c array wrapper struct no malloc allowed

Question

(btw I'm not allowed to malloc in this, I'm writing in c for c99) I'm trying to create a wrapper struct in c for arrays to keep things tidy so I don't need to keep passing the array pointer and the length around, and can just use a struct:

#define MAX_LEN 64

typedef struct {
    uint8_t data[MAX_LEN];
    size_t   len;
} byteArray_t;

Which is fine if MAX_LEN is known, but is there a way to make the length variable although known at compile time, so that for example I could have something like:

typedef struct {
    byteArray_t tag;
    byteArray_t length;
    byteArray_t value;
} tlv_t;

In which the array corresponding to tag would have size MAX_TAG_LEN and so on for the others - so I'd need to tell the compiler somehow that that was the case...

Anyone got any ideas? Thanks!

Edit

Alright, sorry for the confusion. Here's what I'm trying to do. I basically have the following structures at present:

// tag object
typedef struct {
    uint8_t  data[MAX_TAG_LENGTH_IN_BYTES];
    uint32_t len;
} tlvTag_t;

// length object
typedef struct {
    uint8_t  data[MAX_LENGTH_OF_LENGTH_IN_BYTES];
    uint32_t len;
} tlvLength_t;

typedef struct tlv tlv_t;

// value object definition
typedef struct {
    // can consist of a byte array, or a list of sub TLVs
    union {
        uint8_t data[MAX_VALUE_LENGTH_IN_BYTES];
        // data can be parsed into subTLVs
        tlv_t* subTLVs;
    };

    // need to store the number of subTLVs
    uint32_t numberOfSubTLVs;

    // len is the total length of the data represented:
    // the total length of the subTLVs placed end to end
    // or the length of the data array.
    uint32_t len;

} tlvValue_t;

// tlv object definition
struct tlv {
    tlvTag_t    tag;
    tlvLength_t len;
    tlvValue_t  value;

    // total length of entire tlv block (not value length)
    // (if there are sub TLVs, place them end to end)
    uint32_t totalLen;
};

I thought the design would be better if I could wrap the arrays in another struct to avoid all the code duplication and be able to pass fewer arguments around, but I can't because I don't know how to tell the compiler to create different sized byte arrays - maybe it's possible using macros? Hope that makes sense.

Compilers accept a switch the defines a macro. Check the documentation for your compiler which would permit this (/D for MSVC for example). — hmjd
– hmjd, Commented May 28, 2013 at 14:06
How exactly is this more tidy than just passing a plain pointer and a plain size variable? What are you trying to achieve? — Lundin
– Lundin, Commented May 28, 2013 at 14:13

anatolyg · Accepted Answer · 2013-05-28 15:56:26Z

It seems you are trying to somehow declare a struct whose contents depend on a parameter. In c++ this could be implemented by a template:

template <size_t MAX_LEN>
struct byteArray_t
{
    uint8_t data[MAX_LEN];
    size_t   len;
};

...
byteArray_t<MAX_TAG_LENGTH_IN_BYTES> tag;
byteArray_t<MAX_LENGTH_OF_LENGTH_IN_BYTES> len;
...

This is as straightforward as it can get.

To accomplish the same in C, you can use macros:

#define DECLARE_BYTE_ARRAY_T(your_type_name, MAX_LEN) \
typedef struct { \
    uint8_t data[MAX_LEN]; \
    size_t   len; \
} your_type_name

DECLARE_BYTE_ARRAY_T(tlvTag_t, MAX_TAG_LENGTH_IN_BYTES);
DECLARE_BYTE_ARRAY_T(tlvLenght_t, MAX_LENGTH_OF_LENGTH_IN_BYTES);

...
tlvTag_t tag;
tlvLength_t len;

Or (the same) without declaring types (good if you don't need names for your structs):

#define BYTE_ARRAY_T(MAX_LEN) \
struct { \
    uint8_t data[MAX_LEN]; \
    size_t   len; \
}

BYTE_ARRAY_T(MAX_TAG_LENGTH_IN_BYTES) tag;
BYTE_ARRAY_T(MAX_LENGTH_OF_LENGTH_IN_BYTES) len;

This may be marginally better than the code you already have. However, in my opinion, this is not worth the effort, because any non-trivial macro decreases readability of code.

ugoren · Accepted Answer · 2013-05-28 14:31:43Z

1

If you make a struct like this and then pass it by value to a function, then the whole array gets passed by value. You do not want that.

Actually you don't need an array inside the struct, just declare it elsewhere.

typedef struct {
    uint8_t* data;
    size_t   len;
} byteArray_t;


int main()
{
  uint8_t some_array[X];
  ...
  byteArray_t wrapper = {some_array, X};
  some_function (&wrapper);
}

edited May 28, 2013 at 14:31

ugoren

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answered May 28, 2013 at 14:09

Lundin

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5 Comments

Lundin Over a year ago

Keep in mind that this code probably does little good in the real world, outside the scope of this question. The question is trying to find a solution, without a problem to solve.

Matthew Over a year ago

Thanks for your responses. I'm not sure if you've understood what I'm trying to do, maybe I didn't make it clear... I can make a byteArray_t* pbytes and pass that around right? Just seems to me having one less argument makes the api simpler... The main point however was to be able to define the tlv_t objects somehow

Lundin Over a year ago

@Matthew And that's exactly what this code does. You get one function parameter instead of two. Overall, I believe the program gets far less readable because of this.

Matthew Over a year ago

I appreciate your help, but it seems you're not getting what I'm trying to do. Please re-read the question - I can define a byteArray_t struct no problem. The main point however was to be able to define the tlv_t objects somehow. My apologies if this isn't clear, seems to me there ought to be an elegant solution but needs some outside of the box thinking. I'm used to writing in Java and c++ so maybe c ethos is different.

anatolyg Over a year ago

@Matthew I read your question too and didn't get what you're trying to do either. This seems to be an interesting question; please add more info so everyone would benefit from it. (maybe add c++ or java equivalent of what you are trying to do in c?)

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