IPC using multiple pipes and forked processes to run Python programs - c++

Question

I am stuck with a problem for my assignment. I am trying to execute 3 concurrent processes (in C++) out of which 2 of them are Python programs and one of them is C++ program.
My C++ program (sample.cpp):
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <string>
#include <stdlib.h>
#include <iostream>
#include <vector>
#include <signal.h>
using namespace std;
int main()
{
while (true)
{
cout << "lol" << endl;
sleep(2);
}
return 0;
}
My Python program 1 (sample.py):
import sys
while True:
line = sys.stdin.readline().strip()
print "Python says: " + str(line)
My Python program 2 (sample2.py):
import sys
while True:
line = sys.stdin.readline().strip()
print "Python 2 says: " + str(line)
Here is my driver C++ program which forks processes:
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <string>
#include <stdlib.h>
#include <iostream>
#include <vector>
#include <signal.h>
using namespace std;
int main()
{
vector<pid_t> kids;
int fd[2];
if (pipe(fd) < 0)
{
cout << "Error";
return 1;
}
int fd2[2];
if (pipe(fd2) < 0)
{
cout << "Error";
return 1;
}
pid_t pid;
pid = fork();
if (pid == 0)
{
dup2(fd[1], STDOUT_FILENO);
close(fd[1]);
close(fd[0]);
while (true)
{
execvp("./sample", NULL);
}
}
else
{
kids.push_back(pid);
pid = fork();
if (pid == 0)
{
dup2(fd[0], STDIN_FILENO);
close(fd[0]);
close(fd[1]);
dup2(fd2[1], STDOUT_FILENO);
close(fd2[1]);
close(fd2[0]);
char * python = "/usr/bin/python";
char * pythonProgram = "./sample.py";
char * pythonArgs[] = {python, pythonProgram, NULL, NULL};
execvp(python, pythonArgs);
}
else
{
kids.push_back(pid);
pid = fork();
if (pid == 0)
{
dup2(fd2[0], STDIN_FILENO);
close(fd2[0]);
close(fd2[1]);
char * python = "/usr/bin/python";
char * pythonProgram = "./sample2.py";
char * pythonArgs[] = {python, pythonProgram, NULL, NULL};
execvp(python, pythonArgs);
}
else
{
kids.push_back(pid);
}
}
}
close(fd[0]);
close(fd[1]);
close(fd2[0]);
close(fd2[1]);
for (pid_t k : kids)
{
int status;
//kill (k, SIGTERM);
waitpid(k, &status, 0);
}
}
When I run this program, I am expected to see "Python 2 says: Python says: lol". However, I see nothing (complete blank)... it just hangs. What am I doing wrong? I tried looking up a lot of things but no luck.

Answer 1

The while loop around the start of ./sample is pointless unless you expect execvp to fail. A successful call to exec* will never return. The actual call to execvp is wrong too:
execvp("./sample", NULL);
the second argument should be a char *const[].
You should add error handling for execvp:s (like a line with std::exit(1)). Otherwise if execvp fails, you'll have child processes running in the main flow of the program.
The python programs needs to be run unbuffered or else it will take a long time for the messages to appear. You should also check if the readline succeeded.
sample.py
import sys
while True:
line = sys.stdin.readline().strip()
if not line: break
print "Python says: " + str(line)
sample2.py
import sys
while True:
line = sys.stdin.readline().strip()
if not line: break
print "Python 2 says: " + str(line)
driver.cpp
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <string>
#include <stdlib.h>
#include <iostream>
#include <vector>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
using namespace std;
int main()
{
vector<pid_t> kids;
int fd[2];
if (pipe(fd)==-1)
{
clog << "Error\n";
return 1;
}
int fd2[2];
if (pipe(fd2)==-1)
{
clog << "Error\n";
return 1;
}
pid_t pid;
pid = fork();
if (pid == 0)
{
dup2(fd[1], STDOUT_FILENO);
close(fd[1]);
close(fd[0]);
char* const args[] = { NULL };
execvp("./sample", args);
std::clog << "sample failed\n";
std::exit(1);
}
else
{
kids.push_back(pid);
pid = fork();
if (pid == 0)
{
dup2(fd[0], STDIN_FILENO);
close(fd[0]);
close(fd[1]);
dup2(fd2[1], STDOUT_FILENO);
close(fd2[1]);
close(fd2[0]);
char const* python = "/usr/bin/python";
char const* pythonProgram = "./sample.py";
char const* pythonArgs[] = {python, "-u", pythonProgram, NULL};
execvp(python, const_cast<char* const*>(pythonArgs));
std::clog << "sample.py failed\n";
std::exit(1);
}
else
{
kids.push_back(pid);
pid = fork();
if (pid == 0)
{
dup2(fd2[0], STDIN_FILENO);
close(fd2[0]);
close(fd2[1]);
char const* python = "/usr/bin/python";
char const* pythonProgram = "./sample2.py";
char const* pythonArgs[] = {python, "-u", pythonProgram, NULL};
execvp(python, const_cast<char* const*>(pythonArgs));
std::clog << "sample2.py failed\n";
std::exit(1);
}
else
{
kids.push_back(pid);
}
}
}
close(fd[0]);
close(fd[1]);
close(fd2[0]);
close(fd2[1]);
for (pid_t k : kids)
{
int status;
//kill (k, SIGTERM);
waitpid(k, &status, 0);
}
}