Question

现在我尝试了解子进程的 stdin / out / err 的分支/重新绑定，并正确管理资源（文件句柄，套接字）而不泄漏任何内容资源。

还有一些问题：在我创建 socketpair 和fork之后，我在父5个文件描述符和子文件中（ stdin / out / err / socket1 / socket2 ）。在子进程中，我需要关闭socketpair的“父”端。我在fork之后关闭（） stdin / out / err ，然后将 dup（）套接字的“客户端”关闭三次。在 dup（）之后，我是否需要关闭dup的“来源”？我想是的......但我是对的吗？

当我以这种方式创建（见下文）第二个孩子时，资源处理是对的吗？我试图严重依赖RAII而不泄漏任何fds，但这是对的吗？我想念一件大事吗？

再见，谢谢！

乔治

编辑：我修复了rebind_and_exec_child中的错误。

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>

#include <memory>
#include <cassert>

// this handle takes a fd, behaves like an int and makes sure the fd is closed again
class fdhandle {
public:
    explicit fdhandle(int fd) {
        mp_fd = std::shared_ptr<int>(new int, [=](int* pfd) {
            close(*pfd);
            delete pfd;
        });
        assert(mp_fd);
        *mp_fd = fd;
    }
    operator int() {
        assert(mp_fd);
        return *mp_fd;
    }
private:
    std::shared_ptr<int>    mp_fd;
};

void rebind_and_exec_child(fdhandle fd, std::string exe) {
    // now close the std fds and connect them to the given fd
    close(0);   close(1);   close(2);

    // dup the fd three times and recreate stdin/stdout/stderr with fd as the target
    if (dup(fd) != 0 || dup(fd) != 1 || dup(fd) != 2) {
        perror("error duplicating socket for stdin/stdout/stderr");
        exit(EXIT_FAILURE);
    }

    // now we can exec the new sub process and talk to it through 
    // stdin/stdout/stderr
    char *arguments[4] = { exe.c_str(), exe.c_str(), "/usr/bin", NULL };
    execv(exe.c_str(), arguments);

    // this could should never be reached
    perror("error: executing the binary");
    exit(EXIT_FAILURE);
}

fdhandle fork_connected_child(std::string exe) {
    // create the socketpair
    int fd[2];
    if (-1 == socketpair(PF_LOCAL, SOCK_STREAM, 0, fd)) {
        perror("error, could not create socket pair");
        exit(EXIT_FAILURE);
    }
    fdhandle fdparent(fd[0]);   fdhandle fdchild(fd[1]);

    // now create the child
    pid_t pid = fork();
    switch (pid) {
    case -1:    // could not fork
        perror("error forking the child");
        exit(EXIT_FAILURE);
        break;

    case 0: // child
        rebind_and_exec_child(fdchild);
        break;

    default:    // parent
        return fdparent;
        break;
    }       
}

int main(int argc, const char** argv) {
    // create 2 childs
    fdhandle fdparent1 = fork_connected_child("/bin/ls");
    fdhandle fdparent2 = fork_connected_child("/bin/ls");   
}

Answer 1

我想，我找到了解决方案。对于socketpair()来电中每个创建的套接字，我设置了FD_CLOEXEC。这样，我可以确定内核关闭所有文件描述符。由我的代码处理的所有其他套接字将被fdhandle类调用close()关闭。重新绑定stdin / stdout / stderr，我将dup()替换为dup2()因为它确实关闭并重复进行重复操作。

提示是这个页面： http://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html

在调用过程映像中打开的文件描述符应保持打开状态在新的过程映像中，除了那些close-on-exec标志 FD_CLOEXEC已设置。对于那些保持打开的文件描述符，全部打开文件描述的属性保持不变。对于任何文件由于这个原因关闭的描述符，文件锁被删除为关闭的结果，如close()中所述。锁定未删除通过关闭文件描述符保持不变。

现在是我调整后的代码：

编辑：调整后的结构

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/wait.h>

#include <memory>
#include <cassert>
#include <iostream>

// this handle takes a fd, behaves like an int and makes sure the fd is closed again
class fdhandle {
public:
    fdhandle()  {}
    explicit fdhandle(int fd) {
        mp_fd = std::shared_ptr<int>(new int, [=](int* pfd) {
            close(*pfd);
            delete pfd;
        });
        assert(mp_fd);
        *mp_fd = fd;

        // set FD_CLOEXEC on fd
        int flags;
        flags = fcntl(fd, F_GETFD);
        if (-1 == flags) {
            perror("error, could not get flags from filedescriptor");
            exit(EXIT_FAILURE);
        }
        flags |= FD_CLOEXEC;
        if (fcntl(fd, F_SETFD, flags) == -1) {
            perror("error, could not set FD_CLOEXEC");
            exit(EXIT_FAILURE);
        }
    }
    operator int() {
        assert(mp_fd);
        return *mp_fd;
    }

    void show_fd_status() {
        if (!mp_fd)
            return;

        int fd = *mp_fd;

        using namespace std;
        char buf[256];
        int fd_flags = fcntl(fd, F_GETFD);
        if (fd_flags == -1)
            return;
        int fl_flags = fcntl(fd, F_GETFL);
        if (fl_flags == -1)
            return;
        char path[256];
        sprintf(path, "/proc/self/fd/%d", fd);
        memset(&buf[0], 0, 256);
        ssize_t s = readlink(path, &buf[0], 256);
        if (s == -1) {
            cerr << " (" << path << "): " << "not available";
            return;
        }
        cerr << fd << " (" << buf << "): ";
        // file status
        if (fd_flags & FD_CLOEXEC)  cerr << "cloexec ";
        if (fl_flags & O_APPEND)  cerr << "append ";
        if (fl_flags & O_NONBLOCK)  cerr << "nonblock ";

        // acc mode   
        if (fl_flags & O_RDONLY)  cerr << "read-only ";
        if (fl_flags & O_RDWR)  cerr << "read-write ";
        if (fl_flags & O_WRONLY)  cerr << "write-only ";
        if (fl_flags & O_DSYNC)  cerr << "dsync ";
        if (fl_flags & O_RSYNC)  cerr << "rsync ";
        if (fl_flags & O_SYNC)  cerr << "sync ";

        struct flock fl;
        fl.l_type = F_WRLCK;
        fl.l_whence = 0;
        fl.l_start = 0;
        fl.l_len = 0;
        fcntl(fd, F_GETLK, &fl);
        if (fl.l_type != F_UNLCK)
        {
            if (fl.l_type == F_WRLCK)
                cerr << "write-locked";
            else
                cerr << "read-locked";
            cerr << "(pid:" << fl.l_pid << ") ";
        }
    }
private:
    std::shared_ptr<int>    mp_fd;
};

struct child
{
    pid_t       pid;
    fdhandle    fd;
};

void rebind_and_exec_child(fdhandle fd, std::string exe) {
    // unset FD_CLOEXEC 
    int flags, oflags;
    flags = oflags = fcntl(fd, F_GETFD);
    if (-1 == flags) {
        perror("error, could not get flags from filedescriptor");
        exit(EXIT_FAILURE);
    }
    flags &= ~FD_CLOEXEC;
    if (fcntl(fd, F_SETFD, flags) == -1) {
        perror("error, could not unset FD_CLOEXEC");
        exit(EXIT_FAILURE);
    }

    // close and rebind the stdin/stdout/stderr
    // dup the fd three times and recreate stdin/stdout/stderr with fd as the target
    if (dup2(fd, STDIN_FILENO) != 0 || dup2(fd, STDOUT_FILENO) != 1 || dup2(fd, STDERR_FILENO) != 2) {
        perror("error duplicating socket for stdin/stdout/stderr");
        exit(EXIT_FAILURE);
    }

    // restore the old flags
    if (fcntl(fd, F_SETFD, oflags) == -1) {
        perror("error, could not set FD_CLOEXEC");
        exit(EXIT_FAILURE);
    }

    // now we can exec the new sub process and talk to it through 
    // stdin/stdout/stderr
    char path[256];
    char argv[256];
    sprintf(path,"%s",exe.c_str());
    sprintf(argv,"%d",30);
    execlp(path, path, argv, 0);

    // this should never be reached
    perror("error: executing the binary");
    exit(EXIT_FAILURE);
}

child fork_connected_child(std::string exe) {
    // create the socketpair
    int fd[2];
    if (-1 == socketpair(PF_LOCAL, SOCK_STREAM, 0, fd)) {
        perror("error, could not create socket pair");
        exit(EXIT_FAILURE);
    }
    fdhandle fdparent(fd[0]);   fdhandle fdchild(fd[1]);

    // now create the child
    pid_t pid = fork();
    switch (pid) {
    case -1:    // could not fork
        perror("error forking the child");
        exit(EXIT_FAILURE);
        break;

    case 0: // child
        rebind_and_exec_child(fdchild, exe);
        break;

    default:    // parent
        std::cout << "forked " << exe << std::endl;
        return child { pid, fdparent };
        break;
    }       
}

int main(int argc, const char** argv) {
    // setup the signal handler prior to forking
    sleep(20);

    // create 2 childs
    {
        child child1 = fork_connected_child("/usr/bin/sleep");
        child child2 = fork_connected_child("/usr/bin/sleep");

        int status;
        waitpid(child1.pid, &status, 0);
        waitpid(child2.pid, &status, 0);
    }

    sleep(20);
}

linux：fork / socketpair / close和多个子进程

编辑：我修复了rebind_and_exec_child中的错误。

1 个答案:

编辑：调整后的结构