using signal SIGALRM for timeout

/*******************************************************
* timeout *
* Program to demonstrate a timeout routine *
*******************************************************/
#include < stdio.h >
#include < signal.h >
#include < setjmp.h >
/*
* The environment for setjmp.
*/
jmp_buf env;
main()
{
int i;
char buf[160];
extern void timeout();
/*
* Inform the system we want to catch the
* alarm signal
*/
signal(SIGALRM, timeout);
/*
* The code inside the if gets executed the first
* time through setjmp, the code inside the else
* the second time.
*/
if (setjmp(env) == 0) {
/*
* Issue a request for an alarm to be delivered
* in 5 seconds. alarm() is a system call.
*/
alarm(5);
/*
* Prompt for input. Waits for it forever. If
* it's still waiting and an alarm signal comes,
* we come back to the setjmp() and never get
* any further.
*/
printf("Type a word; if you don't in 5 ");
printf("seconds I'll use 'WORD' instead:");
(void) fgets(buf, sizeof(buf), stdin);
/*
* Turn off the alarm
*/
alarm(0);
}
else {
/*
* Assume the default.
*/
strcpy(buf, "WORD");
}
printf("\n The word is: %s\n", buf);
exit(0);
}
/*
* timeout -- catch the signal
*/
void timeout(sig)
int sig;
{
int i;
/*
* Ignore the signal for the duration of this
* routine.
*/
signal(sig, SIG_IGN);
/*
* We would perform any timeout-related
* functions here.
*/
printf("...Timeout!\n");
/*
* Restore the action of the alarm signal
*/
signal(SIGALRM, timeout);
/*
* Return to the main routine at setjmp,
* and make setjmp return 1.
*/
longjmp(env, 1);
}

gethostbyname() simple example code

#include < stdio.h >
#include < netdb.h >
#include < netinet.h >
#include < arpa.h >

int main()
{
struct hostent *host;
host = gethostbyname("localhost");
char **alias;
alias = host->h_aliases;
while(*alias != NULL)
{
printf("%s\n",*alias);
alias++;
}
switch(host->h_addrtype)
{
case AF_INET:
printf("IP ver 4 \n");
break;
case AF_INET6:
printf("IP ver 6 \n");
break;
}
char buff[INET_ADDRSTRLEN];
sockaddr_in addr;
addr.sin_addr = *(struct in_addr *)host->h_addr_list[0];
addr.sin_family = host->h_addrtype;
inet_ntop(AF_INET,&(addr.sin_addr.s_addr),buff,sizeof(buff));
printf("%s\n",buff);
return 0;
}