Never seen before C method of initialization of an array of structs found in the Linux kernel source

Question

55 typedef struct pidmap {
56         atomic_t nr_free;
57         void *page;
58 } pidmap_t;
59 
60 static pidmap_t pidmap_array[PIDMAP_ENTRIES] =
61          { [ 0 ... PIDMAP_ENTRIES-1 ] = { ATOMIC_INIT(BITS_PER_PAGE), NULL } };

The code snippet above shows the initialization of an array of a structs that I found in the Linux kernel source. I have never seen this form of initialization before and I couldn't simulate the same thing on my own. What am I missing actually?

Source of the code

The Linux kernel uses lot of GCC specific extensions. See e.g. http://gcc.gnu.org/onlinedocs/gcc-4.6.3/gcc/index.html#toc_C-Extensions — Some programmer dude, Mar 29 '12 at 06:45

score 6 · Accepted Answer · answered Mar 29 '12 at 06:44

6

It is a GNU/GCC extension called Designated Initializers. You can find more information about it in the GCC documentation.

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

answered Mar 29 '12 at 06:44

ThiefMaster

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Just for accuracy, designated initializers aren't a GCC extension ([see this question](http://stackoverflow.com/questions/9849719/what-do-square-brackets-mean-in-array-initialization-in-c)), but the range syntax is. – huon Mar 29 '12 at 07:05

score 5 · Answer 2 · answered Mar 29 '12 at 06:43

5

It is done by using a Designated Initializer.

It is a gcc extension and not standard c construct. Using it results in non portable code, So avoid using such compiler extensions unless portability is least of your concerns.

answered Mar 29 '12 at 06:43

Alok Save

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Never seen before C method of initialization of an array of structs found in the Linux kernel source

2 Answers2