getifaddrs源码

以下是getifaddrs函数的源码实现，它定义在头文件<ifaddrs.h>中：

#include <sys/socket.h>
#include <ifaddrs.h>

int getifaddrs(struct ifaddrs **ifap);

void freeifaddrs(struct ifaddrs *ifa);

struct ifaddrs {
    struct ifaddrs  *ifa_next;    /* Next item in list */
    char            *ifa_name;    /* Name of interface */
    unsigned int     ifa_flags;   /* Flags from SIOCGIFFLAGS */
    struct sockaddr *ifa_addr;    /* Address of interface */
    struct sockaddr *ifa_netmask; /* Netmask of interface */
    union {
        struct sockaddr *ifu_broadaddr;
        struct sockaddr *ifu_dstaddr;
    } ifa_ifu;
#define              ifa_broadaddr ifa_ifu.ifu_broadaddr
#define              ifa_dstaddr   ifa_ifu.ifu_dstaddr
    void            *ifa_data;    /* Address-specific data */
};

getifaddrs函数返回系统中所有网络接口的基本信息，包括接口名称、地址和标志等。它的参数ifap是一个指向ifaddrs结构体链表的指针，函数将通过该指针返回结果。freeifaddrs函数用于释放该链表。

以下是getifaddrs函数的具体实现：

#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <net/if.h>
#include <ifaddrs.h>

int getifaddrs(struct ifaddrs **ifap)
{
    int fd, err, n;
    struct ifreq ifr;
    struct ifconf ifc;
    char buf[1024];

    ifc.ifc_len = sizeof(buf);
    ifc.ifc_buf = buf;
    if ((fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_IP)) < 0)
        return -1;
    if (ioctl(fd, SIOCGIFCONF, &ifc) < 0) {
        close(fd);
        return -1;
    }
    *ifap = NULL;
    for (n = 0; n < ifc.ifc_len; ) {
        struct ifaddrs *ifa;
        struct sockaddr_in *sin;
        ifr = ((struct ifreq *)(ifc.ifc_buf + n))->ifr_name;
        n += sizeof(ifr);
        if (ioctl(fd, SIOCGIFFLAGS, &ifr) < 0) {
            err = errno;
            close(fd);
            return err;
        }
        if ((ifr.ifr_flags & IFF_UP) == 0)
            continue;
        ifa = malloc(sizeof(struct ifaddrs));
        if (ifa == NULL) {
            close(fd);
            return -1;
        }
        memset(ifa, 0, sizeof(struct ifaddrs));
        ifa->ifa_flags = ifr.ifr_flags;
        ifa->ifa_name = strdup(ifr.ifr_name);
        sin = (struct sockaddr_in *)&ifr.ifr_addr;
        memcpy(&ifa->ifa_addr, sin, sizeof(struct sockaddr));
        if (ioctl(fd, SIOCGIFNETMASK, &ifr) < 0) {
            err = errno;
            freeifaddrs(ifa);
            close(fd);
            return err;
        }
        sin = (struct sockaddr_in *)&ifr.ifr_addr;
        memcpy(&ifa->ifa_netmask, sin, sizeof(struct sockaddr));
        if (ioctl(fd, SIOCGIFBRDADDR, &ifr) < 0) {
            if (ifr.ifr_flags & IFF_POINTOPOINT) {
                memcpy(&ifa->ifa_dstaddr, sin, sizeof(struct sockaddr));
            }
        } else {
            sin = (struct sockaddr_in *)&ifr.ifr_addr;
            memcpy(&ifa->ifa_broadaddr, sin, sizeof(struct sockaddr));
        }
        if ((*ifap) != NULL) {
            ifa->ifa_next = (*ifap);
        }
        *ifap = ifa;
    }
    close(fd);
    return 0;
}

void freeifaddrs(struct ifaddrs *ifa)
{
    if (ifa == NULL)
        return;
    freeifaddrs(ifa->ifa_next);
    free(ifa->ifa_name);
    free(ifa);
}