the function
int inet_pton(int af, const char *src, void *dst);
is declared in header file:
#include <arpa/inet.h>
if this is Windows (Vista or later) there is Winsock analog to this ANSI version:
INT WSAAPI InetPton(
_In_ INT Family,
_In_ PCTSTR pszAddrString,
_Out_ PVOID pAddrBuf
);
try #include <Ws2tcpip.h>
add Ws2_32.lib