[SOLVED] What difference between these initializing array?

chaoxifer · 10-15-2013, 09:02 PM

I'm studying linux kernel source and I found the definition like this:

static pidmap_t pidmap_array[PIDMAP_ENTRIES] =
{ [ 0 ... PIDMAP_ENTRIES-1 ] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } };

Why maintainer coded the source like this?
I think they could make the code like this:
static pidmap_t pidmap_array[PIDMAP_ENTRIES] = { ATOMIC_INIT(BITS_PER_PAGE), NULL };

I want to know the difference between them.

rtmistler · 10-18-2013, 02:31 PM

The first style is a C extension to initialize all elements of the array. Are you saying that if one is initializing all array elements that the second notation works? I don't think the second one works, I think you initialize only two array elements with that second variation.

chaoxifer · 10-21-2013, 02:42 AM

Thanks for reply.

I thought that

Code:

static pidmap_t pidmap_array[PIDMAP_ENTRIES] = { ATOMIC_INIT(BITS_PER_PAGE), NULL };

is possible because all entry has same value as ATOMIC_INIT(BITS_PER_PAGE),NULL -> That's because pidmap_t is a structure defined as

Code:

typedef struct pidmap {
	atomic_t nr_free;
	void *page;
} pidmap_t;

As we can initialize some array with the way like

Code:

int arr[arrSize]={0};

I thought that the code can be simplified using that way... In my thought, the developer use a C extension you mentioned for explicitly initializiing the value.... Am I wrong??

rtmistler · 10-21-2013, 09:10 AM

Quote:

Originally Posted by chaoxifer

Am I wrong??

Unfortunately yes, but my initial answer didn't settle the issue completely either. So in the future we both can remember that one can code simple stuff like this and validate or invalidate an area of question.

Test source code. Note the differences in the initializations of test_array_1 versus test_array_2:

Code:

#include <stdio.h>
#include <stdlib.h>

typedef struct {
    int a;
    char bee[8];
}sTest_t;

void main(void)
{
    sTest_t test_array_1[5] = { [ 0 ... 4 ] = { 1, "one" } };
    sTest_t test_array_2[10] = { 2, "two" };
    int i;

    for(i = 0; i < 5; i++) {
        printf("Element: %d Values: %d %s\n", i, test_array_1[i].a, test_array_1[i].bee);
    }
    for(i = 0; i < 10; i++) {
        printf("Element: %d Values: %d %s\n", i, test_array_2[i].a, test_array_2[i].bee);
    }
}

The results:

Code:

~/testcode$ ./extension 
Element: 0 Values: 1 one
Element: 1 Values: 1 one
Element: 2 Values: 1 one
Element: 3 Values: 1 one
Element: 4 Values: 1 one
Element: 0 Values: 2 two
Element: 1 Values: 0 
Element: 2 Values: 0 
Element: 3 Values: 0 
Element: 4 Values: 0 
Element: 5 Values: 0 
Element: 6 Values: 0 
Element: 7 Values: 0 
Element: 8 Values: 0 
Element: 9 Values: 0 
~/testcode$

chaoxifer · 10-22-2013, 01:45 AM

Thanks for reply.

Now I can understand!!..

Although it is basical grammer(?), I didn't know about that...shamefully.. oTL

I should learn C language, including GCC compiler extention.

Thank you rtmistler!!