The issue is calling xmas() in main before it's defined. Compiling with GCC on macOS gives the error: xmas.c:16:5: error: call to undeclared function 'xmas';

The issue is calling xmas() in main before it's defined. Compiling with GCC on macOS gives the error: xmas.c:16:5: error: call to undeclared function 'xmas'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration] xmas(2, 2, ""); Moving main() to the bottom compiles and executes with the proper output.