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#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <stdbool.h>
#include <fcntl.h>
#include <stdlib.h>
#include <arpa/inet.h>
#include <netinet/in.h>
// BEEJ NETWORKING 5.1
int main (int argc, char *argv[]) {
if (argc > 2 || argc == 1) {
fprintf(stderr, "Too many or not enough arguments! \n");
fprintf(stderr, "USAGE: getip [DOMAIN]\n");
exit(1);
}
char ipstr[INET_ADDRSTRLEN]; // We store the IP address in text form here. INET_ADDRSTRLEN is the length of ipv4 addresses.
int status;
struct addrinfo hints; // Struct where we put information in to pass it to getaddrinfo.
struct addrinfo *res; // The result of getaddrinfo. Not sure why its a pointer but ill figure that out later
struct addrinfo *rescpy; // Another rescpy struct pointer. This one is used to point to the next element when iterating in a for loop.
memset(&hints, 0, sizeof hints); // Set all values to 0 in hints.
hints.ai_family = AF_INET; // Add the necessary info to ai_family, ai_socktype, ai_flags.
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
status = getaddrinfo(argv[1], "62222", &hints, &res); // get addrinfo. Address to hints and res passed.
if (status != 0) {
fprintf(stderr, "Error: %s\n", gai_strerror(status));
exit(1);
}
// rescpy = res. Both point to the same result.
// rescpy != NULL. If its NULL then the next struct in the linked list is the last one.
// rescpy = rescpy->ai_next. After it did the loop it goes back, checks if its null. If not then rescpy is assigned the pointer to ai_next. And so on and so on.
// A visual representation:
//
// res --> GAI_RESULT_STRUCT (res points to the result addrinfo struct)
// rescpy = res (BOTH POINT TO GAI_RESULT_STRUCT)
// IF rescpy is not NULL
//
// rescpy = rescpy --> ai_next (Its easier for me to imagine this line with some different syntax: rescpy = rescpy.ai_next. This is technically not correct but this is for my eyes only.)
// ai_next is a pointer to another addrinfo struct. Which is the next item in the linked list. rescpy = rescpy --> ai_next, this basically means we access the ai_next member
// which is inside GAI_RESULT_STRUCT. We basically make rescpy point to the next item.
//
//
for(rescpy = res ; rescpy != NULL ; rescpy = rescpy->ai_next) {
struct sockaddr_in *sa1 = (struct sockaddr_in *)rescpy->ai_addr; // struct sockaddr_in sa1 pointer assigned ai_addr. sa1 now points to ai_addr.
// ai_addr may be a sockaddr_in or a sockaddr_in6.
// We cast it to sockaddr_in which is supposed to just work.
void *addr = &(sa1->sin_addr); // Address of sin_addr. sa1 now points to ai_addr so we access the item sin_addr and make addr point to it.
inet_ntop(rescpy->ai_family, addr, ipstr, rescpy->ai_addrlen);
fprintf(stdout, "%s\n", ipstr);
}
freeaddrinfo(res);
return 0;
}
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