C Struct array member without specific length

Question

I have encountered this piece of code:

struct test                   
{                                        
 uint32       num_fields;            
 char array_field [];               
};

How can I understand array_field? Is this a gcc extension for the C language?

Please revert the update and post a new question. – undur_gongor Jun 19 '13 at 08:49 — undur_gongor, Jun 19 '13 at 08:49

score 22 · Accepted Answer · edited May 23 '17 at 12:15

22

It's a C99 feature, called flexible array member which is typically used to create a variable length array.

It can only be specified as the last member of a struct without specifying the size (as in array_field [];).

For example, you can do the following and the member arr will have 5 bytes allocated for it:

struct flexi_example
{
int data;
char arr[];
};


struct flexi_example *obj;

obj = malloc(sizeof (struct flexi_example) + 5);

Its pros/cons discussed here:

Flexible array members in C - bad?

edited May 23 '17 at 12:15

Community

1
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answered Jun 19 '13 at 07:41

P.P

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Any idea why this was added to C instead of just using pointers? I hardly see why to use arrays at all except to allocate some pre-determined space on the stack. This doesn't give you that benefit. – xaxxon Jun 19 '13 at 08:03
I finally got the point when I printf the sizeof of the struct. Actually, the array_field does not occupy any bytes, it's the same thing as array_field[0], which is used to create a variable length array in gcc. – FaceBro Jun 19 '13 at 08:06
@undur_gongor sorry for the delay. I was commuting ;). – P.P Jun 19 '13 at 08:47
@xaxxon It's not an irreplaceable feature. I believe it was added based on the GNU extension zero length member. Details here: http://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html – P.P Jun 19 '13 at 08:48
2

@xaxxon It's added because with this feature, you can have true VLAs *inside* a struct, and you don't have to mallocate memory *twice.* This increases readability and decreases memory fragmentation and the number of pointer traversals (and thus perhaps boosts performance a little bit). – Jun 19 '13 at 08:50
3

@H2CO3: Flexible array members are not VLAs. In particular, `sizeof` applied to the struct doesn't include the size of the array member; you just have to keep track of that yourself. – Keith Thompson Jun 24 '13 at 16:35
@KeithThompson Yes I know that, still the reasoning is correct I believe. – Jun 24 '13 at 21:08
@H2CO3: VLAs and flexible array members have similarities, but they're distinct features. I suggest that referring to flexible array members as VLAs is likely to cause confusion. – Keith Thompson Jun 24 '13 at 21:19
@KeithThompson Yes, you're correct with regards to wording, sorry about that. Under "reasoning" I meant the response I gave to xaxxon. – Jun 24 '13 at 21:21
1

The flexible array member was added to formalize the [long existing and relied upon "struct hack"](http://stackoverflow.com/search?tab=votes&q=%2b[c]%20%22struct%20hack%22) (that search really isn't complete, as some instances are left off, but without the quotes a lot of junk creeps in). – dmckee --- ex-moderator kitten Nov 08 '13 at 02:53

score 3 · Answer 2 · edited Jun 19 '13 at 11:56

Such structures are usually allocated on the heap with a calculated size, with code such as the following:

#include <stddef.h>

struct test * test_new(uint32 num_fields)
{
    size_t sizeBeforeArray = offsetof(struct test, array_field);
    size_t sizeOfArray = num_fields * sizeof(char);
    struct test * ret = malloc(sizeBeforeArray + sizeOfArray);
    if(NULL != ret)
    {
        ret->num_fields = num_fields;
    }
    return ret;
}

C Struct array member without specific length

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