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OS calls back to application
Indicates something went wrong or is ready
Common signals (see man 7 signal):
- SIGSEGV/SIGBUS - access memory it shouldn't
- SIGILL - running corrupt code/non-code
- SIGFPE - math error
- SIGABRT - Something detected an error
Often will 'dump core'
All Unix(tm) like systems use signals. A program can cause a piece of code to be run when a signal is received (this would be a signal handler).
Sometimes programs do evil things with signals that would appear to be bad - e.g. Using sigsegv and attaching memory allocation schemes to it.