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my goal is to successfully call the function inet_pton in C. I am using Windows 7. So far I learned that inet_pton has 3 arguments. Besides, its job is to translate human IP representation into binary representation for the computer.

Details on my understanding of the arguments of inet_pton(a,b,c):

a: An address_family. For example AF_INET

b: A pointer to the human readable representation of the IP address (e.g."127.0.0.1").

c: A pointer to the "thing" where the translated IP address (binary form) is stored.

In a tutorial I read it is said inet_pton will store the translation inside the array c points to.

This page

https://msdn.microsoft.com/de-de/library/windows/desktop/cc805844(v=vs.85).aspx

tells me, that the third argument has the type PVOID.

INT WSAAPI inet_pton(__in   INT  Family,__in   PCTSTR pszAddrString,__out  PVOID pAddrBuf);

In my opinion the receiving array should be an char array. But PVOID is not equal to char. The last idea I have is that a cast is involved, though I am only familiar with a cast in a situation like 

buffer = (char*) malloc (i+1);

At the moment my code Looks like this:

#include <stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <windows.h>
#include <Winsock2.h>
#include <Ws2tcpip.h>
int main(int argc, char *argv[])
{
    char *ptr2="127.0.0.1";
    char buffe[512];
    PVOID *ptr3;
    PVOID ptr3=&buffe;
    inet_pton(AF_INET,ptr2,ptr3);
    system("PAUSE");    
    return 0;
}

How can I initialize the 3rd argument correctly (if it's necessary)? How to deal with the fact that the array storing the binary representation of the IP address should have the type char while the pointer should have the type PVOID?

My compiler is complaining about conflicting types for ptr3. Thank you so much for your help.

indiv
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Heilmann
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    Are you using a C++ compiler by accident? Conversions to and from `void *` don't require a cast in C. – Carl Norum Jan 22 '15 at 18:59
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    Just curious, what is `InetPton` in `c`(in that particular CaSe)? – Sourav Ghosh Jan 22 '15 at 19:00
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    The page you linked says that the object pointed to by the third argument `pAddrBuf` must be large enough to hold one of two possible data structures, depending on the first argument. So I suggest you follow that page's links to those structure definitions. – Weather Vane Jan 22 '15 at 19:10
  • @norum: I am using DEV C++. I believe it comes with a C++ compiler, but who knows^^ – Heilmann Jan 23 '15 at 21:30
  • @Gosh: Can not tell you yet, because I have not yet successfully called inet_pton. so sorry :( – Heilmann Jan 23 '15 at 21:32

1 Answers1

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PVOID is a typedef for void * which means that the third argument expects a generic pointer. PCTSTR is ultimately typedefed to either const char * (if UNICODE is not defined) or const wchar_t * (if UNICODE is defined). See Windows Data Types for more information on those specific types.

Instead of trying to figure out how many bytes to allocate using malloc() (hint: sizeof(IN_ADDR)), you can just directly use the data type it expects: IN_ADDR. According to the documentation, the third argument pAddrBuf needs to be large enough to hold an IN_ADDR when the first argument Family is AF_INET.

Try doing something like this:

#include <WinDef.h>
#include <Winsock2.h>
#include <Ws2tcpip.h>

int main(int argc, char *argv[]) {
    char ip_string[] = "127.0.0.1";
    IN_ADDR ip_value;
    inet_pton(AF_INET, ip_string, &ip_value);
    system("PAUSE");    
    return 0;
}
Uyghur Lives Matter
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  • Thanks for the answer. Is the code supposed to not compile? Because I got an error message that there is an undefined reference to inet_pton. – Heilmann Jan 23 '15 at 21:42
  • @Heilmann I guessed at the includes from the documentation. I don't have a Windows machine to test the compilation. Try using the includes you provided in your question. Otherwise, ['inet_pton': identifier not found](http://stackoverflow.com/q/15660203/369450) might help. – Uyghur Lives Matter Jan 24 '15 at 14:59