pointers to pointers

Atomical · #1 30-Jul-2005, 18:07

I've run this through the debugger and the problem I continue to have is that the pointers inside my function are not passed back to the main program. I took screenshots of it to you show you what I mean.

harddrivecaddy.com

After I learned about pointers to pointers I rewrote my function but got the same result. I've tried a hundred variations but all return the same result. Here's my code:

CPP / C++ / C Code:

int hostfromurl ( char *url, char **getpath ,int& port,char **host, char **query) {

string s_url = reinterpret_cast<const char*>(url);

url_string foo(s_url);

if ( foo.get_scheme() != "http" )
return -1;


//alright, so it works inside this function
host = (char **)tochar(foo.get_net_path());

}

char ** tochar ( string s ) {
char **retptr;
retptr = (char **)malloc(sizeof(char)*2000);
char *t = strdup (s.c_str() );
*retptr = t;
return retptr;
}

WaltP · #2 30-Jul-2005, 19:06

Please read the Guidelines and pay close attention to guideline #1.

You really need to explain what you're trying to do because without the background of what and why, the how is unknowable.

It looks like you have some serious memory leakage if you ever do get this to work the way you're going. Using a convoluted pointer to pointer hodgepodge like this can cause a lot of grief in the long run.

Also, because strdup() is non-standard it's compiler-dependant. Information I have is it already does a malloc() and you must remember to free it before you lose track of the buffer it creates. You probably should not use functions that internally malloc buffers because it's easy to lose control of your memory.

Atomical · #3 30-Jul-2005, 19:25

My whole program is written with character arrays. The opensource URL class I downloaded is written with the string library. I'm trying to convert the string values to character arrays.

WaltP · #4 30-Jul-2005, 19:56

Quote:

Originally Posted by Atomical

My whole program is written with character arrays. The opensource URL class I downloaded is written with the string library. I'm trying to convert the string values to character arrays.

This tells us nothing. What is the code supposed to do? What is the purpose for this code's existence?

Zorachus · #5 30-Jul-2005, 20:02

I don't know much about pointers, but the following code does what you are trying to do, as long as the string is not more than one line long. As I do not know the commands to create files in c++, I used the system command.

CPP / C++ / C Code:

//*Converts string Example to char converted[100] without pointers*//
#include <iostream>
#include <cstdlib>
#include <fstream>
#include <cstring>
using namespace std;
int main()
{
string example="Example text";
char converted[100];
system ("echo a > String.txt");
fstream file ("String.txt");
cout << "Input a string value: ";
cin >> example;
file << example << endl;
file.close();
fstream readfile ("String.txt");
readfile.getline (converted,100);
file.close();
cout << converted << endl;
system ("pause");
return 0;
}

Atomical · #6 30-Jul-2005, 20:39

Quote:

Originally Posted by WaltP

This tells us nothing. What is the code supposed to do? What is the purpose for this code's existence?

The code's purpose is to parse a URL and update the host, get, and query pointer arguments. Then I'm going to use that data to create a socket and perform a request to a web server. I want this function to act as a wrapper to the string class.

Class is here: freshmeat.net

WaltP · #7 30-Jul-2005, 20:55

Quote:

Originally Posted by Atomical

The code's purpose is to parse a URL and update the host, get, and query pointer arguments. Then I'm going to use that data to create a socket and perform a request to a web server. I want this function to act as a wrapper to the string class.

Class is here: http://freshmeat.net/projects/liburl/

It does? I don't see any parsing what-so-ever. All I see is 2000 pointers to pointers being malloc()ed and one string being duplicated and returned somewhere in this buffer of pointers.

Let me rephrase the question. What is

CPP / C++ / C Code:

char ** tochar ( string s ) 
{
    char **retptr;
    retptr = (char **)malloc(sizeof(char)*2000);
    char *t = strdup (s.c_str() );
    *retptr = t;
    return retptr;
}

supposed to be doing? Given the string s, what does it return? This is after all what you asked about...

Atomical · #8 30-Jul-2005, 21:13

Quote:

Originally Posted by WaltP

It does? I don't see any parsing what-so-ever. All I see is 2000 pointers to pointers being malloc()ed and one string being duplicated and returned somewhere in this buffer of pointers.

Let me rephrase the question. What is

CPP / C++ / C Code:

char ** tochar ( string s ) 
{
    char **retptr;
    retptr = (char **)malloc(sizeof(char)*2000);
    char *t = strdup (s.c_str() );
    *retptr = t;
    return retptr;
}

supposed to be doing? Given the string s, what does it return? This is after all what you asked about...

string -> const char * -> char *
I want it to return a pointer to the converted string that is now a character array. In other words, takes a string and returns a char.

WaltP · #9 30-Jul-2005, 21:38

So just use the c_str() method in the main routine and forget all this pointer business. if foo.get_net_path() returns a string type

CPP / C++ / C Code:

foo.get_net_path().c_str();

should return a character buffer. Use strcpy() to put it where you want it. Simple is better.

Or better yet, find a character version of the function in C instead of using the C++ string version.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Rate This Thread
Rate This Thread:

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Help with an array using pointers	glulu76	C++ Forum	1	01-May-2005 11:21
[Tutorial] Pointers in C (Part II)	Stack Overflow	C Programming Language	0	27-Apr-2005 17:36
array of pointers to ......	kai85	C++ Forum	1	18-Apr-2005 17:06
[Tutorial] Pointers in C (Part I)	Stack Overflow	C Programming Language	1	08-Apr-2005 18:35
Help with C++ pointers	Mjkramer21	C++ Forum	23	18-Apr-2004 07:53