C - function that allocates memory

Question

I've written a procedure, that creates 2 matrices and multiplies them. However I have to divide my code into functions, and I have a problem with declaring a function that will allocate the memory for matrices.

Here's my code:

void matrixMult(){
int **A, **B, **C; //matrices
int rowA, colA, rowB, colB; //rows and columns 
int i, j, k; //for iterations

printf("Number of rows in matrix A: \n");
scanf("%d", &rowA);
printf("Number of columns in matrix A: \n");
scanf("%d", &colA);

printf("Number of rows in matrix B: \n");
scanf("%d", &rowB);
printf("Number of columns in matrix B: \n");
scanf("%d", &colB);

//memory allocation
A = (int**)malloc(rowA * sizeof(int));
B = (int**)malloc(rowB * sizeof(int));
C = (int**)malloc(rowA * sizeof(int));

for (i = 0; i < rowA; i++)
  {
   A[i] = (int*)malloc(colA * sizeof(int));
  }

for (i = 0; i < rowB; i++)
  {
   B[i] = (int*)malloc(colB * sizeof(int));
  }

for (i = 0; i < rowA; i++)
  {
   C[i] = (int*)malloc(colB * sizeof(int));
  } 
/* 
the rest of code
*/
}

What should this function look like?

What's wrong with it? Does it not work? Is it slow? Does it cause memory leaks? Is it ugly? — Beta, May 29 '16 at 20:44
http://stackoverflow.com/questions/35542391/segmentation-fault-but-unable-to-reason-how-memory-allocation-looks-fine-to-me — n. m. could be an AI, May 29 '16 at 20:46
http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — n. m. could be an AI, May 29 '16 at 20:48
@Beta well, the bad indentation sure does not make it pretty. — Martin James, May 29 '16 at 20:48
One of the usual suspects can provide the link: Don't cast the result of `malloc`. — Scott Hunter, May 29 '16 at 20:49
There are no matrices (i.e. 2D arrays) in your code. Nor something that can point to them. — too honest for this site, May 29 '16 at 23:45

score 3 · Answer 1 · answered May 29 '16 at 20:48

You have two ways of doing it that are both used in practice:

Write a function that allocates the matrix and returns the result - your function would have the following signature: int **alloc_matrix(size_t rows, size_t columns)
Write a function that takes a pointer to matrix pointer, and returns a status code - the signature would be int alloc_matrix(int*** res, size_t rows, size_t columns)

The value returned from the second function would indicate a success or a failure in allocating the memory.

Actually for a matrix/2d array, this is wrong. OP should use a correct 2D array. — too honest for this site, May 29 '16 at 23:46

ruirodrigues1971__ · Answer 2 · 2016-05-29T21:14:22.540

0

The first allocation you have an error. You have to alocate an array of pointers to Ints.

A= (int**)malloc(rowA * sizeof(int*));
B = (int**)malloc(rowB * sizeof(int*));
C = (int**)malloc(rowA * sizeof(int*));

for the function problem

int** alocatingArray(size_t nRows, size_t nCols){
...
}

this function will be used to each array returning the primary pointer to the matrix

edited May 29 '16 at 21:14

answered May 29 '16 at 21:09

ruirodrigues1971__

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C - function that allocates memory

2 Answers2