如何传递/访问结构的双指针？

Question

A 10x10 matrix is created as follows:

double **c = (double **)pvPortMalloc(ROW * sizeof(double*));
for (i = 0; i < ROW; i++)
    c[i] = (double *)pvPortMalloc(COL * sizeof(double));

I would like to pass double **c to a struct :

struct AMessage {
    char cMessageID;
    double (**doublePtr);
} xMessage;

At the end I want to access the struct and print the matrix on a screen, however, I am unsure how I should go about this using double pointers...

Answer 1

Firstly, in direct answer to your question, if you define a matrix in that way and want to store its pointer in a struct, you can do it via a direct assignent. Carrying on from your example, you could make the doublePointer member of a struct AMessage point to the memory you've allocated at c simply via something like this:

struct AMessage am;
am.doublePtr = c;

But if you're wanting to create a matrix, where every row is guaranteed to have the same number of elements as any other row a better fit for representing this is a 2D array, rather than an array of pointers to other arrays.

A 2D array in C can be declared like so:

double matrix[ROW][COL];

this declares a variable called matrix , with ROW rows and COL columns.

There is nothing to stop you including this definition straight in the definition of your struct , making the definition of your struct like this:

struct AMessage
{
     char MessageId;
     double matrix[ROW][COL];
};

(Thanks to chqrlie for suggesting this)

If you definitely want a pointer to a 2D array to be in your struct (for example, you might want to have one large array referenced by two of your struct s) you need to know that a 2D array in C is really a 1D array, only addressed in a certain way.

You can dynamically allocate the memory to be used for the matrix with one call to malloc:

double *c = malloc(ROW * COL * sizeof(*c));

One option for accessing element the element in column j , row i is:

double a = c[ROW * j + i];

but this is obviously a bit clunky, really, it is more convenient to access element i , j by something like this:

double a = d[j][i];

To define the pointer d that we can dereference like this we need to think of what type d[j] is. It is not, as many think a double * , rather it is a pointer to an array of ROW double s. In C syntax you would declare d like so:

double (*d)[ROW];

Finally resolving your question, I would define your struct like this:

struct AMessage 
{
     char MessageId;
     double (*matrix)[ROW];
}

Assigning to this struct would look something like this:

struct AMessage am;
am.matrix = malloc(ROW * COL * sizeof(*am.matrix));
am.matrix[j][i] = 42; // Assigns 42 to row j, column i