Since your code could have been anywhere in the execution path you can only do very limited amount of things within the signal handler that won't cause catastrophic issues otherwise.
I'm pretty familiar with signals, but I don't really know what the problem is here. Why would you rely on signals to interrupt system calls? Maybe if I know why you would do that, I will see why it's a problem if the signal comes before the syscall happens.
So we have to rely upon SIGALRM (via setitimer()) to interrupt the fcntl() call so we don't hang indefinitely.
For example, lets say you want Ctrl + C to quit the program (bare with me, very simple example), and you have a select() call. So you set up your signal handlers, and then start filling the required structures for the select() call, before select() is called though you receive a SIGINT, your signal handler simply sets a global flag that is checked when select() returns (with errno == EINTR). So now the flag is set, the handler has done its job, and select() gets called and your program now starts waiting on file descriptors.
What you really wanted to happen is that the program would see the flag, clean up nicely and quit. Now the user has to send a second Ctrl + C to interrupt the select() syscall and to have the code executed that checks for errno == EINTR and the global flag that was set in the signal handler.
This is but a simple example of a race condition that can exist. The SIGALRM example given by the other HN user is also an excellent example of when things can go awry when not intentioned, and unless you program your signal handlers with that in mind you may get results you weren't expecting.
That example doesn't live up to the hype that was mentioned by a previous poster and that had me worried. However, that hype was probably overstated.
For example, "there are certain race conditions if you rely on signals to interrupt system calls and the interrupt happens before the syscall is called" -> seems to imply there are race conditions inherent to the situation, not race conditions that can be introduced by programmer mistake (which is pretty obvious, IMO).
Also "since your code could have been anywhere in the execution path you can only do very limited amount of things within the signal handler that won't cause catastrophic issues otherwise" -> I just don't believe that; you can do lots of things in the signal handler if you know what you're doing.