Runtime initialization of structure member in C ( member name decided at run )

Question

I'm wondering for a solution to the below problem. Please help.

Problem:

struct s{
int a;
int b;
}st;

I want a function to initialize the values at runtime. The problem is that I want to make it generic, so I want to pass the member name as input to the function and get it initialized.

fun_init(char* mem, int val)
{
  // Find offset of the member variable in struct 'st'
  // Assign the value
}

One straight solution is to use string comparision on the member name. But if I happen to add some extra variables at a later time, I'll have to modify the function, which I don't want. Hope I was able to frame the ques clearly.

Thanks

C names don't exist at run-time. The compiled code doesn't need them. — rici, Nov 28 '13 at 06:19
If you only need this for one or two structures, you could use a code generator to set up the appropriate data structures or generate suitable code for accessing the fields. Of course this has the usual issues associated with code generators. — creichen, Nov 28 '13 at 07:44

score 2 · Answer 1 · edited May 23 '17 at 11:52

2

C does not provide a way to find a symbol by name. See this thread for more information.

The simplest solution here is to use an associative array.

Read this thread if you need to mix value-types. (In your example, all value types are int, so you might not need this.)

edited May 23 '17 at 11:52

Community

1
1

answered Nov 28 '13 at 06:20

Domi

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score 1 · Accepted Answer · answered Nov 28 '13 at 07:35

1

void fun_init(int *storage, int val) {
    *storage = val;
}
void something_else(void) {
    struct s {
        int a;
        int b;
    } st;
    fun_init(&st.a, 42);
}

However, if you need to dynamically determine the key name, you are doing something wrong. If you need to store key/value pairs, perhaps you would be interested in the hashtable.

answered Nov 28 '13 at 07:35

Konrad Borowski

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Thanks all for the comments. – ssin Nov 28 '13 at 09:30

score 0 · Answer 3 · answered Nov 28 '13 at 08:10

I'm guessing you want to initialize struct from either user input or persistency.

A solution involves creating an associative array as mentioned by @Domi.

The array is filled with key/value pairs such as (const char*, unsigned). The key is the name of struct member and the value is the offset from the start of the struct. Each struct will need to have a function that initializes the above array. You can get an offset to a member via the offsetof macro.

This will NOT work with structs that have bit fields (sub byte named members).

Runtime initialization of structure member in C ( member name decided at run )

3 Answers3