Program to get IP Address in C

An Internet Protocol address (IP address) is a numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.

Algorithm

1. Create a character array called hostbuffer with a size of 256, a pointer to a character called IPbuffer, and a struct hostent pointer called host_entry.
2. Call the gethostname() function and store the result in a variable called hostname. Pass hostbuffer and its size as arguments to the function.
3. Check the value of hostname using the hostnamechk() function and exit the program if it is equal to -1.
4. Call the gethostbyname() function and pass hostbuffer as an argument. Store the result in host_entry.
5. Check the value of host_entry using the hostentrychk() function and exit the program if it is NULL.
6. Use the inet_ntoa() function to convert the Internet network address stored in host_entry to an ASCII string and store the result in IPbuffer.

Code

// C program to print IP address
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

// Returns hostname for the local computer
void hostnamechk(int hostname) {
    if (hostname == -1) {
        perror("gethostname");
        exit(1);
    }
}

// getting host information of the host name
void hostentrychk(struct hostent * hostentry) {
    if (hostentry == NULL) {
        perror("gethostbyname");
        exit(1);
    }
}

// Converting space-delimited IPv4 address
// to dotted-decimal format
void ipbufferchk(char *IPbuffer) {
    if (NULL == IPbuffer) {
        perror("inet_ntoa");
        exit(1);
    }
}

// Driver code
int main() {
    char hostbuffer[256];
    char *IPbuffer;
    struct hostent *host_entry;
    int hostname;

    hostname = gethostname(hostbuffer, sizeof(hostbuffer));
    hostnamechk(hostname);
    // To get host information
    host_entry = gethostbyname(hostbuffer);
    hostentrychk(host_entry);

    // Converting Internet network address to ASCII string
    IPbuffer = inet_ntoa(*((struct in_addr*)host_entry->h_addr_list[0]));
    printf("Host IP: %s", IPbuffer);
    return 0;
}

Output

Host IP: 192.168.0.1

Use of terms used

1. The <unistd.h> header defines miscellaneous symbolic constants and types, and declares miscellaneous functions.
2. <errno.h> is a header file in the standard library of the C programming language. It defines macros for reporting and retrieving error conditions using the symbol errno.
3. The <netdb.h> header shall define the hostent structure
4. The <netinet/in. h> header also defines the IN6ADDR_ANY_INIT macro. This macro must be constant at compile time and can be used to initialize a variable of type struct in6_addr to the IPv6 wildcard address. This variable is initialized by the system to contain the loopback IPv6 address.
5. <arpa/inet.h> – definitions for internet operations
6. The inet_ntoa() function converts the specified Internet host address to a string in the Internet standard dot notation.
7. perror is used in C and C++ to print an error message to stderr.