Question

I have this piece of code

#include <stdio.h> #include <stdlib.h> #include <stdint.h> #include <string.h>  int main(){     void *a, *b;      a = malloc(16);     b = malloc(16);     printf('\n   block size (for a): %p-%p : %li', b, a, b-a);      a = malloc(1024);     b = malloc(1024);     printf('\n   block size (for a): %p-%p : %li', b, a, b-a);   } 

Shouldn’t this print the last allocated block size (16 or 1024)? It instead prints 24 and 1032, so the amount of memory allocated seems to have 8 extra bytes.

My problem is (before making this test case) that I do malloc() in a function (1024 bytes), and return the allocated result. When checking the block size on the function return I get 516 blocks… and I don’t understand why. I guess this might be the reason for the memory corruption that occurs after doing some processing on the allocated buffers:)

Edit: I’ve seen How can I get the size of an array from a pointer in C? and seems to ask the same thing, sorry for reposting.

I’ve redone my example to my more specific code:

#include <stdio.h> #include <stdlib.h> #include <stdint.h> #include <string.h>  short int * mallocStuff(long int number, short int base){     short int *array;     int size=1024;      array=(short int*)calloc(1,size);     //array=(short int*)malloc(size);      return array; }  int main(){     short int **translatedArray;      translatedArray=malloc(4*sizeof(short int));      int i;     for(i=0;i<4;i++){         translatedArray[i]=mallocStuff(0,0);          if(i>0)             printf('\n   block size (for a): %p-%p : %i',                 translatedArray[i], translatedArray[i-1], translatedArray[i]-translatedArray[i-1]);     }      return 0; } 

And the output is

   block size (for a): 0x804a420-0x804a018 : 516    block size (for a): 0x804a828-0x804a420 : 516    block size (for a): 0x804ac30-0x804a828 : 516 

According to the above post that is bigger than 1024. Am I wrong?

Answer 1

You have a bug. Instead of:

translatedArray=malloc(4*sizeof(short int)); 

You should have

translatedArray=malloc(4*sizeof(short int*)); 

Note the missing pointer in your code. I suspect this is where your observed behavior stems from.

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Also notice that 0x804a420 - 0x804a018 = 1032, not 516. The formula translatedArray[i] - translatedArray[i - 1] gives you the number of elements (short ints, or more simply, shorts) in between the two addresses, not the number of bytes.