Changing the address value of a function pointer

Question

I have the following code in C:

int addInt(int n, int m) {
    return n + m;
}

int (*functionPtr)(int, int);

functionPtr = &addInt;

functionPtr is a function pointer and it points to the specific address of the function addInt. I want to change 1 bit of its value, but I can't figure it out how.

Let's say functionPtr points to 0xABC0 (assuming a 16-bit address) after the last statement. I want to change its value to 0xABC1. I tried to OR the value with 0x1, but I guess that something is wrong with the operand conversion:

functionPtr = &addInt | 0x00000001; // addresses are of 32 bits

I know that messing around with pointers is risky, but I have to change the LSB of the address in order to enter into the Thumb state of an ARM Cortex-M4 MCU.

Answer 1

To modify the value of a pointer via arithmetic operations, you would need to convert it to an integer type, perform the operation, and then convert it back. C does not define behavior for converting a function pointer to anything other than another function pointer, however, so there is no defined way to do that.

You might nevertheless write this:

typedef int (*fptr)(int, int);

functionPtr = (fptr)(((intptr_t) functionPtr) | 1);

The behavior you are looking for is one of the more plausible results, but again, the behavior of both casts is undefined. Therefore the behavior to be expected from performing a function call via the modified pointer -- if your program can even get that far -- is also undefined. The compiler is not required even to accept the code.