What causes signal 'SIGILL'?

Solution 1:

Make sure that all functions with non-void return type have a return statement.

While some compilers automatically provide a default return value, others will send a SIGILL or SIGTRAP at runtime when trying to leave a function without a return value.

Solution 2:

It means the CPU attempted to execute an instruction it didn't understand. This could be caused by corruption I guess, or maybe it's been compiled for the wrong architecture (in which case I would have thought the O/S would refuse to run the executable). Not entirely sure what the root issue is.

Solution 3:

It could be some un-initialized function pointer, in particular if you have corrupted memory (then the bogus vtable of C++ bad pointers to invalid objects might give that).

BTW gdb watchpoints & tracepoints, and also valgrind might be useful (if available) to debug such issues. Or some address sanitizer.

Solution 4:

LeetCode's online compiler and dev environment generates SIGILL errors for mistakes that do not generate the same error in my desktop IDE.

For example, array access with an out-of-bounds index:

["foo", "bar"][2]

LeetCode's compiler shows only the error:

Runtime Error process exited with signal SIGILL

in a local Xcode playground this same code instead results in the error:

error: Execution was interrupted, reason: EXC_BREAKPOINT (code=1, subcode=0x18f2ea5d8).
The process has been left at the point where it was interrupted, use "thread return -x" to return to the state before expression evaluation.

Only in a full Xcode project compilation and run does it report the actual error:

Thread 1: Fatal error: Index out of range