进程间传递文件描述符——sendmsg和recvmsg函数

Posted 2021-10-25 mered1th

先引入一个例子，该程序的目的是子进程向父进程传递文件描述符，并通过该文件描述符读取buf。

#include <func.h>

int main(){
    int fds[2];
    pipe(fds);
    if(!fork()){
        close(fds[1]);
        int fd;
        read(fds[0], &fd, sizeof(fd));
        printf("child fd = %d\n", fd);
        char buf[128] = {0};
        read(fd, buf, sizeof(buf));
        printf("buf = %s\n", buf);
        return 0;
    }
    else{
        close(fds[0]);
        int fd;
        fd = open("file", O_RDWR);
        printf("parent fd = %d\n", fd);
        write(fds[1], &fd, sizeof(fd));
        wait(NULL);
        return 0;
    }
}

编译测试，发现结果不正确，通过ps aux查看到程序卡在了等待管道写数据，原因是卡在了第二个read读取buf处。我们再来看一下程序（见注释）：

#include <func.h>

int main(){
    int fds[2];
    pipe(fds);
    if(!fork()){
        close(fds[1]); //子进程关闭文件描述符4，但fds[0]为3
        int fd;
        read(fds[0], &fd, sizeof(fd)); //通过fds[0]读出管道内容，写入fd中
        printf("child fd = %d\n", fd); //输出为3
        char buf[128] = {0};
        read(fd, buf, sizeof(buf)); //fds[0]与fd同时为3，读阻塞
        printf("buf = %s\n", buf);
        return 0;
    }
    else{
        close(fds[0]); //父进程关闭文件描述符3
        int fd;
        fd = open("file", O_RDWR); //打开文件的描述符为fd = 3
        printf("parent fd = %d\n", fd);
        write(fds[1], &fd, sizeof(fd));  //通过fds[1]写入管道内容
        wait(NULL); //回收子进程
        return 0;
    }
}

所以我们必须借助内核传递文件描述符，sendmsg和recvmsg函数登场。

进程间传递文件描述符

步骤如下：

1. 初始化socketpair类型描述符

2. sendmsg发送描述符

   ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);

   参1：sockfd指socket创建的FILENO

   参2：结构体（见下）

   参3：The flags argument is the bitwise OR of zero or more of the following flags.这里暂时不需要参数，先填0

   使用的sockfd即sockpair初始化的描述符fds[1]

   • 结构体 struct msghdr msg;

   struct msghdr {
        void         *msg_name;       /* optional address */
        socklen_t     msg_namelen;    /* size of address */
        struct iovec *msg_iov;        /* scatter/gather array */
        size_t        msg_iovlen;     /* # elements in msg_iov */
        void         *msg_control;    /* ancillary data, see below 关键，即下面cmsghdr结构体地址 */
        size_t        msg_controllen; /* ancillary data buffer len cmsghdr结构体的长度*/
        int           msg_flags;      /* flags (unused) */
   };

   • 结构体 struct cmsghdr

   struct cmsghdr{
       socklen_t cmsg_len; /* data byte count, including header */
    int cmsg_level; /* originating protocol */
    int cmsg_type; /* protocol-specific type */
    /* followed by unsigned char cmsg_data[]; */
   }

   • 结构体msg_iov

    struct iovec {
        void  *iov_base;    /* Starting address */
        size_t iov_len;     /* Number of bytes to transfer */
    };

cmsghdr结构体的初始化

 int len = CMSG_LEN(sizeof(int));//通过CMSG_LEN计算cmsg_len，传递的fd的大小为整型四个字节
 cmsg = (struct cmsghdr *)calloc(1, len);
 cmsg->cmsg_len = len;
 cmsg->cmsg_level = SOL_SOCKET;
 cmsg->cmsg_type = SCM_RIGHTS;
 int *fdptr;
 fdptr = (int*)CMSG_DATA(cmsg);
 *fdptr = fd;

msg_iov结构体初始化

    struct iovec iov[2];
    char buf1[10]="hello";
    char buf2[10]="world";
    iov[0].iov_base=buf1;
    iov[0].iov_len=5;
    iov[1].iov_base=buf2;
    iov[1].iov_len=5;

msghdr结构体初始化

    /* iovec必须赋值 */
    struct msghdr msg;
    memset(&msg,0,sizeof(msg));
    msg.msg_iov = iov;
    msg.msg_iovlen = 2;
    msg.msg_control = cmsg;
    msg.msg_controllen = len;

最后就可以通过sendmsg来发送文件描述符，完整代码如下：

int sendFd(int sfd,int fd)
{
    struct msghdr msg;
    memset(&msg,0,sizeof(msg));
    struct iovec iov[2];
    char buf1[10]="hello";
    char buf2[10]="world";
    iov[0].iov_base=buf1;
    iov[0].iov_len=5;
    iov[1].iov_base=buf2;
    iov[1].iov_len=5;
    msg.msg_iov=iov;
    msg.msg_iovlen=2;
    struct cmsghdr *cmsg;
    int len=CMSG_LEN(sizeof(int));
    cmsg=(struct cmsghdr *)calloc(1,len);
    cmsg->cmsg_len=len;
    cmsg->cmsg_level=SOL_SOCKET;
    cmsg->cmsg_type=SCM_RIGHTS;
    *(int*)CMSG_DATA(cmsg)=fd;
    msg.msg_control=cmsg;
    msg.msg_controllen=len;
    int ret;
    ret=sendmsg(sfd,&msg,0);
    ERROR_CHECK(ret,-1,"sendmsg");
    return 0;
}

3.recvmsg接受文件描述符，接收的msghdr结构体初始化和sendmsg类似。

int recvFd(int sfd,int *fd)
{
    struct msghdr msg;
    memset(&msg,0,sizeof(msg));
    struct iovec iov[2];
    char buf1[10];
    char buf2[10];
    iov[0].iov_base=buf1;
    iov[0].iov_len=5;
    iov[1].iov_base=buf2;
    iov[1].iov_len=5;
    msg.msg_iov=iov;
    msg.msg_iovlen=2;
    struct cmsghdr *cmsg;
    int len=CMSG_LEN(sizeof(int));
    cmsg=(struct cmsghdr *)calloc(1,len);
    cmsg->cmsg_len=len;
    cmsg->cmsg_level=SOL_SOCKET;
    cmsg->cmsg_type=SCM_RIGHTS;
    msg.msg_control=cmsg;
    msg.msg_controllen=len;
    int ret;
    ret=recvmsg(sfd,&msg,0);
    ERROR_CHECK(ret,-1,"sendmsg");
    *fd=*(int*)CMSG_DATA(cmsg);
    return 0;
}

• writev和readv

#include <func.h>

int main(){
    int fd = open("file", O_RDWR);
    struct iovec iov[2];
    char buf1[10] = "hello";
    char buf2[10] = "world";
    iov[0].iov_base = buf1;
    iov[0].iov_len = 5;
    iov[1].iov_base = buf2;
    iov[1].iov_len = 5;
    writev(fd, iov, 2);//注意这里是2
    close(fd);
}