为什么'接受(2)`需要`sockaddr`长度作为一个单独的指针? [英] Why does `accept(2)` need the `sockaddr` length as a separate pointer?
问题描述
我努力理解的原型接受(2)
。
我有接受连接,并返回一个简单的服务器的Hello world!\\ n来的客户。系统调用接受(2)
需要一个指向结构sockaddr
和一个指向 socklen_t的
来存储结构的长度。但的sockaddr
已经有一个叫场 sa_len
,这似乎是刚刚完成的。
另外,我有这个简单的服务器(OSX下的编译,希望Linux的太)打印出我自己的 socklen_t的
传递给接受,然后价值 sa_len
:它们是相同的,在这种情况下28,在OSX
编辑:一个多一点的测试后,似乎 sa_len
不一定一样存储在指针长度
为什么接受(2)
所需要的长度作为一个单独的指针?
有关参考,我张贴下面的例子服务器。你可以编译:
的gcc -Wall -Wextra的main.c
然后运行:
./ a.out的
在另一端,连接到它:
127.0.0.1的telnet 3000
的#include<&errno.h中GT;
#包括LT&; SYS / socket.h中>
#包括LT&;&unistd.h中GT;
#包括LT&;&netdb.h中GT;
#包括LT&;&stdio.h中GT;
#包括LT&; netinet / in.h中>
#包括LT&;&strings.h GT;//如果没有,如果传递的程序参数默认端口
#定义FTP_PORT_DEFAULT(3000)//连接数允许等待呼叫接受
的#define FTP_BACKLOG(5)INT主(INT交流,焦炭** AV)
{
结构addrinfo中的提示; //提示用于获取地址信息
结构addrinfo中的*赛; //服务器地址信息
INT SS; //服务器套接字
字符*口; 端口=(AC&GT = 2)? AV [1]:FTP_PORT_DEFAULT;
bzero(安培;提示,sizeof的(提示)); 使用AF_INIT6代替PF_INET6 //
// http://stackoverflow.com/questions/6729366/
hints.ai_family = AF_INET6;
hints.ai_socktype = SOCK_STREAM; //用于与`bind`使用
hints.ai_flags = AI_PASSIVE; //传递`NULL`作为主机名告诉`getaddrinfo`使用
//任何可用的本地IP地址
如果(的getaddrinfo(NULL,端口&放大器;提示,&放大器;!SAI)= 0)
返回(-1); 如果((SS =
插座(SAI-> ai_family,SAI-> ai_socktype,SAI-> ai_protocol))== -1)
返回(-1); 如果(绑定(SS,SAI-> ai_addr,SAI-> ai_addrlen)== -1 ||
听(SS,FTP_BACKLOG)== -1)
{
关闭(SS);
返回(-1);
} 而(1)
{
INT CS;
结构sockaddr csockaddr;
无符号csockaddr_len; bzero(安培; csockaddr,sizeof的(csockaddr));
CS =接受(SS,和放大器; csockaddr,&安培; csockaddr_len);
//处理致命我不认为我可以从错误中恢复
//比通过重新启动服务器的其他
如果(CS == EBADF || CS == || ECONNABORTED CS == || ENOTSOCK
CS == EOPNOTSUPP)
{
突破;
}
的printf(我的LEN%U /内部LEN%U \\ N,
csockaddr_len,csockaddr.sa_len); 如果(叉()== 0)
{
写(CS,的Hello world \\ n!,13);
关闭(CS);
}
打破;
}
关闭(SS); 返回0;
}
sa_len
是不可移植。据只加于1988年BSD ,4.3BSD几年后,但它不是在 POSIX ,它不是在Linux中。
I am struggling to understand the prototype of accept(2)
.
I have a simple server that accepts connections and returns "Hello world!\n" to the clients. The system call accept(2)
takes a pointer to struct sockaddr
and a pointer to an socklen_t
to store the length of the struct. But sockaddr
already has a field called sa_len
, which seems to be done just for that.
In addition, I have this simple server (compiles under OSX, hopefully Linux too) that prints out my own socklen_t
passed to accept and then the value of sa_len
: they are the same, 28 in this case, on OSX.
EDIT: After a bit more testing, it seems that sa_len
is not necessarily the same as the length stored in the pointer.
Why does accept(2)
need the length as a separate pointer?
For reference, I'm posting the example server below. You can compile with:
gcc -Wall -Wextra main.c
And then run:
./a.out
In another terminal, connect to it:
telnet 127.0.0.1 3000
#include <errno.h>
#include <sys/socket.h>
#include <unistd.h>
#include <netdb.h>
#include <stdio.h>
#include <netinet/in.h>
#include <strings.h>
// default port if none if passed in the program arguments
#define FTP_PORT_DEFAULT ("3000")
// number of connections allow to wait for a call to accept
#define FTP_BACKLOG (5)
int main(int ac, char **av)
{
struct addrinfo hints; // hints used to get address information
struct addrinfo *sai; // server address information
int ss; // server socket
char *port;
port = (ac >= 2) ? av[1] : FTP_PORT_DEFAULT;
bzero(&hints, sizeof(hints));
// using AF_INIT6 instead of PF_INET6
// http://stackoverflow.com/questions/6729366/
hints.ai_family = AF_INET6;
hints.ai_socktype = SOCK_STREAM;
// intended for use with `bind`
hints.ai_flags = AI_PASSIVE;
// passing `NULL` as the hostname tells `getaddrinfo` to use
// any available local IP address
if (getaddrinfo(NULL, port, &hints, &sai) != 0)
return (-1);
if ((ss =
socket(sai->ai_family, sai->ai_socktype, sai->ai_protocol))== -1)
return (-1);
if (bind(ss, sai->ai_addr, sai->ai_addrlen) == -1 ||
listen(ss, FTP_BACKLOG) == -1)
{
close(ss);
return (-1);
}
while (1)
{
int cs;
struct sockaddr csockaddr;
unsigned csockaddr_len;
bzero(&csockaddr, sizeof(csockaddr));
cs = accept(ss, &csockaddr, &csockaddr_len);
// handle "fatal" errors that I don't think I can recover from
// other than by relaunching the server
if (cs == EBADF || cs == ECONNABORTED || cs == ENOTSOCK ||
cs == EOPNOTSUPP)
{
break ;
}
printf("my len %u / internal len %u\n",
csockaddr_len, csockaddr.sa_len);
if (fork() == 0)
{
write(cs, "Hello world!\n", 13);
close(cs);
}
break;
}
close(ss);
return 0;
}
sa_len
is not portable. It was added to BSD only in 1988, a few years after 4.3BSD, but it's not in POSIX and it's not in Linux.
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