How to Get the Local Computer's IP Address and Subnet Mask in C ?

Determining Local Computer's IP Address and Subnet Mask in C

Acquiring the local computer's IP address and subnet mask is essential for various networking operations. In C , this process can be accomplished by leveraging platform-specific system calls.

1. Unix/MacOS

For Unix and macOS systems, the getifaddrs function allows retrieval of a linked list of network interface addresses. Iterating through this list provides access to each interface's IP address, subnet mask, and other related information:

#include <sys/types.h>
#include <ifaddrs.h>

struct ifaddrs *ifaddr;
int s;

if ((s = getifaddrs(&ifaddr)) == -1) {
    // Error handling
}

for (struct ifaddrs *ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
    if (ifa->ifa_addr->sa_family == AF_INET) {
        char addr[INET_ADDRSTRLEN];
        inet_ntop(AF_INET, &((struct sockaddr_in *)ifa->ifa_addr)->sin_addr, addr, INET_ADDRSTRLEN);
        printf("IP Address: %s\n", addr);

        char netmask[INET_ADDRSTRLEN];
        inet_ntop(AF_INET, &((struct sockaddr_in *)ifa->ifa_netmask)->sin_addr, netmask, INET_ADDRSTRLEN);
        printf("Subnet Mask: %s\n", netmask);
    }
}

2. Windows

For Windows systems, the GetAdaptersAddresses function provides a similar functionality. This function fills a buffer with information for all network adapters on the local computer:

#include <iphlpapi.h>

PIP_ADAPTER_ADDRESSES addresses;
ULONG outBufLen = 0;
DWORD res = GetAdaptersAddresses(AF_INET, 0, NULL, addresses, &outBufLen);

if (res != ERROR_SUCCESS) {
    // Error handling
}

for (PIP_ADAPTER_ADDRESSES adapter = addresses; adapter != NULL; adapter = adapter->Next) {
    char ip[MAX_PATH + 1];
    InetNtopA(AF_INET, &adapter->FirstUnicastAddress->Address.Ipv4.sin_addr, ip, MAX_PATH + 1);
    printf("IP Address: %s\n", ip);

    char netmask[MAX_PATH + 1];
    InetNtopA(AF_INET, &adapter->FirstUnicastAddress->NetMask.Ipv4.sin_addr, netmask, MAX_PATH + 1);
    printf("Subnet Mask: %s\n", netmask);
}

By following these steps, you can retrieve both the IP address and subnet mask for the local computer in C , allowing you to establish network connections and perform various communication operations effectively.

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