CODEWITHSUNDEEP
cpointersfunction-pointers
j b
j b

Reputation: 5296

Are C functions guaranteed to have a fixed memory address?

If I store a pointer to a function, and then at some later point during my program's execution, compare it to the address of the same function, are the two addresses guaranteed to be equal.

E.g.

int foo(void){return 0;}
int (*foo_p)(void) = &foo;

assert(foo_p == &foo);

In the above code is the assertion always guaranteed to succeed? Are there any circumstances under which the address of a function can change?

Upvotes: 12

Views: 531

Answers (2)

Emil Romanus
Emil Romanus

Reputation: 794

A function's address will never change. Many programs are built around the concept of callbacks, which would not work if a function's address could change.

If, hypothetically, a function's location changed, for example by a self-modifying program, then all calls to that function would cause a segfault or very undefined behaviour any way. Edit: Clarification - function symbols are like pointers, if you free memory pointed to by a pointer, that will not zero the actual pointer variable, it will still point there, just as your function calls will still point to the old location of the moved function.

Self-modifying programs are very big exceptions though, and in these days the code section of a binary is write protected rendering this very, very hard.

Upvotes: 0

R.. GitHub STOP HELPING ICE
R.. GitHub STOP HELPING ICE

Reputation: 215261

Per 6.5.9:

Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.

(Boldface added for emphasis.)

Upvotes: 17

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