OP here. This is not actually compliant with the C standard, just a gcc extension. It is documented at [1] and according to the documentation, the code seems p

OP here. This is not actually compliant with the C standard, just a gcc extension. It is documented at [1] and according to the documentation, the code seems perfectly legal. However, calling it twice add5(2); add5(2); leads to an "Illegal instruction. Core dumped" Even stranger, calling it twice but with a different argument leads to a a segfault instead of illegal instruction: add5(5); add5(5); [1] https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html