因此，我为我的操作系统类分配了一项任务，其中我要创建一个与管道连接的进程环，以便在它们之间传递消息。我找到了一些示例代码，我希望根据我的需要调整（或至少理解）这些代码。示例代码（稍作修改）是：
/* Program 4.1 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
/* Sample C program for generating a unidirectional ring of processes.Invoke this program
with a command-line arg ument indicating the number of processes on the ring. Communication
is done via pipes that connect the standard output of a process to the standard input of
its successor on the ring. After the ring is created, each process identifies itself with
its process ID and the process ID of its parent. Each process then exits. */
void main(int argc, char *argv[ ])
{
int master_pid = getpid();
printf("master pid: %i\n", master_pid);
int i; /* number of this process (starting with 1) */
int childpid; /* indicates process should spawn another */
int nprocs; /* total number of processes in ring */
int fd[2]; /* file descriptors returned by pipe */
int error; /* return value from dup2 call */
/* check command line for a valid number of processes to generate */
if ( (argc != 2) || ((nprocs = atoi (argv[1])) <= 0) ) {
fprintf (stderr, "Usage: %s nprocs\n", argv[0]);
exit(1);
}
/* connect std input to std output via a pipe */
if (pipe (fd) == -1) {
perror("Could not create pipe");
exit(1);
}
printf("%s\n", "test");
//this section is blocking printf()?
if ((dup2(fd[0], STDIN_FILENO) == -1) ||
(dup2(fd[1], STDOUT_FILENO) == -1)) {
perror("Could not dup pipes");
exit(1);
}
printf("%s\n", "test");
if ((close(fd[0]) == -1) || (close(fd[1]) == -1)) {
perror("Could not close extra descriptors");
exit(1);
}
/* create the remaining processes with their connecting pipes */
for (i = 1; i < nprocs; i++) {
if (pipe (fd) == -1) {
fprintf(stderr,"Could not create pipe %d: %s\n",
i, strerror(errno));
exit(1);
}
if ((childpid = fork()) == -1) {
fprintf(stderr, "Could not create child %d: %s\n",
i, strerror(errno));
exit(1);
}
if (childpid > 0) /* for parent process, reassign stdout */
error = dup2(fd[1], STDOUT_FILENO);
else
error = dup2(fd[0], STDIN_FILENO);
if (error == -1) {
fprintf(stderr, "Could not dup pipes for iteration %d: %s\n",
i, strerror(errno));
exit(1);
}
if ((close(fd[0]) == -1) || (close(fd[1]) == -1)) {
fprintf(stderr, "Could not close extra descriptors %d: %s\n",
i, strerror(errno));
exit(1);
}
if (childpid)
break;
}
/* say hello to the world */
fprintf(stderr,"This is process %d with ID %d and parent id %d\n",
i, (int)getpid(), (int)getppid());
wait(1);
exit (0);
} /* end of main program here */
输出：
master pid: 30593
test
This is process 1 with ID 30593 and parent id 30286
This is process 2 with ID 30594 and parent id 30593
当我给 2 作为 argv[1]
所以，我想知道，为什么 dup2 部分会阻止 printf() 执行？如果我什至不能打印某些东西，我不确定我是否可以正确传递消息。另外，为什么 fprintf() 已经在那里工作了，但我不会放在那里？
编辑：我会把这个带给我的教授/助教，但他们都在城外，从现在到截止日期之前都无法到达......