What causes signal 'SIGILL'?

Question

I'm porting some C++ code to Android using NDK and GCC. The code basically runs. At one point, when debugging in Eclipse, the call

Dabbler::Android::Factory* pFactory = new Dabbler::Android::Factory;

causes this error:

Thread [1] (Suspended: Signal 'SIGILL' received. Description: Illegal instruction.) 
    1 <symbol is not available> 0x812feb44

What does that mean? Has the compiler generated illegal code for some reason? I have a breakpoint in the constructor (which does nothing), and it's not hit. I have already done a full rebuild.

What could I be doing wrong to cause this problem?

Outmaneuver

Answer 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.

Anele

You have no idea to what lengths I went searching for a reason behind SIGILL (QT + Android) just to find a missing return statement in a function (inspired by your reply). Thank you. – Crimson

@Crimson What compiler you were using? Thanks! – Andradite

Answer 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.

Kaden

Another cause of execing an invalid instruction is jumping to an address that isn't in the program area. – Estuary

Weird, I got this, when I compiled with clang, but it worked fine with gcc – Ashtonashtonunderlyne

Answer 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.

Emancipated

For me, I wasn't returning any value from a function that was supposed to return an int. So somewhat related to this answer. No idea how it managed to compile. – Poetics

Answer 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

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