Question about passing pointers

This is a discussion on Question about passing pointers within the C Programming forums, part of the General Programming Boards category; Can someone tell me what I am doing wrong? I want to pass a NULL pointer to a function which ...

  1. #1
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    Question about passing pointers

    Can someone tell me what I am doing wrong? I want to pass a NULL pointer to a function which will allocate some memory to it and assign it values. Then I want to be able to access the values outside of the function. This is probably an easy fix, but the solution is escaping me.


    Code:
    void test(int* tmp);
    
    void main(void)
    {
          int* data = NULL;
    
          test(data);
    
          printf("%d %d\n",data[0], data[1]);
    }
    
    void test(int* tmp)
    {
          tmp = (int *)malloc(sizeof(int)*2);
    
          *tmp[0] = 1;
          *tmp[1] = 2;
    }

  2. #2
    ZuK
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    Your function receives and modifies a copy of the original pointer. You have to pass a pointer to a pointer
    Code:
    void test(int** tmp);
    
    void main(void)
    {
          int* data = NULL;
    
          test(&data);
    
          printf("%d %d\n",data[0], data[1]);
    }
    
    void test(int** tmp)
    {
          *tmp = malloc(sizeof(int)*2);  // don't cast
    
          (*tmp)[0] = 1;   // this dereferences twice
          (*tmp)[1] = 2;
    }
    Kurt

    EDIT: don't cast the returnvalue of malloc. Look in the FAQ.
    Last edited by ZuK; 04-21-2006 at 09:50 AM.

  3. #3
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    And another point: main always returns an int. Check the faq for the reason.

  4. #4
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    or you can return the pointer value from the function:
    Code:
    int* test(void);
    
    int main(void)
    {
          int* data = NULL;
    
          data = test();
    
          printf("%d %d\n",data[0], data[1]);
          return 0;
    }
    
    int* test(void)
    {
          int *tmp = malloc(sizeof(int)*2);
    
          tmp[0] = 1;
          tmp[1] = 2;
          return tmp;
    }

  5. #5
    Frequently Quite Prolix dwks's Avatar
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    You should free your malloc()d memory.
    dwk

    Seek and ye shall find. quaere et invenies.

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  6. #6
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    Quote Originally Posted by bithub
    or you can return the pointer value from the function:
    Code:
    int* test(void);
    
    int main(void)
    {
          int* data = NULL;
    
          data = test();
    
          printf("%d %d\n",data[0], data[1]);
          return 0;
    }
    
    int* test(void)
    {
          int *tmp = malloc(sizeof(int)*2);
    
          tmp[0] = 1;
          tmp[1] = 2;
          return tmp;
    }

    thankyou bithub.. this helped me a lot

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