Passing struct pointer to function in c

Question

I'm having a problem with passing a pointer to a struct to a function. My code is essentially what is shown below. After calling modify_item in the main function, stuff == NULL. I want stuff to be a pointer to an item struct with element equal to 5. What am I doing wrong?

void modify_item(struct item *s){    struct item *retVal = malloc(sizeof(struct item));    retVal->element = 5;    s = retVal; }  int main(){    struct item *stuff = NULL;    modify_item(stuff); //After this call, stuff == NULL, why? } 

Answer 1

Because you are passing the pointer by value. The function operates on a copy of the pointer, and never modifies the original.

Either pass a pointer to the pointer (i.e. a struct item **), or instead have the function return the pointer.

Answer 2

void modify_item(struct item **s){    struct item *retVal = malloc(sizeof(struct item));    retVal->element = 5;    *s = retVal; }  int main(){    struct item *stuff = NULL;    modify_item(&stuff); 

or

struct item *modify_item(void){    struct item *retVal = malloc(sizeof(struct item));    retVal->element = 5;    return retVal; }  int main(){    struct item *stuff = NULL;    stuff = modify_item(); }