.out). A solution is called correct iff, provided (exercise_code.in) as input, output produced by the program equals verbatim (exercise_code.out). If a solution is correct, it will be inserted in the database. So a student can see other solutions and learn. Or practice, in order to participate to ACM International Collegiate Programming Contest. As you can see, the entire process is automatizable. So far so good.

Here's a pseudo code description of my submit script.

get solution's source code from web form
get a valid temp file and store solution in it

# Here's my rudimental flocking system
while ( -e "exercise_code.in" ) {}

get from the database exercise_code.in and exercise_code.out
compile solution's source code (using gcc)
run the solution (it will produce executable_filename.out)
File::Compare executable_filename.out and exercise_code.out
if they are the same
        insert solution in the database
else
        wrong answer!
[download]

I feel I could do something better. Now, I am going to examine your Simple Locking code
See you