Segmentation fault when looking up host name and IP address

Question

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 during compilation:

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

Then there is a segmentation fault when I run the code:

./host 192.168.1.4
Hostname : 192.168.1.4
Segmentation fault

What is the error in the code?

Answer 1

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.