Here’s a simple example using gethostbyname() that emulates the host command.
I wrote it to debug a DNS problem. On the network I’m on right now, gethostbyname() returns a different answer than host and dig. It is overaggressively caching other hostnames. i.e. nsloookup shrub.ca is returning 17.250.248.64, which is mail.mac.com (!).
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv) {
if (argc < 2) {
printf("Usage: %s hostname", argv[0]);
exit(-1);
}
struct hostent *hp = gethostbyname(argv[1]);
if (hp == NULL) {
printf("gethostbyname() failed\n");
} else {
printf("%s = ", hp->h_name);
unsigned int i=0;
while ( hp -> h_addr_list[i] != NULL) {
printf( "%s ", inet_ntoa( *( struct in_addr*)( hp -> h_addr_list[i])));
i++;
}
printf("\n");
}
}
Thank you for the example Paul!
Oops hit the return button to quickly
3) exit (-1) doest not make much sense, what you get is the same as if you had written exit(255), since -1 representation is 0xffffffff in general on a 32bit system and the status returned to the parent process is truncated to the last 8 bits, exit(1) is what is used in general
To summarize all those cosmetics changes I would recommend the following:
/* for inet_ntoa() */ #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #include <netdb.h> #include <stdlib.h> #include <stdio.h> int main(int argc, char **argv) { if (argc < 2) { fprintf(stderr,"Usage: %s hostname\n", argv[0]); exit(1); } struct hostent *hp = gethostbyname(argv[1]); if (hp == NULL) { fprintf(stderr,"gethostbyname() failed\n"); exit(1); } else { printf("%s = ", hp->h_name); unsigned int i=0; while ( hp -> h_addr_list[i] != NULL) { printf( "%s ", inet_ntoa( *( struct in_addr*)( hp -> h_addr_list[i]))); i++; } printf("\n"); exit(0); } }Thank you, I used this for Windows, for those, who will find this page, here is what you need to get it working under Windows
[code]
#include
#include
WSADATA wsaData;
WORD version;
int error;
version = MAKEWORD( 2, 0 );
error = WSAStartup( version, &wsaData );
if (argc < 2) {
printf("Usage: %s hostname", argv[0]);
exit(-1);
}[/code]
i need the program for the gethostbyname
The code for Windows (based on previous responses to this post) is below. Remember to add -lws2_32 to the linker arguments. I compiled using MinGW using Eclipse on Windows 7.
#include <stdio.h>
#include <winsock2.h>
WSADATA wsaData;
WORD version;
int error;
int main(int argc, char **argv) {
if (argc < 2) {
fprintf(stderr,"Usage: %s hostname\n", argv[0]);
exit(1);
}
struct hostent *hp = gethostbyname(argv[1]);
if (hp == NULL) {
fprintf(stderr,"gethostbyname() failed\n");
exit(1);
} else {
printf("%s = ", hp->h_name);
unsigned int i=0;
while ( hp -> h_addr_list[i] != NULL) {
printf( "%s ", inet_ntoa( *( struct in_addr*)( hp -> h_addr_list[i])));
i++;
}
printf("\n");
exit(0);
}
}
Ignore my previous post – I missed a bit out
The code for Windows (based on previous responses to this post) is below. Remember to add -lws2_32 to the linker arguments. I compiled using MinGW using Eclipse on Windows 7.
#include <stdio.h>
#include <winsock2.h>
WSADATA wsaData;
WORD version;
int error;
int main(int argc, char **argv) {
version = MAKEWORD( 2, 0 );
error = WSAStartup( version, &wsaData );
if (argc < 2) {
fprintf(stderr,"Usage: %s hostname\n", argv[0]);
exit(1);
}
struct hostent *hp = gethostbyname(argv[1]);
if (hp == NULL) {
fprintf(stderr,"gethostbyname() failed\n");
exit(1);
} else {
printf("%s = ", hp->h_name);
unsigned int i=0;
while ( hp -> h_addr_list[i] != NULL) {
printf( "%s ", inet_ntoa( *( struct in_addr*)( hp -> h_addr_list[i])));
i++;
}
printf("\n");
exit(0);
}
}
Hi Kevin,
You mentioned “Remember to add -lws2_32 to the linker arguments. I compiled using MinGW using Eclipse on Windows 7”
I selected the project -> Properties -> C/C== Building -> Settings -> MinGW C++ Linker -> Libraries , then at Libraries (-l) I typed “ws2_32”, then OK
When I built project I got an errors “cannot find -lws2_32” , I wonder what did I miss?
Thanks,
Yin