Question

i have written the following code to copy a string “hello world” to another char array using fork and pipes instead of using standard library functions or standard i/o streams. The program is compiling successfully but i am not getting any output. Even, the printf’s output are not being shown.

# include <string.h>
# include <unistd.h>
# include <stdio.h>

char string[] = "hello world";

int main()

{

        int count, i;
        int toPar[2], toChild[2];
        char buf[256];
        pipe(toPar);
        pipe(toChild);

        if (fork() == 0)
        {
                printf("\n--- child process ---");
                close(0);
                dup(toChild[0]);
                close(1);
                dup(toPar[1]);
                close(toPar[1]);
                close(toChild[0]);
                close(toPar[0]);
                close(toChild[1]);
                for (;;)
                {
                        if ((count = read(0, buf, sizeof(buf))) == 0)
                                break;
                        printf("\nChild buf: %s", buf);
                        write(1, buf, count);
                }
        }

        printf("\n--- parent process ---");
        close(1);
        dup(toChild[1]);
        close(0);
        dup(toPar[0]);
        close(toPar[1]);
        close(toChild[0]);
        close(toPar[0]);
        close(toChild[1]);
        for (i = 0; i < 15; i++)
        {
                write(1, string, strlen(string));
                printf("\nParent buf: %s", buf);
                read(0, buf, sizeof(buf));
        }
        return 0;

   }

Answer 1

Your printfs are writing to stdout – but in both the parent and child, you’ve redirected file descriptor 1 to a pipe, so that’s where the printf output will go.

Instead of printf(...), use fprintf(stderr, ...) – then you’ll be able to see the output, since stderr is still pointing to your terminal.

Note that you have a couple of bugs:

• the child should call _exit(0) when it is done, otherwise it will drop into the parent code;
• the write should use strlen(string) + 1, so that it writes the nul terminator.