Compiler error: incompatible types when assigning to 'struct' from type 'void *' during malloc

Question

EDIT -- can the down voter explain? I have a clear question, with supporting evidence, and proof of prior investigation. I would like to understand why you are down voting me...?

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I am getting this error when I compile with gcc:

error: incompatible types when assigning to type ‘struct cell’ from type ‘void *

The problem lines are:

    struct cell* cells = NULL;
    cells = malloc(sizeof(struct cell) * length);
    for (i = 0; i < length; i++) {
            cells[i] = malloc(sizeof(struct cell) * width);

I believe I have followed the proper protocol, as described here and also here. What am I missing?

Solution

For a multidimensional array, you want an array of type struct cell** cells:

struct cell** cells = NULL;
cells = malloc(sizeof(struct cell*) * length);
for(int i = 0; i < length; i++) {
  cells[i] = malloc(sizeof(struct cell)*width);
}

Now cells is a multidimensional array, where the first index range is the length and the second index range is the width.

Comments

[d0rmLife]

Great, the compiler is happy :) Makes sense, thanks for explaining.

[Keith Thompson]

Strictly speaking, a multidimensional array is simply an array of arrays. You can create a data structure that acts like a multidimensional array (with more flexibility and more need to manage your own memory) using an array of pointers.

[d0rmLife]

@KeithThompson Is the alternate data structure you are referring to an array that would result from a command something like: struct cell* cells = malloc(sizeof(struct cell) * length * width); ?

[John Colanduoni]

@d0rmLife What my code above gives you is an array of pointers to arrays. struct cell* cells = malloc(sizeof(struct cell) * length * width) gives you a one dimensional array, but it can be indexed using something along the lines of cells[i*width+j].

[Keith Thompson]

@d0rmLife: Partly. To emulate a 2D array like that, you need to allocate memory both for an array of pointers (each element of which points to a row of the 2D array), and for each row. Basically what you're doing in the code in your question, apart from the type error.