Initialization from incompatible pointer type warning when assigning to a header given array

Question

GCC gives me an 'Initialization from incompatible pointer type' warning when I use the following code.

 #include "mnist.h"
 #include <stdio.h>

 int main(void)
 {

    double** inputLayer = train_image; 

}

train_image is a bidimensional array given by mnist header file that according to the autor is like that

While array decays to pointers to their first element, in this case the array `train_image` decays to a pointer to an *array* (which does *not* decay further). A pointer to an array is not the same as a pointer to a pointer. [This old answer of mine](https://stackoverflow.com/questions/18440205/casting-void-to-2d-array-of-int-c/18440456#18440456) tries to show the difference. — Some programmer dude, Dec 09 '19 at 17:27

score 0 · Answer 1 · answered Dec 09 '19 at 17:40

0

array is not pointer to pointer.

int main()
{
    double *pointer = &train_image[0][0];
    double **inputLayer = &pointer; 
}

answered Dec 09 '19 at 17:40

0___________

score 0 · Answer 2 · answered Dec 09 '19 at 17:57

If you have an array like this

double train_image[60000][784];

then a pointer to its first element should be declared like

double ( *inputLayer )[784] = train_image;

That is the expression train_image used as an initializer has the type double ( * )[784].

If it is needed then you can reinterpret the two-dimensional array as a one dimensional array using the following pointer declaration

double *inputLayer = ( double * )train_image;

2 Answers2