Linxu的struct ifaddrs 与getifaddrs（）函数

来自man函数手册中的介绍：

1，关于struct ifaddrs的说明：

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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; /* Broadcast address of interface */

struct sockaddr *ifu_dstaddr; /* Point-to-point destination address */

} ifa_ifu;

#define ifa_broadaddr ifa_ifu.ifu_broadaddr

#define ifa_dstaddr ifa_ifu.ifu_dstaddr

void *ifa_data; /* Address-specific data */

};

2，关于getifaddrs()

The getifaddrs() function creates a linked list of structures describing the network interfaces of the local system, and stores the address of the first
item of the list in *ifap.
The list consists of ifaddrs structures, defined as follows:

The ifa_next field contains a pointer to the next structure on the list, or
NULL if this is the last item of the list.

The ifa_name points to the null-terminated interface name.

The ifa_flags field contains the interface flags

The ifa_addr field points to a structure containing the interface address.

The ifa_netmask field points to a structure containing the netmask associated with ifa_addr, if applicable for the address family.

Depending on whether the bit IFF_BROADCAST or IFF_POINTOPOINT is set in ifa_flags (only one can be set at a time), either ifa_broadaddr will contain the broadcast address associated with ifa_addr (if applicable for the address family) or ifa_dstaddr will contain the destination address of the point-to-point interface.

The ifa_data field points to a buffer containing address-family-specific data;this field may be NULL if there is no such data for this interface.

返回值：
On success, getifaddrs() returns zero; on error, -1 is returned, and errno is set appropriately.

3，注意：
The data returned by getifaddrs() is dynamically allocated and should be freed using freeifaddrs() when no longer needed.

4, man中的实例代码：

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#include <arpa/inet.h>

#include <sys/socket.h>

#include <netdb.h>

#include <ifaddrs.h>

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

int main(int argc, char *argv[])

{

struct ifaddrs *ifaddr, *ifa;

int family, s;

char host[NI_MAXHOST];

if (getifaddrs(&ifaddr) == -1) {

perror("getifaddrs");

exit(EXIT_FAILURE);

}

/* Walk through linked list, maintaining head pointer so we

* can free list later */

for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {

if (ifa->ifa_addr == NULL)

continue;

family = ifa->ifa_addr->sa_family;

/* Display interface name and family (including symbolic

* form of the latter for the common families) */

printf("%s address family: %d%s\n",

ifa->ifa_name, family,

(family == AF_PACKET) ? " (AF_PACKET)" :

(family == AF_INET) ? " (AF_INET)" :

(family == AF_INET6) ? " (AF_INET6)" : "");

/* For an AF_INET* interface address, display the address */

if (family == AF_INET || family == AF_INET6) {

s = getnameinfo(ifa->ifa_addr,

(family == AF_INET) ? sizeof(struct sockaddr_in) :

sizeof(struct sockaddr_in6),

host, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);

if (s != 0) {

printf("getnameinfo() failed: %s\n", gai_strerror(s));

exit(EXIT_FAILURE);

}

printf("\taddress: <%s>\n", host);

}

}

freeifaddrs(ifaddr);

exit(EXIT_SUCCESS);

}