Expression has undefined behavior (left operand modifies code->globals->used, used by right operand): (code->globals)[(((vector_t*)((void *)code->globals)) - 1)->used++] = (code_genstring(code, global->constval.vstring)) Code has unspecified behavior. Order of evaluation of function parameters or subexpressions is not defined, so if a value is used and modified in different places not separated by a sequence point constraining evaluation order, then the result of the expression is unspecified.