cpipedup2

can't read pipe after using dup2 to copy stdout

I am trying to use a pipe to rederect stdout into a pipe and read it later. I will use this later with fork(), where the child process starts a different program that I need to comunicate with. This is my Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>

int main(){
printf("Starting Test\n");

int myPipe[2], nbytes;
char readbuffer[80];

pipe(myPipe);

int backup = dup(1);    //store stdout
if (dup2(1,myPipe[1])< 1){printf("error");}     //copy stdout in the input end of my pipe
printf("in pipe\n");    //print something in stdout -> in my pipe
nbytes = read(myPipe[0],readbuffer,sizeof(readbuffer)); //read output of my pipe

dup2(myPipe[1],backup); // restore stdout
printf("recived: %s",readbuffer);   //prit out what I recived

return 0;
}

I expected it to print out:

Starting Test
recived: in pipe

But the output that I get is:

Starting Test
in pipe
recived: @����U

So I assume that stdout was not copied properly, as I get the "in pipe" before the "recived: ..." But the dup2() call throws no errors.

I read some tutorials, mostly this one https://tldp.org/LDP/lpg/node11.html but I can't find my error... Thank you for your help!

Solution

  • The code has a couple of problems:

    1. In dup2(1,myPipe[1]) the parameters are back to front. That makes mypipe[1] be the same as 1. But instead you need it to be the other way around: dup2(myPipe[1],1)

    2. dup2(myPipe[1],backup) is also wrong. That makes backup be the same as mypipe[1]. What you want instead is to make 1 the same as backup: dup2(backup, 1).

    3. Smaller problem but printf does not output a NUL character. So the read will not result in a valid NUL terminated string. Solve that by initialising: char readbuffer[80] = "";