Looping a fork() - linux

Question

This is a sample of my code for some piping I want to do. The problem is that pid2[0] does not supply me a child process. How do I fix it? pid2[1] and pid2[2] and so on do supply the parent with children.
int numCommands = numPipes + 1; /// not worrying about '>' and '<' right now
int *pipes = newint[numPipes*2]; /// two ends for each pipe
for(int i = 0; i < numPipes*2; i+=2) /// Offset by two since each pipe has two ends
pipe(pipes + i);
int *pid2 = new int [numCommands];
for(int i = 0; i < numCommands; i++)
{
pid2[i] = fork();
if(pid2[i] < 0)
{
std::cerr << "Failure to fork..." << std:endl;
return EXIT_FAILURE
}
if(pid2[i] == 0) /// Child process
{
if(i == 0) /// First Command
{
dup2(pipes[1], 1);
}
else if(i == numCommands -1) /// Last Command
{
dup2(pipes[2*(numCommands-1)-1], 0);
}
else /// Middle commands
{
dup2(pipes[2*(i-1)], 0);
dup2(pipes[(2*i)+1],1);
}
for(int j = 0; j < numPipes*2;j++)
close(pipes[j]);
execvp(pipeCommands[i][0], pipeCommands[i].data()); ///pipeCommands is a vector<vector<char*>>
perror("exec failed");
return EXIT_SUCCESS;
}
else /// The parent
{
for(int j = 0; j <numPipes*2;j++)
close(pipes[j]);
for(int k = 0; k < numCommands; k++)
waitpid(pid2[k],nullptr,0);
}
}

Answer 1

Your indexing into the pipes array is a little bit problematic. For sort file.txt | head | wc I'm assuming that numPipes is 2. Let's walk through your for loop for each value of i.
i==0
dup2(pipes[1], 1); // Send stdout to pipes[1]
i==1
dup2(pipes[2*(i-1)], 0); // dup2(pipes[0], 0), stdin from pipes[0]
dup2(pipes[(2*i)+1],1); // dup2(pipes[3], 1), stdout to pipes[3]
i == 2
dup2(pipes[2*(numCommands-1)-1], 0); //dup2(pipes[3], stdin from pipes[3]
In other words, any stdout from process 0 is going to a dead end. The second process will never read from its standard output. So the debug statement you put in there (cout == stdout, remember) will get lost as well.