How to create and return a 2D matrix from a function in C using malloc?

Question

I'm trying to send a file to a function that creates a matrix of a struct called Entry, populates it based on the file, and returns the matrix. Right now it's a cascade of errors that I can add if needed, but I suspect I'm just making multiple dumb errors. If there's an easier way that doesn't involve malloc, that would work just as well.

What am I doing wrong and how can I do this properly?

Here's the relevant code:

Entry **matCreate(FILE *fp){

// initialize matrix
Entry matrix = malloc(states * N_CC * sizeof(Entry));

// populate the matrix with initial values
for (int i = 0; i < states; i++) {
  for (int j = 0; j < N_CC; j++) {
    matrix[i][j].next = 99;
    matrix[i][j].action = "D";
  }
}
return matrix;
}

and in the main function:

Entry **matrix = matCreat(fp);

I'm starting the function off with

Entry **matCreate(FILE *fp){

Initializing it with

Entry (*matrix)[N_CC] = malloc( states * sizeof *matrix );

Populating it with:

for (int i = 0; i < states; i++) {
  for (int j = 0; j < N_CC; j++) {
    matrix[i][j].next = 99;
    matrix[i][j].action = "D";
  }
}

And getting it from main with: Entry ourMat = (*matCreate(fp))[N_CC];

But still getting the error: tokenize.c:84:1: warning: return from incompatible pointer type [enabled by default]

Where am I going wrong?

Entry ** is not compatible with a 2D array; which do you want? — Carl Norum
– Carl Norum, Commented Oct 14, 2014 at 23:16
see also Declaring a C function to return an array, malloc in C, but use multi-dimensional array syntax — M.M
– M.M, Commented Oct 14, 2014 at 23:43

Community · Accepted Answer · 2017-05-23 10:33:34Z

2

C doesn't have a true multidimensional array type. You have to either return a jagged array, or return an array of arrays. (Also, since arrays cannot be returned by value; in both cases you have to dynamically allocate the array, and return a pointer to it). Further reading.

For the array of arrays option, you declare and allocate it like this:

Entry (*matrix)[N_CC] = malloc( states * sizeof *matrix );

By using this pattern for the argument to malloc, you avoid having to worry about any complicated stuff.

The type of a function to return this pointer is:

Entry (*matCreate(FILE *fp))[N_CC];

In the calling code you can use the function like this:

Entry (*matrix)[N_CC] = matCreate(fp);
bar( matrix[1][2].action );

You could use a typedef to make all of this simpler syntax. Personally I prefer not to, as then it is less obvious when array-pointer decay is happening.

edited May 23, 2017 at 10:33

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answered Oct 14, 2014 at 23:16

M.M

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1 Comment

HB- Over a year ago

Thank you, I confused myself. If I may ask one more question, am I still able to access elements at, for example, "ourMat[0][0].action"? I'm getting the error "subscripted value is neither array nor pointer nor vector".

radar · Accepted Answer · 2014-10-14 23:27:05Z

2

You need to assign malloc return pointer to Entry **

  Entry **matrix;
  matrix= (Entry**)malloc(states * sizeof(Entry *));
  for (i = 0; i < states; i++)
       Entry[i] = malloc(N_CC *   sizeof(Entry));

answered Oct 14, 2014 at 23:27

radar

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