UNIX Signal 4 (at least in x86 Linux), Illegal instruction. This signal is rarely seen, and can usually only be seen when you either use a broken compiler or write some assembly and make a coding mistake.
This is the computer's idea of telling you that it can't run the binary program because it has encountered a syntax error.
A famous example is the
f0 0f code - in Linux (with either a working x86 processor or a bug-correcting version of the kernel) this generates SIGILL, but in buggy environment it halts the processor.
#define SIGILL 6 /* blech */
- Larry Wall in perl.c from the perl source code