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.

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

Code:

store(jump,address,op);

Firstly you print the address of jump, and then print the address of tab, not *tab.

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'.

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);		
}

Originally Posted by Civus

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...

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!

Originally Posted by Civus

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.