defining structure variable during declaration

Question

I have been trying something in structures in C.

struct val{
    unsigned int a : 1 ;
}store[100] ;

Now I want to initialize all the array members value to 1. That is, I want all the array members to have their variable a assigned to 1. I can use a loop for that but how can I do this during the declaration?

struct val
{
    unsigned int a=1 : 1 ;
}store[100];

How can i achieve this? The above syntax is coming out to be wrong in code::blocks.

if you have this in a class you can initialize it in the constructor, by using a simple for with 100 runs and initialize every element — Martin, Feb 05 '15 at 12:15
actually 100 is a test number, the actual number is of 9 digits. So as you can see i can't use a for. — Kapil Garg, Feb 05 '15 at 12:17

score 3 · Answer 1 · edited May 23 '17 at 12:20

struct val
{
unsigned int a : 1;
}store[100];

I initially thought that what you were doing was a bitfield initialization. Further research though suggested I am wrong, and that you are trying to use gcc's designated initializers. What you want to do, is not possible that way. I found another way in your code that you can do it, though:

typedef struct {
    unsigned int a;
} val;

then, where you want to initialize the array, you will do something like that:

val values[100] = {[0].a = 1};

This works by exploiting this behavior of gcc:

If the same field is initialized multiple times, it has the value from the last initialization. If any such overridden initialization has side-effect, it is unspecified whether the side-effect happens or not. Currently, GCC discards them and issues a warning.

My Test Program follows:

#include <stdio.h>

struct val {
    unsigned int a;
};



int
main (void)
{
    struct val value[100] = {[0].a = 1};
    printf ("A random val's value is %d\n", value[40].a);
    return 0;
}

Compiles and works cleanly on my GCC 4.9.1.

the loop is a good solution but only for small numbers. I will have to run the loop a billion times if i want to initialize every array member in my code. So for loop can't be used. — Kapil Garg, Feb 05 '15 at 12:21
@KapilGarg I edited the answer. See if the changes reflect what you want to achieve. — NlightNFotis, Feb 05 '15 at 12:46

score 1 · Answer 2 · answered Feb 05 '15 at 12:22

1

It's not possible.

You can use a loop, or, if by any chance you want to initialize all struct's data members to a specific value, you can also use memset:

struct val {
    unsigned int a : 1 ;
} store[N];

memset(store, value, N * sizeof(struct val));

answered Feb 05 '15 at 12:22

rubikonx9

1,403
15
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ok, but what is we have `unsigned int a : 10;`. What will happen after `memset()`? – Sourav Ghosh Feb 05 '15 at 12:37
I think it depends on the value being set, but appropriate bits should be set the same way as they are defined in a value. Which makes it risky with non-zero value. Unless you know what you're doing. – rubikonx9 Feb 05 '15 at 12:39

haccks · Answer 3 · 2015-02-05T12:36:33.770

0

Before declaring a struct variable you can't initialize its member. Initialization can be done as

struct val{
    unsigned int a : 1;
}store[100] = {[0].a = 1} ; // Designated initializer

but it will initialize a member of other elements of store to 0. To initialize member a of all elements of store you need a loop.

edited Feb 05 '15 at 12:36

answered Feb 05 '15 at 12:19

haccks

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25
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defining structure variable during declaration

3 Answers3