C - dup2 stdin make dup2 stdout and stderr stop printing - c

Question

I'm having a strange problem. I'm currently creating two programs, one will execute on background and the other one will read its output and also allow to make inputs to the other one running on background.
The problem is, when I put the line dup2(in, STDIN_FILENO) the program executing on background stops printing to the files.
The Reader:
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
int main()
{
int out, err, in;
size_t i = 0, j = 0;
int offErr = 0, offOut = 0;
char bufErr[1024];
char bufOut[1024];
char *cFifo = "/tmp/out";
char *cFifoErr = "/tmp/err";
char *cFifoIn = "/tmp/in";
out = open(cFifo, O_RDONLY|O_CREAT, 0600);
err = open(cFifoErr, O_RDONLY|O_CREAT, 0600);
in = open(cFifoIn, O_WRONLY|O_CREAT, 0600);
while(1)
{
memset(bufOut, 0, 1024);
memset(bufErr, 0, 1024);
i=0;
j=0;
while(!i && !j)
{
j = pread(err, bufErr, 1024, offErr);
i = pread(out, bufOut, 1024, offOut);
offOut +=i;
offErr +=j;
}
if(i)
write(STDOUT_FILENO, bufOut, 1024);
if(j)
write(STDOUT_FILENO, bufErr, 1024);
}
return 0;
}
and the Writer (background):
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
int main()
{
int out, in, err;
char *cFifo = "/tmp/out";
char *cInFifo = "/tmp/in";
char *cErrFifo = "/tmp/err";
out = open(cFifo, O_RDWR|O_CREAT|O_TRUNC, 0600);
in = open("/tmp/in", O_RDWR|O_CREAT|O_TRUNC, 0600);
err = open(cErrFifo, O_RDWR|O_CREAT|O_TRUNC, 0600);
dup2(out, STDOUT_FILENO);
dup2(err, STDERR_FILENO);
//dup2(in, STDIN_FILENO); //if I uncomment this it stops printing to out or err
system("python");
while(1);
return 0;
}

Answer 1

When you uncomment:
//dup2(in, STDIN_FILENO); //if I uncomment this it stops printing to out or err
you are making Python read from an empty file (it was created and/or truncated when you opened it), so Python gets nothing to do and exits.
Your reader code then sits in the while (1) ; loop doing nothing useful.
At the very least, add a printf() after the system() statement so you know when the Python command has completed.