c: How not to code #1

(gdb) bt
#0  0x00007f8ac115bebe in __lll_lock_wait_private () from /lib64/libc.so.6
#1  0x00007f8ac10f22be in _L_lock_9876 () from /lib64/libc.so.6
#2  0x00007f8ac10f05c1 in free () from /lib64/libc.so.6
#3  0x0000000000402649 in handle_sig (signo=<optimized out>, info=<optimized out>, context=<optimized out>) at lol.c:158
#4  <signal handler called>
#5  0x00007f8ac10edf03 in _int_malloc () from /lib64/libc.so.6
#6  0x00007f8ac10f06b7 in malloc () from /lib64/libc.so.6
#7  0x00000000004015ba in do_the_boogie (fd=3, gp=<optimized out>) at lol.c:715
#8  0x00000000004023cc in main (argc=5, argv=<optimized out>) at lol.c:810

LOL, it is deadlocked. The signal handler (frame #4) was invoked inside a malloc (frame #5), and the signal handler calls free in frame #3. Of course, the heap lock is held in frame #5 and both are in the same memory arena (see: break space), hence we are screwed.