Share file descriptor between process
在阅读"wikipedia File descriptor # File descriptors as capabilities"时，其中提及了passing file descriptor，我觉得有必要对这个topic进行总结。
在下面章节中，对这个topic进行了探讨:
一、Book-APUE\17-Advanced-IPC\17.4-Passing-File-Descriptors
二、IPC\Unix-domain-socket\man-7-unix
cnblogs 高级进程间通信之传送文件描述符
NOTE: 1、这篇文章非常好
1、经由基于STREAMS的管道传送文件描述符
文件描述符用两个ioctl命令经由STREAMS管道交换，这两个命令是：I_SENDFD和I_RECVFD。为了发送一个描述符，将ioctl的第三个参数设置为实际描述符。
程序清单17-12 STREAMS管道的send_fd函数
#include "apue.h"
#include <stropts.h>
/*
* Pass a file descriptor to another process.
* If fd < 0, then -fd is sent back instead as the error status.
*/
int
send_fd(int fd, int fd_to_send)
{
char buf[2]; /* send_fd()/recv_fd() 2-byte protocol */
buf[0] = 0; /* null bytes flag to recv_fd() */
if(fd_to_send < 0)
{
buf[1] = -fd_to_send; /* nonzero status means error */
if(buf[1] == 0)
buf[1] = 1; /* -256, etc. would screw up protocol */
}
else
{
buf[1] = 0; /* zero status means OK */
}
if(write(fd, buf, 2) != 2)
return(-1);
if(fd_to_send >= 0)
if(ioctl(fd, I_SENDFD, fd_to_send) < 0)
return(-1);
return(0);
}
当接收一个描述符时，ioctl的第三个参数是一指向strrecvfd结构的指针。
struct strrecvfd {
int fd; /* new descriptor */
uid_t uid; /* effective user ID of sender */
gid_t gid; /* effective group ID of sender */
char fill[8];
};
recv_fd读STREAMS管道直到接收到双字节协议的第一个字节（null字节）。当发出I_RECVFD ioctl命令时，位于流首读队列中的下一条消息应当是一个描述符，它是由I_SENDFD发来的，或者是一条出错消息。
程序清单17-13 STREAMS管道的recv_fd函数
#include "apue.h"
#include <stropts.h>
/*
* Receive a file descirpor from another process ( a server ).
* In addition, any data received from the server is passed
* to (*userfunc)(STDERR_FILENO, buf, nbytes). We have a
* 2-byte protocol for receiving the fd from send_fd().
*/
int
recv_fd(int fd, ssize_t (*userfunc)(int, const void *, size_t))
{
itn newfd, nread, flag, status;
char *ptr;
char buf[MAXLINE];
struct strbuf dat;
struct strrecvfd recvfd;
status = -1;
for(;;)
{
dat.buf = buf;
dat.maxlen = MAXLINE;
flag = 0;
if(getmsg(fd, NULL, &dat, &flag) < 0)
err_sys("getmsg error");
nread = dat.len;
if(nread == 0)
{
err_ret("connection closed by server");
return(-1);
}
/*
* See if this is the final data with null & status.
* Null must be next to last byte of buffer, status
* byte is last byte. Zero status means there must
* be a file descriptor to receive.
*/
for(ptr = buf; ptr < &buf[nread]; )
{
if(*ptr++ == 0)
{
if(ptr != &buf[nread - 1])
err_dump("message format error");
status = *ptr & 0xFF; /* prevent sign extension */
if(status == 0)
{
if(ioctl(fd, I_RECVFD, &recvfd) < 0)
return(-1);
newfd = recvfd.fd; /* new descriptor */
}
else
{
newfd = -status;
}
nread -= 2;
}
}
if(nread > 0)
if((*userfunc(STDERR_FILENO, buf, nread) != nread))
return(-1);
if(status >= 0) /* final data has arrived */
return(newfd); /* descriptor, or -status */
}
}
2、经由UNIX域套接字传送文件描述符
为了用UNIX域套接字交换文件描述符，调用sendmsg（2）和recvmsg（2）函数（http://www.cnblogs.com/nufangrensheng/p/3567376.html）。这两个函数的参数中都有一个指向msghdr结构的指针，该结构包含了所有有关收发内容的信息。该结构的定义大致如下：
TODO
stackoverflow Portable way to pass file descriptor between different processes
stackoverflow Can I share a file descriptor to another process on linux or are they local to the process?
gist.github linux file descriptors passing between processes
stackexchange Sharing file descriptors
stackexchange How can same fd in different processes point to the same file?
stackoverflow Sending file descriptor by Linux socket