One sure way to get a lock is to make a directory.

  #!/bin/sh

  if mkdir /your/lockdir
  then trap "rmdir /your/lockdir" EXIT INT ABRT TERM
       ...code goes here...
  else echo somebody else has the lock
  fi

No matter how many processes attempt to make the directory, only one will succeed. That works for my scripting, but I have never used it in C.