error while Using malloc in creating 2D and 1D array

Question

I am creating 1d arrays B and T of n size and a 2D array A of nxn size,where n has been computed earlier. But the program crashes after Pause, what am i possibly doing wrong??

float *B = malloc(sizeof(int) * (n));
float *T = malloc(sizeof(int) * (n));
system("PAUSE");
float **A;                                                    
A = malloc(sizeof(int) * (n));
for(j = 0; j < n; j++)
{ 
    A[j] = malloc(sizeof(int) * (j));
}

i, j and n are integers.

`sizeof(int)` isn't good here, it should be `sizeof(float)` and `sizeof(float*)` for `A`. — Bo Persson, Aug 29 '12 at 07:54

score 3 · Answer 1 · answered Aug 29 '12 at 07:52

3

a 2D array A of nxn size

You're passing the wrong types to sizeof. You should be using sizeof(float) and sizeof(float *). But the more serious (and insidious) problem is:

A[j]=(float*)malloc(sizeof(int)*(j));
                                 ^

You want n instead of j.

answered Aug 29 '12 at 07:52

cnicutar

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Even more serious problem: casting the return value of `malloc()` in C. – Aug 29 '12 at 07:53
But how does allocating a triangular shape cause a crash? – Bo Persson Aug 29 '12 at 08:34
@BoPersson The OP said "a 2D array A of nxn size". So presumably that's how it's being used. – cnicutar Aug 29 '12 at 08:36

score 1 · Accepted Answer · edited May 23 '17 at 11:54

1

Never repeat the type name in the argument to malloc(); doing so makes it easy to say the wrong type, which creates errors.

You should use sizeof, like so:

float *B = malloc(n * sizeof *B);
float **A = malloc(n * sizeof *A);
for(j = 0; j < n; ++j)
  A[j] = malloc(n * sizeof *A[j]);

Also, in C you shouldn't cast the return value of malloc().

edited May 23 '17 at 11:54

Community

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answered Aug 29 '12 at 07:55

unwind

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error while Using malloc in creating 2D and 1D array

2 Answers2