Initialize all struct members of struct array in C, using designated intializers range

Question

I cannot get designated initialiser syntax working when using a range. I know that designated initialisers are possible with arrays, and I want to initialize a struct array so that all members are the same at boot.

EDIT: error is

error: array index range in initializer exceeds array bounds 32 |
[0 ... NODELIST_LEN].dev_status = DW_DEV_DISABLED

typedef struct {
  int dev_status;
}DW_data; 

typedef struct{
  DW_data list[NODELIST_LEN];
}DW_nodelist;

I have tried the following:

DW_nodelist dw_list = {
    .list[0 ... NODELIST_LEN].dev_status = DW_DEV_DISABLED
}

DW_nodelist dw_list = {
    .list = {
        [0 ... NODELIST_LEN].dev_status = DW_DEV_DISABLED
    }
}

I even gave these a whirl just for laughs:

DW_nodelist dw_list = {
    .list = {
        .dev_status[0 ... NODELIST_LEN] = DW_DEV_DISABLED
    }
}

DW_nodelist dw_list = {
    .list = [0 ... NODELIST_LEN].dev_status = DW_DEV_DISABLED
}

What am I doing wrong, is this even possible with a struct array?

I added the gcc tag since this code isn't valid C but a gcc non-standard extension. — Lundin
– Lundin, Commented May 15, 2019 at 10:55

P.W · Accepted Answer · 2019-05-15 11:08:29Z

1

You are exceeding array bounds with 0 ... NODELIST_LEN. You have to stop at NODELIST_LEN - 1.

The below will work:

DW_nodelist dw_list = {
    .list[0 ... NODELIST_LEN-1].dev_status = DW_DEV_DISABLED
};

Note that the usage of ... (specifying a range of elements) is a GCC-specific extension not supported by standard C.

The GCC compiler warns if you use the -Wpedantic option.

warning: ISO C forbids specifying range of elements to initialize [-Wpedantic]
   13 |     .list[0 ... NODELIST_LEN-1].dev_status = DW_DEV_DISABLED

edited May 15, 2019 at 11:08

answered May 15, 2019 at 10:16

P.W

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1 Comment

BitShift Over a year ago

aaaahhhhhhhh, of course... how silly of me... thank you :)

Lundin · Accepted Answer · 2019-05-15 14:27:40Z

In case someone wonders how to do the same with pure standard C language, it is a bit trickier. Given this:

#define DW_DEV_DISABLED 666
#define NODELIST_LEN 32

typedef int DW_data;
typedef struct{
  DW_data list[NODELIST_LEN];
}DW_nodelist;

We could cook up a macro for array initialization like this:

DW_nodelist dw_list = 
{ 
  .list = { INIT(NODELIST_LEN, DW_DEV_DISABLED) } 
};

Where the first argument to the macro is the number of items to initialize and the second is the value. So here we want to set 32 items to some non-zero value. It is possible in standard C, but we need to declare a lot of macros like these:

#define INIT_1(val)  val
#define INIT_2(val)  INIT_1(val),  INIT_1(val)
#define INIT_4(val)  INIT_2(val),  INIT_2(val)
#define INIT_10(val) INIT_4(val),  INIT_4(val),  INIT_2(val)
#define INIT_32(val) INIT_10(val), INIT_10(val), INIT_10(val), INIT_2(val)

And then we can call these with another macro to make it all somewhat variable:

#define PREPROC(n, val) INIT_##n(val)
#define INIT(n, val) PREPROC(n, val)

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