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

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