Problem passing pointers between functions

This is a discussion on Problem passing pointers between functions within the C Programming forums, part of the General Programming Boards category; Hey guys, I'm obviously somewhat new to C and am having problems passing pointers between functions in two different .c ...

  1. #1
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    Angry Problem passing pointers between functions

    Hey guys, I'm obviously somewhat new to C and am having problems passing pointers between functions in two different .c files.

    Basically, I have this in main.c:

    ...
    printf("Address of jump: %d\n", &jump);
    store(&jump, address, op);
    ...

    and in table.c I have:

    void store(table *tab, int val, char* k)
    {
    printf("Address of received table: %d\n", &tab);
    ...

    jump itself is declared in a global header file. It is a typedef struct.

    When I run the program, I get this:
    Address of jumpTable: 17669248
    Address of received table: 3341436

    Why on Earth are the addresses being pointed to changing when I pass them to different functions? I've been looking around everywhere I can for answers but have not been able to find any. Please help me out!

    * Obviously I've removed most of my program to focus on what (I think) are the relevant bits. If more info is needed, please just let me know.

  2. #2
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    Because jump is already a pointer and you're passing in IT's address not it's contents...

    Code:
    store(jump,address,op);

  3. #3
    Programming Wraith GReaper's Avatar
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    Firstly you print the address of jump, and then print the address of tab, not *tab.
    Devoted my life to programming...

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    What I'd like to do is pass the address of jump so the store function can modify it. I tried removing the ampersand in

    Code:
    store(jump,address,op);
    as well as the corresponding splat in

    Code:
    void store(table tab, int val, char* k)
    ...but I still get different address'.

  5. #5
    Registered User gardhr's Avatar
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    Remember, in C even pointers are passed by value, so by taking the address of a pointer passed to a function you are actually getting the address of a local copy of that pointer. The solution is to pass a pointer to a pointer.

    Code:
    void reset(int** ptr, int* adr)
    {
    	*ptr = adr;
    }
    
    int main(void)
    {
    	int a = 1024, b = 512, *p = 0;
    	printf("Address of 'a': %x\n", (unsigned)&a);
    	printf("Value of 'a':   %d\n", a);
    	printf("Address of 'b': %x\n", (unsigned)&b);
    	printf("Value of 'b':   %d\n", b);
    	puts("reset: p -> a");
    	reset(&p, &a);
    	printf("Value of 'p':   %x\n", (unsigned)p);
    	printf("Value of '*p':  %d\n", *p);	
    	puts("reset: p -> b");
    	reset(&p, &b);
    	printf("Value of 'p':   %x\n", (unsigned)p);
    	printf("Value of '*p':  %d\n", *p);		
    }
    Last edited by gardhr; 09-17-2011 at 09:46 PM.

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    Quote Originally Posted by Civus View Post
    What I'd like to do is pass the address of jump so the store function can modify it. I tried removing the ampersand in

    Code:
    store(jump,address,op);
    as well as the corresponding splat in

    Code:
    void store(table tab, int val, char* k)
    ...but I still get different address'.
    Nope... you should have laft it in the store function...

  7. #7
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    Aha, thanks for the help guys. I guess it's just taking me a while to get used to this pass-by-value convention. Thanks again!

  8. #8
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    Quote Originally Posted by Civus View Post
    Aha, thanks for the help guys. I guess it's just taking me a while to get used to this pass-by-value convention. Thanks again!
    Coming from a Turbo Basic and Pascal background I *really hated* pointers when I first started with C ... But once you get the hang of it, it's actually quite simple. Just remember... in C everything is a number and everything passed into a function is a copy of a number.

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