Why Use Zero-Length Arrays in C Structs?

Array of Zero Length

Refactoring old code can uncover questionable practices, like the use of zero-length arrays in structs. This prompts the question: Why create an array with no elements?

Consider the following struct:

struct someData
{
   int nData;
   BYTE byData[0];
}

A zero-length array, as seen in byData, is an unusual choice. Typically, arrays store elements. However, arrays of length 0 can be used as pointers. In this case, byData serves as a placeholder for a variable-length buffer that will be dynamically allocated later.

struct someData* mallocSomeData(int size)
{
    struct someData*  result = (struct someData*)malloc(sizeof(struct someData) + size * sizeof(BYTE));
    if (result)
    {    result->nData = size;
    }
    return result;
}

This function allocates memory for a someData struct and a buffer of the specified length. The zero-length array in the struct allows us to treat it as a pointer, referencing the dynamically allocated buffer.

To refactor this code, consider using:

• Pointers: Replace the zero-length array with a direct pointer to the buffer and allocate the buffer separately.
• Arrays of Length 1: Create an array of length 1 and use the first element as a pointer to the buffer.

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