getaddrinfo(3)
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
int getaddrinfo(const char *node, const char *service,
const struct addrinfo *hints,
struct addrinfo **res);
void freeaddrinfo(struct addrinfo *res);
const char *gai_strerror(int errcode);
NOTE: : redis的
anet.h模块中使用了该函数，可以作为example来参考，我的redis-ae中提供了注释版本的代码；
Description
Given node and service, which identify an Internet host and a service, getaddrinfo() returns one or more addrinfo structures, each of which contains an Internet address that can be specified in a call to bind(2) or connect(2) . The getaddrinfo() function combines the functionality provided by the gethostbyname(3) and getservbyname(3) functions into a single interface, but unlike the latter functions, getaddrinfo() is reentrant and allows programs to eliminate IPv4-versus-IPv6 dependencies.
NOTE: : node往往是IP地址，server则是端口； 如果用于connect，则此时是指定server端的地址； 如果用于bind，则此时应该指定的是什么呢？
getaddrinfo的返回值是从何而来的？
The addrinfo structure used by getaddrinfo() contains the following fields:
struct addrinfo {
int ai_flags;
int ai_family;
int ai_socktype;
int ai_protocol;
socklen_t ai_addrlen;
struct sockaddr *ai_addr;
char *ai_canonname;
struct addrinfo *ai_next;
};
The hints argument points to an addrinfo structure that specifies criteria for selecting the socket address structures returned in the list pointed to by res. If hints is not NULL it points to an addrinfo structure whose ai_family, ai_socktype, and ai_protocol specify criteria that limit the set of socket addresses returned by getaddrinfo(), as follows:
-
ai_family
This field specifies the desired address family for the returned addresses. Valid values for this field include AF_INET and AF_INET6. The value AF_UNSPEC indicates that getaddrinfo() should return socket addresses for any address family (either IPv4 or IPv6, for example) that can be used with node and service.
NOTE: : address family从其名字可知与address相关，在目前的
TCP/IP网络中，我们都是使用IPv4或IPv6网络；**AF_INET**即IPv4，**AF_INET6**即IPv6。
-
ai_socktype
This field specifies the preferred socket type, for example SOCK_STREAM or SOCK_DGRAM. Specifying 0 in this field indicates that socket addresses of any type can be returned by getaddrinfo().
NOTE: : *ai_socktype*和*ai_protocol*存在关系，但是和*ai_family*之间并没有关系；
-
ai_protocol
This field specifies the protocol for the returned socket addresses. Specifying 0 in this field indicates that socket addresses with any protocol can be returned by getaddrinfo().
-
ai_flags
This field specifies additional options, described below. Multiple flags are specified by bitwise OR-ing them together.
All the other fields in the structure pointed to by hints must contain either 0 or a NULL pointer, as appropriate.
Specifying hints as NULL is equivalent to setting ai_socktype and ai_protocol to 0; ai_family to AF_UNSPEC; and ai_flags to (AI_V4MAPPED | AI_ADDRCONFIG). node specifies either a numerical network address (for IPv4, numbers-and-dots notation as supported by inet_aton(3) ; for IPv6, hexadecimal string format as supported by inet_pton(3) ), or a network hostname, whose network addresses are looked up and resolved. If hints.ai_flags contains the AI_NUMERICHOST **flag then node must be a **numerical network address. The AI_NUMERICHOST flag suppresses any potentially lengthy network host address lookups.
NOTE: :在Wikipedia的getaddrinfo中，列举了一个例子：给定domain name，获得其IP地址；
If the AI_PASSIVE flag is specified in hints.ai_flags, and node is NULL, then the returned socket addresses will be suitable for bind(2)ing a socket that will accept(2) connections. The returned socket address will contain the "wildcard address" (INADDR_ANY for IPv4 addresses, IN6ADDR_ANY_INIT for IPv6 address). The wildcard address is used by applications (typically servers) that intend to accept connections on any of the hosts's network addresses. If node is not NULL, then the AI_PASSIVE flag is ignored.
NOTE: : 这段话非常重要，redis 4 network configuration中有如下一段话：
# By default, if no "bind" configuration directive is specified, Redis listens # for connections from all the network interfaces available on the server. # It is possible to listen to just one or multiple selected interfaces using # the "bind" configuration directive, followed by one or more IP addresses.
通过阅读redis的源代码，可以发现它所使用的就是上面这段话中所描述的方法，即在hints.ai_flags中设置AI_PASSIVE，同时将node参数node设置为NULL；这种方法就相当于flask中的flask run --host=0.0.0.0，这是一种特殊情况；查看下面的Server program，其中的hints.ai_flags = AI_PASSIVE; /* For wildcard IP address */是非常具有提示意义的，它指出了这种特殊的用法；
NOTE: :
PASSIVE的含义是被动的，显然它是符合在client/server模型中，server是被动地，server等待client的请求；NOTE: : 不要忽视了上面这段话中的最后一句话： If node is not NULL, then the AI_PASSIVE flag is ignored.在redis源代码中也有这样的类似说法；对于 node is not NULL 的情况，它到底是声明一个server socket还是一个client socket？
20190529：其实上面我的问题就存在错误getaddinfo函数的功能是获取addrinfo，而不是生成一个socket，它的返回值将用于socket；所以对于最后一句话中描述的情况，它就是最最平台的将node转换为socket能够接受的addrinfo。
If the AI_PASSIVE flag is not set in hints.ai_flags, then the returned socket addresses will be suitable for use with connect(2), sendto(2), or sendmsg(2). If node is NULL, then the network address will be set to the loopback interface address (INADDR_LOOPBACK for IPv4 addresses, **IN6ADDR_LOOPBACK_INIT**for IPv6 address); this is used by applications that intend to communicate with peers running on the same host.
NOTE: : 根据上面所总结的，
PASSIVE的含义是被动的，显然一旦设置了AI_PASSIVE就表明这是应用于server端的；对于client，则AI_PASSIVE不应被设置，它表明它不是被动的；上面这一段中的第二句话说明的是当**AI_PASSIVE** flag is not set inhints.ai_flags, 并且 node is NULL的情况下，getaddrinfo函数的处理逻辑，在这种情况下，它将使用loopback interface address。
The getaddrinfo() function allocates and initializes a linked list of addrinfo structures, one for each network address that matches node and service, subject to any restrictions imposed by hints, and returns a pointer to the start of the list in res. The items in the linked list are linked by the ai_next field.
NOTE: : 这段对该函数的功能的总结比较好
There are several reasons why the linked list may have more than one addrinfo structure, including: the network host is multihomed, accessible over multiple protocols (e.g., both AF_INET and AF_INET6); or the same service is available from multiple socket types (one SOCK_STREAM address and another SOCK_DGRAM address, for example). Normally, the application should try using the addresses in the order in which they are returned. The sorting function used within getaddrinfo() is defined in RFC 3484; the order can be tweaked for a particular system by editing /etc/gai.conf (available since glibc 2.5).
If hints.ai_flags includes the AI_CANONNAME flag, then the ai_canonname field of the first of the addrinfo structures in the returned list is set to point to the official name of the host.
The remaining fields of each returned addrinfo structure are initialized as follows:
*
The ai_family, ai_socktype, and ai_protocol fields return the socket creation parameters (i.e., these fields have the same meaning as the corresponding arguments of socket(2)). For example, ai_family might return AF_INET or AF_INET6; ai_socktype might return SOCK_DGRAM or SOCK_STREAM; and ai_protocol returns the protocol for the socket.
*
A pointer to the socket address is placed in the ai_addr field, and the length of the socket address, in bytes, is placed in the ai_addrlen field.
Example
The following programs demonstrate the use of getaddrinfo(), gai_strerror(), freeaddrinfo(), and getnameinfo(3). The programs are an echo server and client for UDP datagrams.
Server program
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>
#include <netdb.h>
#define BUF_SIZE 500
int
main(int argc, char *argv[])
{
struct addrinfo hints;
struct addrinfo *result, *rp;
int sfd, s;
struct sockaddr_storage peer_addr;
socklen_t peer_addr_len;
ssize_t nread;
char buf[BUF_SIZE];
if (argc != 2) {
fprintf(stderr, "Usage: %s port\n", argv[0]);
exit(EXIT_FAILURE);
}
memset(&hints, 0, sizeof(struct addrinfo));
hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */
hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
hints.ai_flags = AI_PASSIVE; /* For wildcard IP address */
hints.ai_protocol = 0; /* Any protocol */
hints.ai_canonname = NULL;
hints.ai_addr = NULL;
hints.ai_next = NULL;
s = getaddrinfo(NULL, argv[1], &hints, &result);
if (s != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
exit(EXIT_FAILURE);
}
/* getaddrinfo() returns a list of address structures.
Try each address until we successfully bind(2).
If socket(2) (or bind(2)) fails, we (close the socket
and) try the next address. */
for (rp = result; rp != NULL; rp = rp->ai_next) {
sfd = socket(rp->ai_family, rp->ai_socktype,
rp->ai_protocol);
if (sfd == -1)
continue;
if (bind(sfd, rp->ai_addr, rp->ai_addrlen) == 0)
break; /* Success */
close(sfd);
}
if (rp == NULL) { /* No address succeeded */
fprintf(stderr, "Could not bind\n");
exit(EXIT_FAILURE);
}
freeaddrinfo(result); /* No longer needed */
/* Read datagrams and echo them back to sender */
for (;;) {
peer_addr_len = sizeof(struct sockaddr_storage);
nread = recvfrom(sfd, buf, BUF_SIZE, 0,
(struct sockaddr *) &peer_addr, &peer_addr_len);
if (nread == -1)
continue; /* Ignore failed request */
char host[NI_MAXHOST], service[NI_MAXSERV];
s = getnameinfo((struct sockaddr *) &peer_addr,
peer_addr_len, host, NI_MAXHOST,
service, NI_MAXSERV, NI_NUMERICSERV);
if (s == 0)
printf("Received %ld bytes from %s:%s\n",
(long) nread, host, service);
else
fprintf(stderr, "getnameinfo: %s\n", gai_strerror(s));
if (sendto(sfd, buf, nread, 0,
(struct sockaddr *) &peer_addr,
peer_addr_len) != nread)
fprintf(stderr, "Error sending response\n");
}
}
NOTE: : 这段程序终止的时候，只能够由OS来负责清理
sfd所占用的资源；另外可以通过atexit函数来注册一个exit handler负责清理；显然c语言缺乏finally机制；
Client program
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define BUF_SIZE 500
int
main(int argc, char *argv[])
{
struct addrinfo hints;
struct addrinfo *result, *rp;
int sfd, s, j;
size_t len;
ssize_t nread;
char buf[BUF_SIZE];
if (argc < 3) {
fprintf(stderr, "Usage: %s host port msg...\n", argv[0]);
exit(EXIT_FAILURE);
}
/* Obtain address(es) matching host/port */
memset(&hints, 0, sizeof(struct addrinfo));
hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */
hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
hints.ai_flags = 0;
hints.ai_protocol = 0; /* Any protocol */
s = getaddrinfo(argv[1], argv[2], &hints, &result);
if (s != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
exit(EXIT_FAILURE);
}
/* getaddrinfo() returns a list of address structures.
Try each address until we successfully connect(2).
If socket(2) (or connect(2)) fails, we (close the socket
and) try the next address. */
for (rp = result; rp != NULL; rp = rp->ai_next) {
sfd = socket(rp->ai_family, rp->ai_socktype,
rp->ai_protocol);
if (sfd == -1)
continue;
if (connect(sfd, rp->ai_addr, rp->ai_addrlen) != -1)
break; /* Success */
close(sfd);
}
if (rp == NULL) { /* No address succeeded */
fprintf(stderr, "Could not connect\n");
exit(EXIT_FAILURE);
}
freeaddrinfo(result); /* No longer needed */
/* Send remaining command-line arguments as separate
datagrams, and read responses from server */
for (j = 3; j < argc; j++) {
len = strlen(argv[j]) + 1;
/* +1 for terminating null byte */
if (len + 1 > BUF_SIZE) {
fprintf(stderr,
"Ignoring long message in argument %d\n", j);
continue;
}
if (write(sfd, argv[j], len) != len) {
fprintf(stderr, "partial/failed write\n");
exit(EXIT_FAILURE);
}
nread = read(sfd, buf, BUF_SIZE);
if (nread == -1) {
perror("read");
exit(EXIT_FAILURE);
}
printf("Received %ld bytes: %s\n", (long) nread, buf);
}
exit(EXIT_SUCCESS);
}