UNIX两大贡献者贝尔实验室和BSD,在进程之间通信侧重不同,前者基于内核对进程之间的通信手段进行了改进,形成了“System V IPC”,而后者则是基于网络形成了套接字。
而POSIX则是IEEE制定的标准,目的是为运行在不同操作系统上的软件提供统一的接口,实现者则是不同的操作系统内核开发人员。
System V 以及POSIX 对信号量、共享内存、消息队列等进程之间共享方式提供了自己的解决方案。
在观察使用进程间通信手段后,会发现在多线程中使用的基本是POSIX标准提供的接口函数,而多进程则是基于System V。
pipe和fifo用的不多了,让我们从Nginx那里学一个 全双工的管道: socketpair
#include <iostream>
#include <unistd.h>
#include <sys/uio.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <stdio.h>
ssize_t
sock_fd_write(int sock, void *buf, ssize_t buflen, int fd)
{
ssize_t size;
struct msghdr msg;
struct iovec iov;
union {
struct cmsghdr cmsghdr;
char control[CMSG_SPACE(sizeof (int))];
} cmsgu;
struct cmsghdr *cmsg;
iov.iov_base = buf;
iov.iov_len = buflen;
msg.msg_name = NULL;
msg.msg_namelen = 0;
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
if (fd != -1) {
msg.msg_control = cmsgu.control;
msg.msg_controllen = sizeof(cmsgu.control);
cmsg = CMSG_FIRSTHDR(&msg);
cmsg->cmsg_len = CMSG_LEN(sizeof (int));
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
printf ("passing fd %d\n", fd);
*((int *) CMSG_DATA(cmsg)) = fd;
} else {
msg.msg_control = NULL;
msg.msg_controllen = 0;
printf ("not passing fd\n");
}
size = sendmsg(sock, &msg, 0);
if (size < 0)
perror ("sendmsg");
return size;
}
ssize_t
sock_fd_read(int sock, void *buf, ssize_t bufsize, int *fd)
{
ssize_t size;
if (fd) {
struct msghdr msg;
struct iovec iov;
union {
struct cmsghdr cmsghdr;
char control[CMSG_SPACE(sizeof (int))];
} cmsgu;
struct cmsghdr *cmsg;
iov.iov_base = buf;
iov.iov_len = bufsize;
msg.msg_name = NULL;
msg.msg_namelen = 0;
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
msg.msg_control = cmsgu.control;
msg.msg_controllen = sizeof(cmsgu.control);
size = recvmsg (sock, &msg, 0);
if (size < 0) {
perror ("recvmsg");
exit(1);
}
cmsg = CMSG_FIRSTHDR(&msg);
if (cmsg && cmsg->cmsg_len == CMSG_LEN(sizeof(int))) {
if (cmsg->cmsg_level != SOL_SOCKET) {
fprintf (stderr, "invalid cmsg_level %d\n",
cmsg->cmsg_level);
exit(1);
}
if (cmsg->cmsg_type != SCM_RIGHTS) {
fprintf (stderr, "invalid cmsg_type %d\n",
cmsg->cmsg_type);
exit(1);
}
*fd = *((int *) CMSG_DATA(cmsg));
printf ("received fd %d\n", *fd);
} else
*fd = -1;
} else {
size = read (sock, buf, bufsize);
if (size < 0) {
perror("read");
exit(1);
}
}
return size;
}
void
child(int sock)
{
int fd;
char buf[16];
ssize_t size;
sleep(1);
for (;;) {
size = sock_fd_read(sock, buf, sizeof(buf), &fd);
if (size <= 0)
break;
printf ("read %d\n", (int) size);
if (fd != -1) {
write(fd, "hello, world\n", 13);
close(fd);
}
}
}
void
parent(int sock)
{
ssize_t size;
int i;
int fd;
fd = 1;
size = sock_fd_write(sock, (void *) "1", 1, 1);
printf ("wrote %d\n", (int) size);
}
int
main(int argc, char **argv)
{
int sv[2];
int pid;
if (socketpair(AF_UNIX, SOCK_DGRAM, 0, sv) < 0) {
perror("socketpair");
exit(1);
}
switch ((pid = fork())) {
case 0:
close(sv[0]);
child(sv[1]);
break;
case -1:
perror("fork");
exit(1);
default:
close(sv[1]);
parent(sv[0]);
break;
}
return 0;
}