MPI dynamic array using malloc

Question

I am having problem while creating dynamic array using both malloc and calloc.

    int main() {
      float *xd_real_send;
      int Nooflines_Real;
      int *X;
      float test[500];

      Nooflines_Real = count_lines(infile);
      printf("Nooflines_Real: %d\n", Nooflines_Real);

      X = (int *) malloc(Nooflines_Real*sizeof(int));
      xd_real_send = (float *) calloc (Nooflines_Real,sizeof(float));

      printf("size of X %d, test %d and size of xd_real_send %d\n",
      sizeof(X)/sizeof(int),sizeof(test)/sizeof(float),
      sizeof(xd_real_send)/sizeof(float));fflush(stdout);

    }

And the output is

    Nooflines_Real: 40
    size of X 2, test 500 and size of xd_real_send 2

Could you please tell what Am I doing wrong.

Don't cast the result of malloc or calloc.

Kerrek SB
– Kerrek SB

2012-11-15 16:47:11 +00:00
Commented Nov 15, 2012 at 16:47 — Kerrek SB
– Kerrek SB, Commented Nov 15, 2012 at 16:47
sizeof(X) is the sizeof(int*).

hmjd
– hmjd

2012-11-15 16:48:33 +00:00
Commented Nov 15, 2012 at 16:48 — hmjd
– hmjd, Commented Nov 15, 2012 at 16:48
Answers: stackoverflow.com/a/605858/694576 @dreamcrash

alk
– alk

2012-11-15 16:53:13 +00:00
Commented Nov 15, 2012 at 16:53 — alk
– alk, Commented Nov 15, 2012 at 16:53

alk · Accepted Answer · 2012-11-15 16:53:52Z

1

X and xd_real_send are defined as pointers.

The sizeof operator applied returns the amount of memory use by the pointer, not the size of what the pointer refers to.

It not possible (in any portable way) to request the size of a memory block once allocated dynamically and refered by some pointer.

For dynamically allocated memory the application needs to take care of keeping track of how large those memory blocks are.

test is defined expliciltly as an array, so sizeof is able to determine the array's size.

edited Nov 15, 2012 at 16:53

answered Nov 15, 2012 at 16:48

alk

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3 Comments

rkrara Over a year ago

Could you please tell how can i check if array of size 40 is created for xd_real_send.

alk Over a year ago

Test ((Nooflines_Real == 40) && (NULL != (xd_real_send = calloc(Nooflines_Real, sizeof(float)))) @user1733911

rkrara Over a year ago

if((Nooflines_Real == 40) && (NULL != (xd_real_send = calloc(Nooflines_Real, sizeof(float))))){ printf("ok"); } else { printf("not ok"); } output was "not ok"

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