Question on select() : why am I getting absurd output ?

[Manish Jain]

Hi All,
I was trying to write a small demo code using the select() system call. 
Here are the sources :

#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <iostream>
#include <cstring>
#include <cassert>

int nice_child(int * fd, int * fd_close)
{
        close(fd[0]);
        close(fd_close[0]);
        close(fd_close[1]);

        char buffer[32];
        
        while (1)
        {
                sleep(3);
                strcpy(buffer, "I love my wife !");
                write(fd[1], buffer, strlen(buffer) + 1);
        }

        return 0;
}

int naughty_child(int * fd, int * fd_close)
{
        close(fd[0]);
        close(fd_close[0]);
        close(fd_close[1]);

        char buffer[32];
        
        while (1)
        {
                sleep(4);
                strcpy(buffer, "I love your wife !");
                write(fd[1], buffer, strlen(buffer) + 1);
        }

        return 0;
}

int main()
{
        int fd_nice[2];
        int fd_naughty[2];

        pipe(fd_nice);
        pipe(fd_naughty);

        if (fork() == 0)
        {
                return nice_child(fd_nice, fd_naughty);
        }
        else
        {
                if (fork() == 0)
                {
                        return naughty_child(fd_naughty, fd_nice);
                }
        }

        close(fd_nice[1]);
        close(fd_naughty[1]);

        fd_set fdset;
        char buffer[64];
        int fd = (*fd_naughty > *fd_nice) ? *fd_naughty : *fd_nice;

        FD_ZERO(&fdset);
        FD_SET(fd_nice[0], &fdset);
        FD_SET(fd_naughty[0], &fdset);

        while (1)
        {
                int result = select(fd + 1, &fdset, 0, 0, 0);
                assert(result > 0);

                if (FD_ISSET(fd_nice[0], &fdset))
                {
                        int result = read(fd, buffer, sizeof(buffer));
                        buffer[result] = 0;

                        std::cout << "Nice child sent : " << buffer << 
std::endl;
                }

                if (FD_ISSET(fd_naughty[0], &fdset))
                {
                        int result = read(fd, buffer, sizeof(buffer));
                        buffer[result] = 0;

                        std::cout << "Naughty child sent : " << buffer << 
std::endl;
                }
        }

        return 0;
}

I was expecting the output to be like :

Nice child sent : I love my wife !
Naughty child sent : I love your wife !
Nice child sent : I love my wife !

But what I actually get is :

Nice child sent : I love your wife !
Nice child sent : I love your wife !
Nice child sent : I love your wife !
Nice child sent : I love your wife !
Nice child sent : I love your wife !
Nice child sent : I love your wife !

Can somebody throw some light on what might be wrong ?


Thank you &
Regards

Manish Jain
invalid.poin...@gmail.com

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