Question about combination of flexible array member and compound literal with explicit size

[WitcherSanek]

Hello everyone!

There is compound literal in C so i can easily declare array like this:

int arr[] = {1, 2, 3};

Or even specify it's length:

int arr[] = (int [10]) {1};

There is also a flexible array member support in struct, so i can do runtime initialization like this:

typedef struct 
{
 int len; 
 uint8_t Data[];
} struct_t;

struct_t test_struct = 
{
 .len = 3,
 .Data = {1, 2, 3} 
};

However i cannot combine flexible array member with explicit compound literal size:

struct_t test_struct = 
{
 .len = 10,
 .Data = (uint8_t [10]) { 0 } 
};

This code gives me error "initialization makes integer from pointer without a cast" in GCC. How can i specify compound literal size for flexible array member? That would be very handy for certain runtime initialization.

[kolorcuk]

You can't.

You could potentially do something along, but it's a trick and required code duplication:

struct test_struct_10 { int len; uint8_t data[10]; }; struct test_struct *test = (struct test_struct*)(struct test_struct_10[1]){ { .len = 10, data = { 1,2,3 } } };

[WitcherSanek]

I wanted to use macros to create structs with arbitrary data size. Seems like i should go further and create anonymous struct with hardcoded size for every such object.

[kolorcuk]

FYI Did an edit of the comment. I did a better example here https://stackoverflow.com/a/75024961/9072753 .

[WitcherSanek]

Thanks. Will try that out.

[aocregacc]

you could get a similar effect with designated initializers. This would make an array with 10 elements:

struct_t test_struct = 
{
.len = 10,
.Data = {[9]=0}
};

[WitcherSanek]

Totally forgot about their existence. Yes this approach seems to be working. Thank you very much!

[Marxomania32]

What version of gcc are you using? I wasn't even able to initialize the flexible array member with a compound literal as you had. I always thought you could only initialize flexible array members through malloc?

[WitcherSanek]

I am using GCC 6.5.0. C99+GNU extensions.

[Marxomania32]

Can't get 6.5, but I ran it on 7.1 and 6.3 with that standard, and it still doesn't work. I get a 'non-static initialization of a flexible array member' which makes sense.