Pointers as function arguments in C

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A reasonable rule of thumb is that you can’t exactly change the exact thing that is passed is such a way that the caller sees the change. Passing pointers is the workaround.

Pass By Value: void fcn(int foo)

When passing by value, you get a copy of the value. If you change the value in your function, the caller still sees the original value regardless of your changes.

Pass By Pointer to Value: void fcn(int* foo)

Passing by pointer gives you a copy of the pointer – it points to the same memory location as the original. This memory location is where the original is stored. This lets you change the pointed-to value. However, you can’t change the actual pointer to the data since you only received a copy of the pointer.

Pass Pointer to Pointer to Value: void fcn(int** foo)

You get around the above by passing a pointer to a pointer to a value. As above, you can change the value so that the caller will see the change because it’s the same memory location as the caller code is using. For the same reason, you can change the pointer to the value. This lets you do such things as allocate memory within the function and return it; &arg2 = calloc(len);. You still can’t change the pointer to the pointer, since that’s the thing you recieve a copy of.

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