C array of structs declaration

Question

In the Linux kernel, I see a declaration of an array of structs that looks like this

struct SomeStructName [] ={
[SOMEWEIRD_NAME] = {
                   .field1 = "some value"
                   },
[SOMEWEIRD_NAME2] = {
                   .field1 = "some value1"
                   },
}

I have never seen a declaration like that, specifically I can't figure out what [SOMEWEIRD_NAME] means, and why it's used.

@ouah I thought C99 only allowed the .field syntax. It seems I was wrong, though GCC does have some extra syntax not in the standard. — Kninnug
– Kninnug, Commented Nov 17, 2015 at 18:53
@Kninnug the range syntax (e.g., [0..255]) is a GNU extension — ouah
– ouah, Commented Nov 17, 2015 at 18:54

ouah · Accepted Answer · 2015-11-17 18:52:36Z

5

It's a C99 designated initializer for arrays.

For example:

/* 
 * Initialize element  0 to 1
 *                     1 to 2
 *                     2 to 3
 *                   255 to 1   
 * and all other elements to 0
 */
int arr[256] = {[0] = 1, 2, 3, [255] = 1};

It allows you to initialize some specific array elements in any order and also allows you to omit some elements.

In your example the expression between [] can be a macro name for an integer constant expression or an enum constant. It cannot be a variable name as it has to be an integer constant expression.

edited Nov 17, 2015 at 18:52

answered Nov 17, 2015 at 18:36

ouah

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jNull · Accepted Answer · 2015-11-17 18:47:01Z

2

Im not sure what you meant but i assume SOMEWEIRD_NAME is a Define value.

Define is a way to give values another name but it wont take space in you'r memorry on runtime, insted, it will be replaced everywhere the name of that defined values is writen in your code during compiling prosses.

The syntax for deifne is as the following: #define NAME_OF_DEFINE 80 in the following example every NAME_OF_DEFINE in your code will be replaced whit the values 80. Notice that you shouldn't end the line whit ;.

In your example i expect SOMEWEIRD_NAME to have a numbric value to set the array's size.

You can fine more information about #define here

edited Nov 17, 2015 at 18:47

answered Nov 17, 2015 at 18:39

jNull

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DBug · Accepted Answer · 2015-11-17 18:36:46Z

1

The "SOMEWEIRD_NAME" is most likely either a #define whose value is a number, or it's an enumeration, whose numeric value is its position in the enumeration.

answered Nov 17, 2015 at 18:36

DBug

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1 Comment

Michael P Over a year ago

Yeah in my case it was actually an enum which is interpreted as an integer by the compiler so SOMEWEIRD_NAME turns into 0

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