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Commit 5d7997f

Browse files
author
Anton Yarkov
committed
Socketpair... fd passing.
1 parent b6e1d66 commit 5d7997f

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1 file changed

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1 file changed

+286
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <netinet/in.h>
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#include <sys/types.h>
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#include <sys/wait.h>
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#include <sys/socket.h>
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#define EXIT_FAILURE 1
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// Compilation:
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// gcc "05 - Socketpair_fd_passing.c" -o socketpairfdpassing
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// Task:
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// Use of FD-passing technology to let 2 processes/programs communicate with each other and send sockets.
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// This technology should be helpful in programming multiprocess server programming to handle socket connections
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// in child processes instead of single parent process.
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void kill_child_handler(int sig)
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{
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int status;
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pid_t done = waitpid(
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-1, // Any child
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&status,
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0); // Blocked mode.
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if (done == -1)
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{
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printf("No more child processes.\n", done);
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}
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else
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{
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short isNormalTermination = WIFEXITED(status);
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if (!isNormalTermination ||
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// WEXITSTATUS should be used only if normal termination = true.
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(isNormalTermination && WEXITSTATUS(status) != 0))
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{
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printf("Zombie for PID -- %d failed.\n", done);
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exit(EXIT_FAILURE);
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}
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else
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{
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printf("Zombie for PID -- %d successfully removed.\n", done);
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}
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}
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}
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// Send information in a buffer via socket and attached additional information with that buffer i.e. our file descriptor.
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// We may send several file descriptors, but this is a simple example.
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ssize_t sock_fd_write(int sock, void *buf, ssize_t buflen, int fd)
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{
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// We should send at least 1 byte of any information except file descriptor itself.
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if (buflen < 0)
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{
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return -1;
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}
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// IOVector - describes information buffer.
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struct iovec iov;
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iov.iov_base = buf;
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iov.iov_len = buflen;
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struct msghdr msg;
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// Send only one buffer IOVector, however sendmsg() may work with several buffers by merging them together.
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msg.msg_name = NULL;
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msg.msg_namelen = 0;
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msg.msg_iov = &iov;
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msg.msg_iovlen = 1;
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if (fd != -1)
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{
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// Declare it here because it may be absent in headers.
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union
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{
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struct cmsghdr cmsghdr;
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char control[CMSG_SPACE(sizeof(int))];
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} cmsgu;
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// System info.
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msg.msg_control = cmsgu.control;
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msg.msg_controllen = sizeof(cmsgu.control);
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// Use constans and macroses to fill info.
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struct cmsghdr *cmsg;
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cmsg = CMSG_FIRSTHDR(&msg);
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cmsg->cmsg_len = CMSG_LEN(sizeof(int));
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cmsg->cmsg_level = SOL_SOCKET;
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cmsg->cmsg_type = SCM_RIGHTS;
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printf("passing fd %d\n", fd);
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*((int *)CMSG_DATA(cmsg)) = fd;
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}
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else
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{
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// We don't need to send file descriptor - work in usual sending mode.
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msg.msg_control = NULL;
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msg.msg_controllen = 0;
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printf("not passing fd\n");
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}
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// Sending data via sockets, similar to sendto.
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ssize_t size = sendmsg(sock, &msg, 0);
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if (size < 0)
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perror("sendmsg");
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return size;
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}
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ssize_t sock_fd_read(int sock, void *buf, ssize_t bufsize,
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// Result file descriptor.
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int *fd)
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{
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ssize_t size;
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if (fd)
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{
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struct iovec iov;
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iov.iov_base = buf;
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iov.iov_len = bufsize;
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struct msghdr msg;
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msg.msg_name = NULL;
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msg.msg_namelen = 0;
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msg.msg_iov = &iov;
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msg.msg_iovlen = 1;
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union
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{
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struct cmsghdr cmsghdr;
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char control[CMSG_SPACE(sizeof(int))];
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} cmsgu;
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msg.msg_control = cmsgu.control;
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msg.msg_controllen = sizeof(cmsgu.control);
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size = recvmsg(sock, &msg, 0);
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if (size < 0)
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{
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perror("recvmsg");
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exit(EXIT_FAILURE);
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}
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struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
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if (cmsg && cmsg->cmsg_len == CMSG_LEN(sizeof(int)))
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{
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if (cmsg->cmsg_level != SOL_SOCKET)
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{
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fprintf(stderr, "invalid cmsg_level %d\n",
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cmsg->cmsg_level);
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exit(EXIT_FAILURE);
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}
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if (cmsg->cmsg_type != SCM_RIGHTS)
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{
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fprintf(stderr, "invalid cmsg_type %d\n",
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cmsg->cmsg_type);
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exit(EXIT_FAILURE);
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}
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*fd = *((int *)CMSG_DATA(cmsg));
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printf("received fd %d\n", *fd);
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}
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else
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*fd = -1;
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}
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else
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{
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size = read(sock, buf, bufsize);
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if (size < 0)
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{
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perror("read");
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exit(EXIT_FAILURE);
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}
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}
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return size;
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}
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void child_read(int sock)
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{
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int fd;
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char buf[16];
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ssize_t size;
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sleep(1);
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for (;;)
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{
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size = sock_fd_read(sock, buf, sizeof(buf), &fd);
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if (size <= 0)
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break;
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printf ("read size: %d, fd: %d\n", size, fd);
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if (fd != -1)
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{
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write(fd, "hello, world\n", 13);
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close(fd);
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}
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}
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}
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void parent_writes(int sock)
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{
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ssize_t size = sock_fd_write(sock, "1", 1, 1);
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printf ("wrote size: %d\n", size);
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}
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/*
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// Multiple writes were made, some with file descriptors and some without
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void parent_writes(int sock)
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{
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ssize_t size = sock_fd_write(sock, "1", 1, -1);
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printf ("wrote %d without fd\n", size);
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size = sock_fd_write(sock, "1", 1, 1);
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printf ("wrote %d with fd\n", size);
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size = sock_fd_write(sock, "1", 1, -1);
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printf ("wrote %d without fd\n", size);
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}
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*/
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/*
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// This shows that the first passed file descriptor is picked up by the first sock_fd_read call, but the file descriptor is closed.
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// The second file descriptor passed is picked up by the second sock_fd_read call.
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void child_read(int sock)
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{
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int fd;
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char buf[16];
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ssize_t size;
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sleep(1);
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size = sock_fd_read(sock, buf, sizeof(buf), NULL);
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if (size <= 0)
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return;
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printf ("read %d\n", size);
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size = sock_fd_read(sock, buf, sizeof(buf), &fd);
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if (size <= 0)
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return;
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printf ("read %d\n", size);
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if (fd != -1)
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{
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write(fd, "hello, world\n", 13);
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close(fd);
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}
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}
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void parent_writes(int sock)
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{
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ssize_t size = sock_fd_write(sock, "1", 1, 1);
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printf ("wrote %d without fd\n", size);
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size = sock_fd_write(sock, "1", 1, 2);
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printf ("wrote %d with fd\n", size);
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}
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*/
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int main(int argc, char **argv)
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{
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int sv[2];
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int pid;
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if (socketpair(AF_LOCAL, SOCK_STREAM, 0, sv) < 0)
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{
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perror("socketpair");
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exit(EXIT_FAILURE);
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}
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// Handle child process killing.
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struct sigaction kill_child_signal;
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kill_child_signal.sa_handler = kill_child_handler;
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sigemptyset(&kill_child_signal.sa_mask);
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kill_child_signal.sa_flags = SA_RESTART; // Permanent handler.
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if (sigaction(SIGCHLD, &kill_child_signal, 0) == -1)
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{
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perror("Error of calling sigaction");
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exit(EXIT_FAILURE);
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}
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switch ((pid = fork()))
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{
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case 0:
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close(sv[0]);
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child_read(sv[1]);
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break;
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case -1:
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perror("fork");
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exit(EXIT_FAILURE);
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default:
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close(sv[1]);
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parent_writes(sv[0]);
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break;
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}
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return 0;
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}

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