c - 为什么将整个 `hints` 变量设置为 0？

Question

我正在阅读 Beej 的网络编程指南和 chapter 5.1 ，在 showip.c 程序中，我看到以下代码行: memset(&hints, 0, sizeof hints);

在 freenode 上的 ##c channel 上进行讨论后，我推断出 memset 调用的原因可能是将 hints.ai_flags 的值设置为 0(请注意程序运行良好，我删除了该行并将 hints.ai_flags 显式初始化为 0)。如果这是真的，为什么他需要将整个结构设置为 0？

这是完整的来源:

/*
** 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

它是 getaddrinfo() 函数文档所必需的(您将 hints 变量作为参数传递)。来自 man getaddrinfo :

All the other fields in the structure pointed to by hints must contain either 0 or a NULL pointer, as appropriate.