Segmentation Fault

Question

Hi All,

I have the following piece of code for getting the hostname and IP address,

#include <stdlib.h>
#include <stdio.h>
#include <netdb.h> /* This is the header file needed for gethostbyname() */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>


int main(int argc, char *argv[])
{
struct hostent *he;

if (argc!=2){
printf("Usage: %s <hostname>\n",argv[0]);
exit(-1);
}

if ((he=gethostbyname(argv[1]))==NULL){
printf("gethostbyname() error\n");
exit(-1);
}

printf("Hostname : %s\n",he->h_name); /* prints the hostname */
printf("IP Address: %s\n",inet_ntoa(*((struct in_addr *)he->h_addr))); /* prints IP address */
}

but i am getting a warning and segmentation fault as

host.c: In function ‘main’: host.c:24: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘int’

$cc host.c -o host ./host 192.168.1.4

Hostname : 192.168.1.4 Segmentation fault

What is the error in the code ??

Thanks

Answer 1

Actually, I just compiled that code on my FreeBSD machine at home and it works.

Answer 2

You could try dumping the value of he->h_addr before trying to dereference it and pass it to inet_ntoa. If it was NULL, that would result in a seg fault.

How about running it through strace?

Answer 3

The warning about the mismatch for the printf format is an important warning. In this case, it comes because the compiler is thinking that the function inet_ntoa returns an int, but you specified to expect a string in the format-string.

The incorrect return-type for inet_ntoa is the result of an old C rule that states that if you try to use a function without a prior declaration, then the compiler must assume the function returns an int and takes an unknown (but fixed) number of arguments. The mismatch between the assumed return type and the actual return type of the function results in undefined behaviour, which manifests itself as a crash in your case.

The solution is to include the correct header for inet_ntoa.

Answer 4

Break this code:

printf("IP Address: %s\n",inet_ntoa(*((struct in_addr *)he->h_addr)));

Into this:

struct in_addr* address = (in_addr*) he->h_addr;
char* ip_address = inet_ntoa(*address);
printf("IP address: %s\n", ip_address);

It also makes it easier to debug and pinpoint the problem.