Problem passing pointer to function

**voidpain()** · 08-10-2011

I am trying to access information within a structure. I am trying to do this by passing a pointer to the structure into a function. The function is declared like so:

Code:

void areaItems(Ptr *sPtr); //typedef Ptr is a pointer to item structure

I am then calling on the function like so:

Code:

areaItems(&startPtr); //is this the right way to pass in the pointer to the first structure in my linked list???

Hopefully I haven't screwed anything up yet.

Here is the code for void areaItems:

Code:

void areaItems(Ptr *sPtr)
{
   printf("%s\n", sPtr->name);
}

...This is clearly not working....but why?

**voidpain()** · 08-10-2011

Also, my sincere apologies for all the questions as of late (and for those of you who've had to repeat yourself throughout my mistakes). This forum has been the best help I've gotten thus far

**quzah** · 08-10-2011

Please tell me you didn't make 'Ptr' a pointer using a typedef? I think I'm going to put you on ignore now.

Quzah.

**voidpain()** · 08-10-2011

Yes, someone else mentioned this earlier. I actually got this straight out of my textbook. I'd be happy not to do it again, but I've been on the same assignment since I've joined the forum. I'm having trouble understanding all of the elements of the assignment, and would be glad to not do the same thing for future assignments once I grasp what the hell is going on in this assignment in my book. (The book is C: How to Program by Deitel)

**AndrewHunter** · 08-10-2011

Because of the typedef pointers. My guess is your function prototype is really areaItems(struct node** sPtr); since Ptr is typedef as a pointer. Hence why you shouldn't do it. You just need to pass the pointer, e.g. like this.

Code:

#include <stdio.h>
#include <stdlib.h>

struct myExample{
	int x;
};

void foo(struct myExample*);

int main(void){

	struct myExample* foobar = malloc(sizeof(*foobar));

	foo(foobar);

	printf("%d", foobar->x);

	free(foobar);
	getchar();
	return(0);
}
void foo(struct myExample* fooPTR){

	fooPTR->x = 5;
}

**TheBigH** · 08-10-2011

If startPtr is a pointer to something, then passing your function the address of startPtr is not going to work. What you're then passing the function is a pointer to a pointer.

Try just

Code:

areaItems(startPtr);

And I agree with quzah. You've been strongly advised many times NOT to typedef pointers. Why are you still doing it?

**voidpain()** · 08-10-2011

TheBigH, see post #4. I'm not trying to ........ anyone off, haha, I really did get this out of my textbook, and I already said I wasn't going to do this again, not to mention apologizing for it (post 2) I've literally been working on the same assignment since I've joined the forum.

Would this declaration work the same?

Code:

areaItems(Item *startPtr); //where Item is the typedef of the item structures?

**AndrewHunter** · 08-10-2011

Yes and then just pass your Item pointer to the function directly, as I showed in the above example. (post #5)

**quzah** · 08-10-2011

When in doubt, test it out:

Code:

#include<stdio.h>
struct foo 
{
    int bar;
};
void bar( struct foo *baz )
{
    if( baz != NULL )
        printf( "baz->bar = %d\n", baz->bar );
}
int main( void )
{
    struct foo d = { 10 }, *l = &d;
    bar( &d );
    bar( l );
    return 0;
}

Quzah.

**voidpain()** · 08-10-2011

Thank you Andrew and quzah (posts 8 and 9), trying now

**voidpain()** · 08-10-2011

Code:

void areaItems (int area, Item *sPtr); //I declare it

//...later on

areaItems(current, startPtr); //I call it

void areaItems(int area, Item *sPtr)
{
   printf("%s\n", sPtr->name);
}

Everything seems to be working correctly, but the print statement seems to print complete garbage. Are there any other errors that would cause this in the above code, or is my problem likely elsewhere?

thanks

**AndrewHunter** · 08-10-2011

Assuming startPtr is a Item*, then my guess is you didn't fix your problem from your other post. That would be you left the name member defined as a char* and then didn't allocate any memory for it. I warned you about this in your other thread.

**quzah** · 08-10-2011

You have been given multiple clear working examples now. Learn from them, and post your code in a small complete example of what you are trying to do.

Quzah.

**AndrewHunter** · 08-10-2011

So you would need to have something like this: (NOTE: inside struct is a char array not a char*, also that strcpy needs to be used to place a value in there).

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct myExample{
	char name[30];
	int x;
};

void foo(struct myExample*);

int main(void){

	struct myExample* foobar = malloc(sizeof(*foobar));

	foobar->x = 5;
	strcpy(foobar->name, "John");

	foo(foobar);

	

	free(foobar);
	getchar();
	return(0);
}
void foo(struct myExample* fooPTR){

	printf ("%d\n%s", fooPTR->x , fooPTR->name);
}

**voidpain()** · 08-10-2011

You were right, that's what was wrong. I am going to have to gut a lot of my code because I am making these same mistakes in multiple places. =/

At the end of your code, what is the getchar();? Is that just holding the program waiting for a keystroke...or something?

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