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how this code works

P: 1
I'm workig on memory allocation and trying to understand this code.

output prints
d=149909512 - How is d getting these numbers?
i=5, d[i]135145.
how can d have these numbers if malloc returns the address of the beginning of allocated memory or NULL if it fails.

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  1. #include<stdio.h>
  3. int main(void)
  4. {
  5.     int i, *d;
  6.         /*printf("%d\n %d",i,*d);*/
  7.     d =  malloc( 5 * sizeof(int) );
  8.     printf("i=%d\n d=%d\n *d=%d\n",i,d,*d);
  10.     for(i = 0; i < 5; i++)
  11.     d[i] = i + 100;
  12.     printf("i=%d\n d[i]%d",i,d[i]);
  14. }
Jul 12 '16 #1
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2 Replies

Expert Mod 5K+
P: 9,197
The for loop exits with i == 5. When i is 4 the ++i has occurred before i < 5 is tested.

I suggest a pair of braces around the loop so both the assignment and printf are inside the loop
Jul 12 '16 #2

Expert 100+
P: 2,396
You are using the "%d" format specifier to print the value of d. That format specifier should only be used with int arguments. Use "%p" for pointers.

You should test the value of d and be careful not to dereference it if the value is NULL.

Your first printf prints the value of *d before it has been initialized. The value of an uninitialized variable is unpredictable. You can avoid this problem by using calloc instead of malloc.
Jul 15 '16 #3

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