Changing address contained by pointer using function

In C, functions arguments are passed by value. Thus a copy is made of your argument and the change is made to that copy, not the actual pointer object that you are expecting to see modified. You will need to change your function to accept a pointer-to-pointer argument and make the change to the dereferenced argument if you want to do this.
For example

 void foo(int** p) {
      *p = NULL;  /* set pointer to null */
 }
 void foo2(int* p) {
      p = NULL;  /* makes copy of p and copy is set to null*/
 }

 int main() {
     int* k;
     foo2(k);   /* k unchanged */
     foo(&k);   /* NOW k == NULL */
 }

If you have the luxury of using C++ an alternative way would be to change the function to accept a reference to a pointer.

Related Contents:

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  2. C Programming: malloc() inside another function
  3. How do you pass a function as a parameter in C?
  4. Function pointers and address of a function
  5. Parameter Passing in C – Pointers, Addresses, Aliases
  6. How can I return a character array from a function in C?
  7. Returning Arrays/Pointers from a function
  8. What is the function of an asterisk before a function name?
  9. Changing array inside function in C
  10. How is the result struct of localtime allocated in C?
  11. How come an array’s address is equal to its value in C?
  12. What is the difference between NULL, ‘\0’ and 0?
  13. strdup() – what does it do in C?
  14. Why use double indirection? or Why use pointers to pointers?
  15. Create a pointer to two-dimensional array
  16. Are there any platforms where pointers to different types have different sizes?
  17. What is the reason for using a double pointer when adding a node in a linked list?
  18. How can I allocate memory and return it (via a pointer-parameter) to the calling function?
  19. Modifying C string constants? [duplicate]
  20. Pointer Arithmetic In C
  21. Pass an array to a function by value
  22. How to increment a pointer address and pointer’s value?
  23. Tools to get a pictorial function call graph of code [closed]
  24. C: pointer to struct in the struct definition
  25. Pointer to a specific fixed address
  26. Why must the variable used to hold getchar’s return value be declared as int?
  27. C function calls: Understanding the “implicit int” rule
  28. What does malloc(0) return? [duplicate]
  29. strcpy() return value
  30. In C, can a const variable be modified via a pointer?
  31. What is the difference between far pointers and near pointers?
  32. Why can I change a local const variable through pointer casts but not a global one in C?
  33. How to include a dynamic array INSIDE a struct in C?
  34. Does the size of pointers vary in C? [duplicate]
  35. Pointer to pointer clarification
  36. Why is a point-to-volatile pointer, like “volatile int * p”, useful?
  37. Function pointer cast to different signature
  38. How to print variable addresses in C?
  39. Using intptr_t instead of void*?
  40. In a C function declaration, what does “…” as the last parameter do?
  41. C typedef of pointer to structure
  42. “int *nums = {5, 2, 1, 4}” causes a segmentation fault
  43. using printf to print out floating values
  44. undefined reference when calling inline function
  45. Can a pointer (address) ever be negative?
  46. How to align a pointer in C
  47. What does getting the address of an array variable mean?
  48. Where in memory are string literals ? stack / heap? [duplicate]
  49. Why can´t we assign a new string to an char array, but to a pointer?
  50. Dereference void pointer

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