New to c, help with (void *)

new to c · 12-03-2008, 09:57 PM

Hi,

I'm pretty new to this, so bear with me.

I'm having a few problems accessing data in a void *. Basically, we had to convert a program that created and performed functions on a linked list of integers to one that used void *, and then could handle any data type.

This is part of my main.c (I'm running it redirecting the input to a file that contains a list of numbers..."main.x < numbers"):

Code:

int main() {
  int cur_size, *number_read;
  void *occurrences, *large_value, *large_key, **key_list;

  Table list= create();
  
  while (!feof(stdin)) {

    scanf("%d ", number_read);
    
    if (lookup(list, number_read, &occurrences) == 1) {
      
      delete(&list, number_read);
      insert(&list, number_read, occurrences);  /* needs to be occurrences + 1 */
      
    } else insert(&list, number_read, (void *)1);
  }

All of the functions it calls I'm pretty sure work, because they've worked in tests that I haven't written the main function. they work with strings, doubles, ints, etc.

My problem is with the insert function. First, is the "else insert(...)" call correct? It should be inserting a pointer to the integer value 1. Second, how do I send in a pointer to the integer value stored at *occurrences + 1?

The call to lookup(...) in the if will store a value in occurrences if it finds number_read in the list.

I've tried casting it to int *, then dereferencing it but I get a seg fault anytime I try to do that.

Any help is awesome...I know this post is probably extremely confusing but I don't have a better way to write it...

Thanks in advance!

slingerland3g · 12-03-2008, 10:07 PM

What does the prototype look like for your insert function call? Also passing variables like that is not best practice unless strictly for testing, you should actually be passing a variable and proper type. Also the real use of void* is to cast to a different type. If you happen to know that you need to cast the response from occurrences variable I would do that somewhere else, but not as a declaration.

new to c · 12-03-2008, 10:15 PM

Thank you so much for the quick reply!

The insert function looks like this:

Code:

int insert(Table *table, void *key, void *item)

So I should declare occurrences as int *?

C_ntua · 12-03-2008, 10:26 PM

I don't see the logic of (void*)1. A void* stores a memory address, as all pointers. So you are casting 1 to a memory address? Meaning that you the memory address with value 1?? That makes no sense. You would do this somewhere:

Code:

int one = 1;
void* p = &one;
insert(..., ..., p);

that would make sense

I dont think either this is correct

Code:

lookup(list, number_read, &occurrences)

I mean, won't you store how many occurrences there where? So you just need to pass a int*. Or a void* (for whatever reason). So you would pass occurrences (WITHOUT the &). You pass void**. But of course you know best, probably you want something else and I am getting it wrong.

new to c · 12-03-2008, 10:58 PM

Haha no I'm not sure why I haven't really questioned the (void *)1 thing yet...thank you for pointing that out to me.

And lookup requires void ** as it's third parameter...lookup doesn't know what type it is dealing with, and since I can't use void as a data type, I have to work with void *. I didn't explain that very well...

Ok so I made a couple changes and now it is running without any segfaults! Thank you for the help, if I can't get it running correctly soon I will definitely be back!

thanks

Elysia · 12-04-2008, 02:46 AM

Quote:

Originally Posted by C_ntua

I don't see the logic of (void*)1. A void* stores a memory address, as all pointers. So you are casting 1 to a memory address? Meaning that you the memory address with value 1?? That makes no sense. You would do this somewhere:

Code:

int one = 1;
void* p = &one;
insert(..., ..., p);

that would make sense.

...That would be a bad idea, as you should be aware of.
You are passing the address of a local variable which is then stored and later the local variable goes out of scope.

12-03-2008, 09:57 PM	#1
new to c Registered User Join Date: Dec 2008 Posts: 3	*New to c, help with (void )** Hi, I'm pretty new to this, so bear with me. I'm having a few problems accessing data in a void . Basically, we had to convert a program that created and performed functions on a linked list of integers to one that used void , and then could handle any data type. This is part of my main.c (I'm running it redirecting the input to a file that contains a list of numbers..."main.x < numbers"): Code: int main() { int cur_size, number_read; void occurrences, large_value, large_key, *key_list; Table list= create(); while (!feof(stdin)) { scanf("%d ", number_read); if (lookup(list, number_read, &occurrences) == 1) { delete(&list, number_read); insert(&list, number_read, occurrences); / needs to be occurrences + 1 / } else insert(&list, number_read, (void )1); } All of the functions it calls I'm pretty sure work, because they've worked in tests that I haven't written the main function. they work with strings, doubles, ints, etc. My problem is with the insert function. First, is the "else insert(...)" call correct? It should be inserting a pointer to the integer value 1. Second, how do I send in a pointer to the integer value stored at occurrences + 1? The call to lookup(...) in the if will store a value in occurrences if it finds number_read in the list. I've tried casting it to int , then dereferencing it but I get a seg fault anytime I try to do that. Any help is awesome...I know this post is probably extremely confusing but I don't have a better way to write it... Thanks in advance!
new to c is offline

12-03-2008, 10:07 PM	#2
slingerland3g Registered User Join Date: Jan 2008 Location: Seattle Posts: 476	What does the prototype look like for your insert function call? Also passing variables like that is not best practice unless strictly for testing, you should actually be passing a variable and proper type. Also the real use of void* is to cast to a different type. If you happen to know that you need to cast the response from occurrences variable I would do that somewhere else, but not as a declaration.
slingerland3g is offline

12-03-2008, 10:15 PM	#3
new to c Registered User Join Date: Dec 2008 Posts: 3	Thank you so much for the quick reply! The insert function looks like this: Code: int insert(Table table, void key, void item) So I should declare occurrences as int ?
new to c is offline

12-03-2008, 10:26 PM	#4
C_ntua Registered User Join Date: Jun 2008 Posts: 1,134	I don't see the logic of (void)1. A void stores a memory address, as all pointers. So you are casting 1 to a memory address? Meaning that you the memory address with value 1?? That makes no sense. You would do this somewhere: Code: int one = 1; void* p = &one; insert(..., ..., p); that would make sense I dont think either this is correct Code: lookup(list, number_read, &occurrences) I mean, won't you store how many occurrences there where? So you just need to pass a int. Or a void (for whatever reason). So you would pass occurrences (WITHOUT the &). You pass void**. But of course you know best, probably you want something else and I am getting it wrong.
C_ntua is offline

12-03-2008, 10:58 PM	#5
new to c Registered User Join Date: Dec 2008 Posts: 3	Haha no I'm not sure why I haven't really questioned the (void )1 thing yet...thank you for pointing that out to me. And lookup requires void * as it's third parameter...lookup doesn't know what type it is dealing with, and since I can't use void as a data type, I have to work with void *. I didn't explain that very well... Ok so I made a couple changes and now it is running without any segfaults! Thank you for the help, if I can't get it running correctly soon I will definitely be back! thanks
new to c is offline

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