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?
}

Oliver Charlesworth · Accepted Answer · 2012-04-08 21:32:57Z

38

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.

answered Apr 8, 2012 at 21:32

Oliver Charlesworth

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1 Comment

user4252523 Over a year ago

Are we not passing the pointer by value in the code below posted by STS Balaji? Does that code work only because we are passing the pointer address (i.e. value) because it's allocated on heap?

Doug Currie · Accepted Answer · 2012-04-08 21:34:31Z

35

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();
}

answered Apr 8, 2012 at 21:34

Doug Currie

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Comments

S.T.S.Balaji · Accepted Answer · 2021-06-07 08:20:33Z

2

I would suggest to modify your code like below if the function 'modify_item' intends to change member of structure which is passed as argument.

void modify_item(struct item *s){
   s->element = 5;
}

int main(){
   struct item *stuff = malloc(sizeof(struct item));
   modify_item(stuff); 
}

answered Jun 7, 2021 at 8:20

S.T.S.Balaji

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1 Comment

user4252523 Over a year ago

This is a very good third alternative to what Doug C mentioned above.

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