Why set the whole `hints` variable to 0? - c

Question

I am reading Beej's guide to network programming and in chapter 5.1, in the showip.c program I see the following line of code:
memset(&hints, 0, sizeof hints);
After a discussion on the ##c channel on freenode I deducted that the reasoning of that memset call could be to set the value of hints.ai_flags to 0(note that the program works fine I remove that line and I explicitly initialize hints.ai_flags to 0). If this is true, why does he need to set the whole struct to 0?
This is the full source:
/*
** showip.c -- show IP addresses for a host given on the command line
*/
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <netinet/in.h>
int main(int argc, char *argv[])
{
struct addrinfo hints, *res, *p;
int status;
char ipstr[INET6_ADDRSTRLEN];
if (argc != 2) {
fprintf(stderr,"usage: showip hostname\n");
return 1;
}
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version
hints.ai_socktype = SOCK_STREAM;
if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
return 2;
}
printf("IP addresses for %s:\n\n", argv[1]);
for(p = res;p != NULL; p = p->ai_next) {
void *addr;
char *ipver;
// get the pointer to the address itself,
// different fields in IPv4 and IPv6:
if (p->ai_family == AF_INET) { // IPv4
struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
addr = &(ipv4->sin_addr);
ipver = "IPv4";
} else { // IPv6
struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
addr = &(ipv6->sin6_addr);
ipver = "IPv6";
}
// convert the IP to a string and print it:
inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);
printf(" %s: %s\n", ipver, ipstr);
}
freeaddrinfo(res); // free the linked list
return 0;
}

Answer 1

It's required by getaddrinfo() function documentation (where you pass your hints variable as parameter). From man getaddrinfo:
All the other fields in the structure pointed to by hints must contain either 0 or a NULL pointer, as appropriate.

Answer 2

It's because you are only going to fill/use/initialize some of the fields of the struct, giving 0 to the other fields prevents reading an uninitialzied variable, and sometimes 0 is the default value for those variables.
note that the program works fine I remove that line and I explicitly initialize hints.ai_flags to 0
Not necessarily, if you are on linux, I suggest using valgrind to detect reads to uninitialized variables, since that causes undefined behavior, the behavior could be that nothing wierd happens.