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how to find the size allocated by malloc

Discussion in 'C' started by naveenrai88, Jul 20, 2008.

  1. naveenrai88

    naveenrai88 New Member

    following is the code that i wrote: :happy:
    #include <stdio.h>
    int main()
            int *ptr;
            ptr = (int *)malloc(256*256*256*256-1);
            printf("Sizeof int is %d\n", sizeof(int));
            if(ptr == NULL)
                    printf("Allocation failed");
                    printf("Allocation succedded");
            return 1;
    i ran this code on gcc complier. now the warning that i get is
    try.c: In function ‘main’:
    try.c:6: warning: incompatible implicit declaration of built-in function ‘malloc’
    try.c:6: warning: integer overflow in expression
    try.c:6: warning: integer overflow in expression

    Q 1) since the sizeof (int) =4 so why integer overflow warning is being displayed???
    Q 2) how can i calculate the size of the memory chunck allocated by the malloc function????
    ( if i write "ptr = (int *)malloc(256*256*256*256+1000);" even though, i get these warnings and get the output of the program as "Allocation succedded: ):surprised
    Kindly explain why am i getting the output as mentioned above even though the value mentioned in malloc is overflowing the unsigned int range.. :cryin:

    thanks in advance
    Last edited by a moderator: Jul 20, 2008
  2. xpi0t0s

    xpi0t0s Mentor

    Why do you want to allocate 4GB of RAM? Are you on a 64-bit system?
    Think about how you might represent 256*256*256*256+1000 in 32 bits.
    Also think about how 256*256*256*256-1 might be represented, and what the value of the MSB will be, and what malloc will do if it's asked to allocate a negative amount of memory.

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