Pass a pointer to a function as read-only in C

Question

Just as the title says, can I pass a pointer to a function so it's only a copy of the pointer's contents? I have to be sure the function doesn't edit the contents.

Thank you very much.

Answer 1

You can use const

void foo(const char * pc)

here pc is pointer to const char and by using pc you can't edit the contents.

But it doesn't guarantee that you can't change the contents, because by creating another pointer to same content you can modify the contents.

So,It's up to you , how are you going to implement it.

Answer 2

Yes,

void function(int* const ptr){
    int i;
    //  ptr = &i  wrong expression, will generate error ptr is constant;
    i = *ptr;  // will not error as ptr is read only  
    //*ptr=10;  is correct 

}

int main(){ 
    int i=0; 
    int *ptr =&i;
    function(ptr);

}

In void function(int* const ptr) ptr is constant but what ptr is pointing is not constant hence *ptr=10 is correct expression!

---

void Foo( int       *       ptr,
          int const *       ptrToConst,
          int       * const constPtr,
          int const * const constPtrToConst )
{
    *ptr = 0; // OK: modifies the "pointee" data
    ptr  = 0; // OK: modifies the pointer

    *ptrToConst = 0; // Error! Cannot modify the "pointee" data
    ptrToConst  = 0; // OK: modifies the pointer

    *constPtr = 0; // OK: modifies the "pointee" data
    constPtr  = 0; // Error! Cannot modify the pointer

    *constPtrToConst = 0; // Error! Cannot modify the "pointee" data
    constPtrToConst  = 0; // Error! Cannot modify the pointer
} 

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