linux - What does [ N ... M ] mean in C aggregate initializers?

Question

From sys.c line 123:

void *sys_call_table[__NR_syscalls] = 
{
    [0 ... __NR_syscalls-1] = sys_ni_syscall,
#include <asm/unistd.h>
};

sys_call_table is a generic pointer to arrays, I can see that. However what is the notation:

[0 ... __NR_syscalls-1]

What is the ...?

---

EDIT:
I learned another C trick here: #include <asm/unistd.h> will be preprocessed and replaced with its content and assigned to [0 ... _NR_syscalls-1].

Answer 1

It is initialization using Designated Initializers.

The range based initialization is a gnu gcc extension.

To initialize a range of elements to the same value, write [first ... last] = value. This is a GNU extension. For example,

 int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

It is not portable. Compiling with -pedantic with tell you so.

How does it work here?
The preprocessor replaces #include <asm/unistd.h> with its actual contents(it defines miscellaneous symbolic constants and types, and declares miscellaneous functions) in the range based construct, which are then further used for initializing the array of pointers.