Question

How to assign c type array to struct member that is also an array of the same type? Here’s my struct:

typedef struct {

  uint8_t type;
  uint8_t data[10];

} MyStruct;

Here’s the creation of the struct:

MyStruct myStruct;

Here’s generation of some array:

uint8_t generatedArray[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

Here’s my assignment:

myStruct.data = generatedArray;

Answer 1

As other answers have stated an array is not directly assignable and you must use a function such as memcpy:

memcpy(myStruct.data, generatedArray, sizeof(myStruct.data));

However…

C is a language with a long history. Early languages had roughly the notion of “little” and “large” types, the former being directly assignable and the latter not. Some later languages made everything assignable (or for functional languages, values).

Original C had primitive types; integer, float, etc.; and arrays of these types. The former, being “little”, were assignable, the latter, being “large”, were not. Subsequently C gained structures, and decided these were “little”…

So, a little strangely, while you cannot directly assign one array to another you can assign on structure to another, and in particular you can assign a structure with just one field which is an array… You can also write literal structure values, including for those with array-valued fields; and being C with its “types are comments” approach you can cast a pointer to an array to a pointer to a structure…

The above means you can write “fun” code like:

typedef struct
{
    uint8_t type;
    uint8_t data[10];
} MyStruct;

typedef struct
{
    uint8_t data[10];
} MyArray;

typedef struct
{
    uint8_t type;
    MyArray array;
} MyStruct2;

void arrayAssignment(int x)
{
    MyStruct myStruct;
    MyArray generatedArray = {{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}}; // structure literal

    *(MyArray *)&myStruct.data = generatedArray;

    uint8_t generatedArray2[] = {11, 12, 13, 14, 15, 16, 17, 18, 19, 20}; // array literal
    *(MyArray *)&myStruct.data = *(MyArray *)generatedArray2;

    MyStruct2 myStruct2;
    myStruct2.array = generatedArray; // no casts at all, but element access is myStruct2.array.data[index]
}

Note that none of the casts, indirections, etc. or in the final example the extra array. in the indexing cost anything in the compiled code – they are all simply ways to persuade the compiler to use its built-in array assignment.

Many would of course argue against regular uses of these techniques!