用c/c++代码举例说明如何使用socket在进程间传递文件句柄

在 Unix/Linux 系统下，我们可以使用 `sendmsg` 和 `recvmsg` 函数来传递文件句柄。下面是一个简单的示例代码：

#### 发送进程代码

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/uio.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>

#define CONTROLLEN CMSG_LEN(sizeof(int))

int send_fd(int fd, int fd_to_send)
{
    struct iovec iov[1];
    struct msghdr msg;
    char buf[2]; // 发送任意数据
    int ret;

    union {
        struct cmsghdr cm;
        char control[CONTROLLEN];
    } control_un;
    struct cmsghdr *pcmsg;

    iov[0].iov_base = buf;
    iov[0].iov_len = 2;

    msg.msg_iov = iov;
    msg.msg_iovlen = 1;
    msg.msg_name = NULL;
    msg.msg_namelen = 0;

    if (fd_to_send >= 0) {
        msg.msg_control = control_un.control;
        msg.msg_controllen = CONTROLLEN;

        pcmsg = CMSG_FIRSTHDR(&msg);
        pcmsg->cmsg_len = CONTROLLEN;
        pcmsg->cmsg_level = SOL_SOCKET;
        pcmsg->cmsg_type = SCM_RIGHTS;

        *(int *)CMSG_DATA(pcmsg) = fd_to_send;
    } else {
        msg.msg_control = NULL;
        msg.msg_controllen = 0;
        buf[1] = 0;
    }

    buf[0] = 0;

    if ((ret = sendmsg(fd, &msg, 0)) < 0) {
        perror("sendmsg");
    }

    return ret;
}

#### 接收进程代码

int recv_fd(int fd)
{
    struct iovec iov[1];
    struct msghdr msg;
    char buf[2];
    int ret;

    union {
        struct cmsghdr cm;
        char control[CONTROLLEN];
    } control_un;
    struct cmsghdr *pcmsg;

    iov[0].iov_base = buf;
    iov[0].iov_len = 2;

    msg.msg_iov = iov;
    msg.msg_iovlen = 1;
    msg.msg_name = NULL;
    msg.msg_namelen = 0;

    msg.msg_control = control_un.control;
    msg.msg_controllen = CONTROLLEN;

    if ((ret = recvmsg(fd, &msg, 0)) < 0) {
        perror("recvmsg");
        return -1;
    }

    if (buf[0] != 0) {
        printf("error: received %d bytes\n", ret);
        return -1;
    }

    if ((pcmsg = CMSG_FIRSTHDR(&msg)) != NULL &&
        pcmsg->cmsg_len == CONTROLLEN) {
        if (pcmsg->cmsg_level != SOL_SOCKET) {
            printf("error: control level != SOL_SOCKET\n");
            return -1;
        }

        if (pcmsg->cmsg_type != SCM_RIGHTS) {
            printf("error: control type != SCM_RIGHTS\n");
            return -1;
        }

        return *(int *)CMSG_DATA(pcmsg);
    } else {
        return -1;
    }
}

使用示例：

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>

int main()
{
    int fd[2];
    int ret;
    char buf[256];

    if (socketpair(AF_UNIX, SOCK_STREAM, 0, fd) < 0) {
        perror("socketpair");
        exit(1);
    }

    int send_fd = open("file.txt", O_RDONLY);
    if (send_fd < 0) {
        perror("open");
        exit(1);
    }

    if ((ret = fork()) < 0) {
        perror("fork");
        exit(1);
    }

    if (ret == 0) { // 子进程
        close(fd[0]);

        int recv_fd = recv_fd(fd[1]);
        if (recv_fd < 0) {
            exit(1);
        }

        printf("received fd: %d\n", recv_fd);
        read(recv_fd, buf, 256);
        printf("read: %s\n", buf);

        close(recv_fd);

        exit(0);
    } else { // 父进程
        close(fd[1]);

        send_fd(fd[0], send_fd);
        close(send_fd);

        waitpid(ret, NULL, 0);

        exit(0);
    }
}

在这个示例中，父进程打开了 `file.txt` 文件，并将其文件描述符通过 `send_fd` 函数发送给了子进程。子进程通过 `recv_fd` 函数接收到了文件描述符，并读取了文件中的内容。注意，这个示例中并没有考虑错误处理，实际应用中需要根据实际情况进行完善。