Why is this not printing to standard output (stdout)? - c

Question

I'm currently creating my own command line shell, and I'm having problems when trying to take in pipes. My program starts with the parent process. It checks to see if the user puts in exit or history. If so it uses those commands, but this is not important. If the user puts in anything other than exit or history, then it creates a child who executes the current task.
However, if the user puts in a command that has a single pipe, then we start at the parent and it creates a child process, call this child, child 1. child 1 see's that there is a pipe and uses fork to create another child, call this child 2. Child 1 creates another child, call it child 3 (Note: child 2 and 3 use shared memory). Now, child 2 executes the first command, and child 3 executes the command using child 2's output. But I'm not getting any output for some reason.
I'm not sure if this is the most efficient way of executing my task, but this is what my professor said to do.
If you want to see all of my code here it is: http://pastebin.com/YNTVf3XP
Otherwise here is my code starting at child 1:
//Otherwise if we have a single pipe, then do
else if (numPipes == 1) {
char *command1[CMD_MAX]; //string holding the first command
char *command2[CMD_MAX]; //string holding the second command
int fds[2]; //create file descriptors for the parent and child
pid_t new_pid; //create new pid_t obj
onePipeSplit(tokens, command1, command2, numTokens); //Getting each command for pipe
if (pipe(fds) < 0) { //use the pipe command. If < 0
perror("Pipe failure."); //we have an error, so exit
exit(EXIT_FAILURE);
}
//Creating child 2
new_pid = fork();
//if pid < 0 then we have an error. Exit.
if (new_pid < 0) {
perror("Fork failure.");
exit(EXIT_FAILURE);
}
//Else we have the child (2) process
else if (new_pid == 0) {
close(fds[0]); //Close read
//sending stdin to the shared file descriptor
if (dup2(fds[1], STDOUT_FILENO)<0) {
perror("Can't dup");
exit(EXIT_FAILURE);
}
execvp(command1[0], command1); //Execute the next command
perror("Exec failure");
exit(EXIT_FAILURE);
}
//Else if we have the parent process
else {
pid_t child = fork(); //Creating child 3
if (new_pid < 0) { //if pid < 0, error, exit
perror("Fork failure.");
exit(EXIT_FAILURE);
}
//else if pid > 0 we have the parent wait until children execute
else if (new_pid > 0) {
wait(NULL);
}
//else we have the child (3)
else {
close(fds[1]); //Close write
//Sending stdout to the shared file descriptor
if (dup2(fds[0], STDIN_FILENO) < 0) {
perror("Can't dup");
exit(EXIT_FAILURE);
}
execvp(command2[0], command2); //Execute the command
perror("Exec failure");
exit(EXIT_FAILURE);
}
}
}
Here is an image that my professor gave me to show how it should work.

Answer 1

The problem is here:
pid_t child = fork(); //Creating child 3
if (new_pid < 0) {
... you keep checking `new_pid` here on down,
... but you should be checking `child` here on down...
Also in onePipeSplit you need to put a NULL at the end of both command lists because execvp needs that. After the first loop add:
command1[i] = NULL;
and after the second:
command2[i] = NULL;
OK, a few more fixes:
after each dup2() you need to close the original fd. One example:
if (dup2(fds[1], STDOUT_FILENO)<0) {
perror("Can't dup");
exit(EXIT_FAILURE);
}
close(fds[1]); /* ADD ME */
and in the parent process:
//else if pid > 0 we have the parent wait until children execute
else if (child > 0) {
close(fds[0]); /* we don't use */
close(fds[1]); /* the pipe */
wait(NULL);
exit(0); /* when 2 & 3 are done, we are too */
}