如何使用 dup2 将 stdin 和 stdout 重定向到管道文件描述符？

Question

I was trying to fork a process and redirect stdout of the parent to the writing end of the pipe and stdin of the child to the reading end of the pipe. The child is supposed to read integers until the parent prints zero. the parent prints from 1 to 3 and then prints 0. Both the parent and the child prints the time when they start and when they finish. since the parent can't print to stdout it sends it's starting and finishing time to the child and the child prints both its starting time and finishing time and parents starting time and finishing time. I could've used dup and redirected stdout to another file descriptor but I chose to make it simple. The program is very simple but the output that I get doesn't make scene.

#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>

int main() 
{ 
    int fd[2];
    int p = pipe(fd);
    int ch = fork();

    if (ch)
    {
        // Parent - Counts from 1 to 3
        int dp = dup2(fd[1], 1);
        printf("Cnt_Started_at_%d\n", time(NULL));
        for (int i = 0; i <= 3; i++)
        {
            printf("Parent %d\n", i);
            sleep(1);
        }
        printf("0\n");
        printf("Cnt_Finished_at_%d\n", time(NULL));
    }
    else 
    {
        // Child - Terminated by 0
        int dp = dup2(fd[0], 0);
        printf("Trm_Started_at_%d\n", time(NULL));
        char buffer[100];
        scanf("%s", buffer);
        printf("%s\n", buffer);

        int i; 
        while (scanf("Parent %d", &i) && i)
            printf("Recieved: %d\n", i);

        scanf("%s", buffer);
        printf("%s\n", buffer);
        printf("Trm_Finished_at_%d\n", time(NULL));
    }
}

Output:

Trm_Started_at_1578295974
Cnt_Started_at_1578295974
Parent
Trm_Finished_at_1578295978

Answer 1

The root issue is the usage of 'scanf %s' to read messages. Recall that '%s' will stop reading when it encounter white space, and will put the white space back into the buffer.

The initial message from the parent is 'Cnt_Started_at_1234\\n'. This child process will read the token, but will leave the trailing '\\n' in the buffer.

Next the parent will send 'Parent 0\\n'. The Child will attempt to parse this is scanf("Parent %d", &i) && i) . Two issues here:

• The 'P' from 'Parent' will not match the left over '\\n' from the initial message
• When the format is updated to skip of leading spaces, the value of 'i' will be zero, which will cause the while to exit after reading 'Parent 0'.

Possible solution: Allow the scanf to skip of spaces, and eliminate the condition on i

    while (scanf(" Parent %d", &i) == 1 )
            printf("Recieved: %d\n", i);

Answer 2

The problem here is with your scanf statement. As suggested by @dash-o, change it to treat spaces.

Another problem is that, first i = 0 . You need to modify your while to accomodate 0.

Since you are only evaluating i in your while loop , you won't be entering for ``i=0``` case.

Below is the modified program and the output, also, please add various checks for return values of the functions / buffer overflows as you deem right --

#include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>

int main()
{
        int fd[2];
        int p = pipe(fd);
        int ch = fork();

        if (ch)
        {
                // Parent - Counts from 1 to 3
                int dp = dup2(fd[1], 1);
                printf("Cnt_Started_at_%d\n", time(NULL));
                for (int i = 0; i <= 3; i++)
                {
                        printf("Parent %d\n", i);
                        sleep(1);
                }
                printf("0\n");
                printf("Cnt_Finished_at_%d\n", time(NULL));
        }
        else
        {
                // Child - Terminated by 0
                int dp = dup2(fd[0], 0);
                printf("Trm_Started_at_%d\n", time(NULL));
                char buffer[100];
                scanf("%s", buffer);
                printf("%s\n", buffer);

                int i;
                while (scanf(" Parent %d", &i) && i >= 0) // notice change here ...
                        printf("Recieved: %d\n", i);

                scanf("%s", buffer);
                printf("%s\n", buffer);
                printf("Trm_Finished_at_%d\n", time(NULL));
        }
}

OUTPUT --
$ ./main.out
Trm_Started_at_1578303662
Cnt_Started_at_1578303662
Recieved: 0
Recieved: 1
Recieved: 2
Recieved: 3
0
Trm_Finished_at_1578303666