Can I put a variable length array inside a struct somehow?

arnold_w

I am working with an embedded microcontroller (ARM) and I'm building with GCC. The following is fully possible for me to:

struct testStruct_s {
int dummy;
const char* string;
};

struct testStruct_s testStructArray[] = {
{0, "Hello World"},
{0, "Hello World Again"}
};

But why isn't the following possble?

struct testStruct_s {
int dummy;
int* array;
};

struct testStruct_s testStructArray[] = {
{0, {0, 1, 2, 3}},
{0, {0, 1, 2, 3, 4}}
};

Is there a workaround to achieve this? I do not want to specify the array values outside the struct so the following is not what I want:

int array1[] = {0, 1, 2, 3};
int array2[] = {0, 1, 2, 3, 4};

struct testStruct_s testStructArray[] = {
{0, array1},
{0, array2}
};

Peter_in_2780

In your first example, the struct member is a char *, that is, a pointer. Its size is fixed, although the string it points to (which is stored elsewhere) may be of variable size. A debugger will show you what lives where at run time.

Software rusts. Simon Stephenson, ca 1994. So does this signature. me, 2012

arnold_w

Yes, char* is a pointer. But I believe int*, in the second example, is also a pointer.

Peter_in_2780

(1, 2, 3} is 3 int's. It is not a pointer to an array of 3 int's. The string/char * fudge dates from the early days of C before they realised they needed string as a first class data type.

Software rusts. Simon Stephenson, ca 1994. So does this signature. me, 2012

CPallini

Using 'compound literals[^]', you can:

#include

struct testStruct_s {
int dummy;
int* array;
};

struct testStruct_s testStructArray[] = {
{ 0, (int[]){0, 1, 2, 3, -1}},
{ 0, (int[]){0, 1, 2, 3, 4, -1}}
};

int main()
{
for (int i=0; i

arnold_w

Is this a compiler-specific feature or is it part of standard C? And is there a way that I can obtain the size of the arrays? The following simply returns the size of the pointer, not the array:

sizeof(testStructArray[0].array)

CPallini

I report you the first sentence of the linked page:

ISO C99 supports compound literals. A compound literal looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer; it is an lvalue. As an extension, GCC supports compound literals in C89 mode and in C++.

So:

C99 compilers support it.
Your compiler suopports it even in C89 mode.

arnold_w

Unfortunately I have very little knowledge about compilers and I don't know what C89 and C99 are. What I'm worried about is that my code won't compile in the future if we move to a different compiler.

CPallini

Quote:

Unfortunately I have very little knowledge about compilers and I don't know what C89 and C99 are

It's time to learn, then.

Quote:

What I'm worried about is that my code won't compile in the future if we move to a different compiler

If you are really worried about then don't use the feature. You might also check in advance if the compiler you are planning to use in future supports such a feature (or, generally C99 features).

Lost User

Then it is easy enough to make your code simpler:

struct testStruct_s {
int dummy;
int* array;
};

int array1[] = {0, 1, 2, 3};
int array2[] = {0, 1, 2, 3, 4};
struct testStruct_s testStructArray[] = {
{0, array1},
{0, array2}
};

jschell

CPallini wrote:

It's time to learn, then.

Keep in mind that the OP is doing embedded code and consequently what is actually available might be less than strict usage would otherwise suggest.

jschell

arnold_w wrote:

an embedded microcontroller (ARM)

Because of that I am going to presume that maybe you can't rely on the newest features (other suggestions). Didn't see anyone mention the old school approach which should work on any C compiler unless perhaps one goes back to the 70s (perhaps even then.) I am not going to try to even create pseudo code for this (way too long for me) but basic outline - Add attributes for the size of each array - All attributes EXCEPT arrays must be before arrays themselves. So arrays are at the end. - Alloc the struct based on the size of the array PLUS the appropriate sizes of the arrays. - Create helper methods that use and OFFSET to access the array pointer based on the sizes (attribute above) for each array. Google for the following for examples. Look for examples that have something like "int[] array" at the end of the structure.

C programming language struct dynamic array

One gotcha which is probably still relevant is that, maybe, C compilers attempt to 'align' attributes in the struct, these days. That means it might add filler that you are unaware of. If so there should be a compiler option, to remove that. This solution means that you cannot treat the array attributes as pointers. Specifically do not try to set them to another pointer. That is because the array storage is in the struct allocated block itself. So you cannot free it. And assignment would mean you couldn't free the assigned block either.

arnold_w

I am basically writing a table (several hundred lines) with lots of information into source code (basically, my source code is the documentation for it) and it makes it so much easier to read if information is where it belongs and not somewhere else. That's why I don't want to use this approach.

arnold_w

It doesn't matter to me where the arrays are actually stored, I just want the data the array is initialized with to be placed where it belong inside structure array. Basically, my source code is kind of a look-up table in this case, so it's purely a visual thing for readability.

leon de boer

For the record on GCC you can control the C standard it uses via the flag -std=C99 or C++ standard -std=C++98 (yes C99 equivalent in C++ is 98) Your GCC compiler should be defaulted to at least C11 or C14, in C or C++ unless it's really old like GCC version 4.7.

In vino veritas