CODEWITHSUNDEEP
cstruct
Tom Lukeywood
Tom Lukeywood

Reputation: 3

How to pass a member of a struct to a function by reference in C?

I am trying to pass a member of a stuct to a function in c by reference and then use it to access the same member of a struct of the same type.

typedef struct Node
{
int a;
int b;
int c;
} Node;

Node mynode; //global

void pass_member(int *member){
   mynode.&member = 1;//gives a compiler error
}

void main()
{
 mynode.a = 0;
 mynode.b = 45;
 mynode.c = 64;

 pass_member(&mynode.a)
}

I can see that mynode.&member is not allowed in c but i think it shows what i am trying to do

is it possible to pass the address of an member of a struct to a function in c
and then use it to access that member in a struct of the same type?

basically what i am trying to achieve is a function somthing like this
void set_member(int value_to_set, int * pointer_to_member_of_which_to_set);

that would set a specific member of a global struct depending on the given member to the function.

Upvotes: 0

Views: 1121

Answers (3)

king_nak
king_nak

Reputation: 11513

You can use offsetof to get the relative address of a member. I am unaware of a reverse operation (don't do much in C), but it can be simulated like this:

#define offsetin(s,a,t) *((t*)(((char*)&s)+a))

Then your code would look like this:

void pass_member(size_t member){
    offsetin(mynode, member, int) = 1;
}

void main()
{
    mynode.a = 0;
    mynode.b = 45;
    mynode.c = 64;

    pass_member(offsetof(Node,a));
}

Upvotes: 0

user2736738
user2736738

Reputation: 30926

I will give you some example first:-

You can do it like this func(&mynode).

And func is void func(Node* aa)

Now you can use it like this

aa->a = aa->a + 1 etc.

Most of the time you will send the address of the whole structure rather than the address of the individiual element.

  • Node aa => aa is of type struct node (typedefed to Node)

  • It's address: &aa

  • aa.a is the element of the structure aa and it's address is &aa.a

Solution-1

void pass_member(int *member){
   *member = 1;
}

  pass_member(&mynode.a) // in main()

Solution-2

Also you can use it like this:-

typedef struct Node
{
int a;
int b;
int c;
} Node;

Node mynode; //global

void pass_member(Node *member){
   (*member).a = 1;//member->a = 1
}

void main()
{
 mynode.a = 0;
 mynode.b = 45;
 mynode.c = 64;

 pass_member(&mynode)
}

You asked is it possible to pass the address of an member of a struct to a function in c?

Ans: Yes it is possible, it is shown in the answer.

and then use it to access that member in a struct of the same type?

You can access that memebr be it of some struct or some other struct. As long as it is a pointer to an appropriate type it's possible.

Upvotes: 1

Gaurav Sehgal
Gaurav Sehgal

Reputation: 7542

You have two options, either pass pointer to the member variable as

void pass_member(int *member){
  *member = 1;

and

 pass_member(&mynode.a);

OR pass pointer to the entire structure

void pass_member(Node *member){
  (*member).a = 1; //or member->a = 1; as pointed out in comments

and

pass_member(&mynode)

Upvotes: 0

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