vex x86->IR: unhandled instruction bytes: 0x........ 0x........ 0x........ 0x........ Your program just tried to execute an instruction that Valgrind did not recognise. This might be because your program has a bug and erroneously jumped to a non-code location. If you are running Memcheck, you might have just seen a warning about a bad jump, which is a good indication that this is so. Or it might be because the instruction is unimplemented in Valgrind; if you think this is the case, or you are not sure, please let us know. Process terminating with default action of signal 4 (SIGILL) Illegal opcode at address 0x........ at 0x........: main (int.c:5)