c++ Process hangs when piping between two child processes - c++

Question

I am trying to pipe data from one child process to another. When I run this, it hangs. If I don't make it wait for the first child process, it goes back to the top of the loop prompting for commands without giving the expected output, and when I prompt it to quit, it dumps all of the output I was expecting. I had it working with just one child process, but then the second execvp killed the parent process, and I didn't get back to the top of the loop prompting for more commands.
#include <iostream>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <sys/wait.h>
using namespace std;
int main(int argc, const char * argv[]) {
bool quit=0;
char quitArray[] = "quit";
int pipeReturnValue, fork1ReturnValue, fork2ReturnValue, pipefd[2], checkForQuit;
//Enter a loop where each iteration prompts for two single-line inputs
while (!quit) {
//Get command 1
char command1[128];
printf("Enter command 1: ");
fgets(command1,128,stdin);
command1[strlen(command1) -1] = 0;
//Exit if user enters quit
checkForQuit = strncmp(command1, quitArray, 4);
if (checkForQuit == 0) {
exit(0);
}
//Get command 2
char command2[128];
printf("Enter command 2: ");
fgets(command2,128,stdin);
command2[strlen(command2) -1] = 0;
//Exit if user enters quit
checkForQuit = strncmp(command2, quitArray, 4);
if (checkForQuit == 0) {
exit(0);
}
//Open pipe
pipeReturnValue = pipe(pipefd);
if (pipeReturnValue < 0) {
perror("Pipe failed");
exit(1);
}
//Fork 1
fork1ReturnValue = fork();
if(fork1ReturnValue < 0) {
perror("Fork failed");
exit(1);
}
else if (fork1ReturnValue == 0) {
//Fork 2
fork2ReturnValue = fork();
if (fork2ReturnValue < 0) {
perror("Fork 2 failed");
}
else if (fork2ReturnValue == 0) {
//close read end of pipe
close(pipefd[0]);
//parse command 1 arguments
//store tokens in array
char *arguments[6] = {};
arguments[0] = strtok(command1, " ");
int tokenCounter = 0;
while (arguments[tokenCounter] != NULL) {
tokenCounter++;
arguments[tokenCounter] = strtok(NULL, " ");
}
//dup stdo to pipe
dup2(pipefd[1], 1);
//execute arguments
execvp(arguments[0], arguments);
}
else {
wait(&fork2ReturnValue);
//close write end of pipe
close(pipefd[1]);
//parse command 2 arguments
//store tokens in array
char *arguments[6] = {};
arguments[0] = strtok(command2, " ");
int tokenCounter = 0;
while (arguments[tokenCounter] != NULL) {
tokenCounter++;
arguments[tokenCounter] = strtok(NULL, " ");
}
//dup stdin to pipe
dup2(pipefd[0], 0);
//exec
execvp(arguments[0], arguments);
}
}
else {
wait(&fork1ReturnValue);
}
}
return 0;
}

Answer 1

I finally figured it out. I needed to open the pipe after the first fork rather than before.
#include <iostream>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <sys/wait.h>
using namespace std;
int main(int argc, const char * argv[]) {
bool quit=0;
char command1[128],
command2[128],
quitArray[] = "quit";
int pipeReturnValue,
fork1ReturnValue,
fork2ReturnValue,
checkForQuit,
pipefd[2];
//Loop where each iteration prompts for two single-line inputs
while (!quit) {
//Get command 1
printf("Enter command 1: ");
fgets(command1,128,stdin);
command1[strlen(command1) -1] = 0;
//Exit if user enters quit
checkForQuit = strncmp(command1, quitArray, 4);
if (checkForQuit == 0) {
quit = 1;
exit(0);
}
//Get command 2 and trim trailing new line character
printf("Enter command 2: ");
fgets(command2,128,stdin);
command2[strlen(command2) -1] = 0;
//Exit if user enters quit
checkForQuit = strncmp(command2, quitArray, 4);
if (checkForQuit == 0) {
quit = 1;
exit(0);
}
//Fork to create 1st child process, return error if fork fails
fork1ReturnValue = fork();
if(fork1ReturnValue < 0) {
perror("Fork 1 failed");
exit(1);
}
//Open pipe, return error if fork fails
pipeReturnValue = pipe(pipefd);
if (pipeReturnValue < 0) {
perror("Pipe failed");
exit(1);
}
//First child process
else if (fork1ReturnValue == 0) {
//Fork to create 2nd child process, return error if fork fails
fork2ReturnValue = fork();
if (fork2ReturnValue < 0) {
perror("Fork 2 failed");
}
//Second child process
else if (fork2ReturnValue == 0) {
//close read end of pipe
close(pipefd[0]);
//Parse command 1 arguments, store tokens in an array
char *arguments[6] = {};
arguments[0] = strtok(command1, " ");
int tokenCounter = 0;
while (arguments[tokenCounter] != NULL) {
tokenCounter++;
arguments[tokenCounter] = strtok(NULL, " ");
}
//Dup standard output to write side of pipe
dup2(pipefd[1], 1);
//Execute arguments from command 1
execvp(arguments[0], arguments);
}
//First child code continued
else {
//Wait for child 2 to to terminate
wait(&fork2ReturnValue);
//Close write end of pipe
close(pipefd[1]);
//Parse command 2 arguments, store tokens in array
char *arguments[6] = {};
arguments[0] = strtok(command2, " ");
int tokenCounter = 0;
while (arguments[tokenCounter] != NULL) {
tokenCounter++;
arguments[tokenCounter] = strtok(NULL, " ");
}
//dup standard input to read side of pipe
dup2(pipefd[0], 0);
//Execute arguments from command 2
execvp(arguments[0], arguments);
}
}
//Parent process continued
else {
//Wait for child 1 to terminate
wait(&fork1ReturnValue);
}
//return to top of loop
}
return 0;
}