Problem with multiple pipes...

This problem works with 2 piped commands (ie ls | grep main) but when I try to do more than 2 commands the program freezes.The int main() basically does the i/o and the execute command function is pretty much where im having all the problems. Thanks for any help. PS ParsedTokens is a class that parses completely the input. It puts the tokens in a vector of strings.

Code:

---


// Program asks user for input and tries to execute linux commands based on that input
// User can use multiple input piped commands ie ls | grep main | grep o.

#define _GNU_SOURCE 1

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <unistd.h>
#include <getopt.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <errno.h>
#include <fcntl.h>
#include "parsedTokens.h"
#include <string>
#include <iostream>
#include <deque>

using namespace std;

void execute_command(string left_command, string right_command);

int main()
{
        char input[256];
        string temp;
        string temp2;
        string temp3;
        ParsedTokens Tokens;
        unsigned int num_tokens;
        vector<string> commands;
    int len, len2;

        // get input
        cout << "enter text: ";
                cin.getline(input, 256, '\n');
        Tokens.Clear();
        Tokens.ParseAndAddTokens(input);
        cout << "you typed: " << input << endl;
        cout << "Parsed: " << endl;

        // lists parsed input
        for(unsigned int i = 0; i < Tokens.GetNumTokens(); i++)
        {
          cout << Tokens[i] << endl;
        }

        // puts the tokens (not including the "|") into a vector
        for(unsigned int i = 0; i < Tokens.GetNumTokens(); i++)
        {
          if(Tokens[i] != "|")
          {
                  if(temp3.empty())
                  {
                          temp3 = Tokens[i];
                          cout << "temp3: " << temp3 << endl;
                          //if not at last token dont push but add the next token that isn't a pipe to temp3
                          if(i == Tokens.GetNumTokens() - 1)
                          {
                                  commands.push_back(temp3);
                  temp3.erase(0, temp3.length());
                          }
                          else if(Tokens[i+1] == "|")    // if the next token is a | then push token in temp3
                          {
                                  commands.push_back(temp3);
                                  temp3.erase(0, temp3.length());
                          }

                  }
                  else  // if temp3 isn't empty then add space to temp3 and add next token to temp3
                  {
                      temp3.insert(temp3.length()," ");
                      temp3.insert(temp3.length(), Tokens[i]);
                          cout << "temp3: " << temp3 << " after getting put together" << endl;
                          if(i == Tokens.GetNumTokens() - 1)  // if you're at last token then push
                          {
                                  commands.push_back(temp3);
                  temp3.erase(0, temp3.length());
                          }
                          else if(Tokens[i+1] == "|")    // if the next token is a | then push token in temp3
                          {
                                  commands.push_back(temp3);
                                  temp3.erase(0, temp3.length());
                          }

                  }
          }
          /*else
          {
                  commands.push_back(temp3);
                  temp3.erase(0, temp3.length());
          }*/
    }
                 
    cout << "all the commands to be executed" << endl;

        for(int i = 0; i < commands.size(); i++)
          cout << commands[i] << endl;

        //execute_command(commands[0], commands[1]);
        for(int i = 0; i < commands.size() - 1; i++)
        {
                cout << "left command: " << commands[i] << endl;
                cout << "right command: " << commands[i+1] << endl;
                execute_command(commands[i], commands[i+1]);
        }
         
    return 0;
}

void execute_command(string left_command, string right_command)
{
  //char *argv[] = {"ls", "ls", 0};
  //char *left_arg[] = {"ls", 0};
  //char *right_arg[] = {"grep", "main", 0};
  int p[2];
  int pid;
  int len, len2;
  ParsedTokens left_comm_toks;
  ParsedTokens right_comm_toks;
     
  // int pipe(int filedes[2]);
  // filedes[0] is for reading, filedes[1] is for writing
  if(pipe(p) < 0)
  {
      cerr << strerror(errno) << endl;
      exit(EXIT_FAILURE);
  }

  pid = fork();
  if(pid < 0)
  {
      cerr << strerror(errno) << endl;
      exit(EXIT_FAILURE);
  }
  if(pid == 0)
  {
      cout << "exec left command" << endl;
          char * tempstring;
          tempstring = new char[left_command.length() + 1];
          strcpy(tempstring, left_command.c_str());
          left_comm_toks.ParseAndAddTokens(tempstring);
          len = left_comm_toks[0].length();
          const char * arg0 = left_comm_toks[0].c_str();
          // converts left_command into left_argv[] which execvp uses
          /*len = left_command.length();
          char * const myArr = new char[len + 1];
          for(int x = 0; x < len; x++)
            myArr[x] = left_command[x];
          myArr[len] = '\0';
          char * const left_argv[] = {myArr,0};*/
     
          delete [] tempstring;

          vector<string> left_args;

      for(int i = 0; i < left_comm_toks.GetNumTokens(); i++)
      {
        left_args.push_back(left_comm_toks[i]);
      }

      vector<char *> left_argv;

      for(int i = 0; i < left_args.size(); i ++)
        left_argv.push_back(const_cast<char *>(left_args[i].c_str()));

          left_argv.push_back('\0');

          close(p[0]);
          dup2(p[1], 1);
      close(p[1]);
      //argv[2] = 0;
      //
          execvp(arg0, &left_argv[0]);
      cerr << "Error on exec of " << arg0 << ": " << strerror(errno) << endl;
      _exit(errno == ENOENT ? 127 : 126);

  }

  pid = fork();
  if(pid < 0)
  {
      cerr << strerror(errno) << endl;
      exit(EXIT_FAILURE);
  }
  if(pid == 0)
  {
      cout << "executing right command" << endl;
      // converts left_command into left_argv[] which execvp uses
          /*len2 = right_command.length();
          char * const myArr2 = new char[len +1];
          for(int x = 0; x < len2; x++)
            myArr2[x] = right_command[x];
          myArr2[len2] = '\0';

          char *const right_argv[] = {myArr2, 0};*/

      char * tempstring2;
          tempstring2 = new char[right_command.length() + 1];
          strcpy(tempstring2, right_command.c_str());
          right_comm_toks.ParseAndAddTokens(tempstring2);
          len = right_comm_toks[0].length();
          const char * arg02 = right_comm_toks[0].c_str();

          delete [] tempstring2;

          vector<string> right_args;

      for(int i = 0; i < right_comm_toks.GetNumTokens(); i++)
      {
        right_args.push_back(right_comm_toks[i]);
      }

      vector<char *> right_argv;

      for(int i = 0; i < right_args.size(); i ++)
        right_argv.push_back(const_cast<char *>(right_args[i].c_str()));

          right_argv.push_back('\0');
          close(p[1]);
          dup2(p[0], 0);
      close(p[0]);
      //execvp(right_argv[0], right_argv);
      execvp(arg02, &right_argv[0]);
          cerr << "Error on exec of " << right_argv[0] << ": " << strerror(errno) << endl;
      _exit(errno == ENOENT ? 127 : 126);
  }
  close(p[0]);
  close(p[1]);

  wait(0);
  wait(0);
 
}

---