How to find out whether child process still is running? - c++

Question

I am spawning a process in my application:
int status = posix_spawnp(&m_iProcessHandle, (char*)strProgramFilepath.c_str(), NULL, NULL, argsWrapper.m_pBuffer, NULL);
When I want to see if the process is still running, I use kill:
int iReturn = kill(m_iProcessHandle,0);
But after the spawned process has finished its work, it hangs around. The return value on the kill command is always 0. Not -1. I am calling kill from within the code, but if I call it from the command line, there is no error - the spawned process still exists.
Only when my application exits does the command-line kill return "No such process".
I can change this behavior in my code with this:
int iResult = waitpid(m_iProcessHandle, &iStatus, 0);
The call to waitpd closes down the spawned process and I can call kill and get -1 back, but by then I know the spawned process is dead.
And waitpd blocks my application!
How can I test a spawned processes to see if it is running, but without blocking my application?
UPDATE
Thanks for the help! I have implemented your advise and here is the result:
// background-task.cpp
//
#include <spawn.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <signal.h>
#include "background-task.h"
CBackgroundTask::CBackgroundTask()
{
// Initialize member variables
m_iProcessHandle = 0;
}
CBackgroundTask::~CBackgroundTask()
{
// Clean up (kill first)
_lowLevel_cleanup(true);
}
bool CBackgroundTask::IsRunning()
{
// Shortcuts
if (m_iProcessHandle == 0)
return false;
// Wait for the process to finish
int iStatus = 0;
int iResult = waitpid(m_iProcessHandle, &iStatus, WNOHANG);
return (iResult != -1);
}
void CBackgroundTask::Wait()
{
// Wait (clean up without killing)
_lowLevel_cleanup(false);
}
void CBackgroundTask::Stop()
{
// Stop (kill and clean up)
_lowLevel_cleanup(true);
}
void CBackgroundTask::_start(const string& strProgramFilepath, const string& strArgs, int iNice /*=0*/)
{
// Call pre-start
_preStart();
// Split the args and build array of char-strings
CCharStringAarray argsWrapper(strArgs,' ');
// Run the command
int status = posix_spawnp(&m_iProcessHandle, (char*)strProgramFilepath.c_str(), NULL, NULL, argsWrapper.m_pBuffer, NULL);
if (status == 0)
{
// Process created
cout << "posix_spawn process=" << m_iProcessHandle << " status=" << status << endl;
}
else
{
// Failed
cout << "posix_spawn: error=" << status << endl;
}
// If process created...
if(m_iProcessHandle != 0)
{
// If need to adjust nice...
if (iNice != 0)
{
// Change the nice
stringstream ss;
ss << "sudo renice -n " << iNice << " -p " << m_iProcessHandle;
_runCommand(ss.str());
}
}
else
{
// Call post-stop success=false
_postStop(false);
}
}
void CBackgroundTask::_runCommand(const string& strCommand)
{
// Diagnostics
cout << "Running command: " << COUT_GREEN << strCommand << endl << COUT_RESET;
// Run command
system(strCommand.c_str());
}
void CBackgroundTask::_lowLevel_cleanup(bool bKill)
{
// Shortcuts
if (m_iProcessHandle == 0)
return;
// Diagnostics
cout << "Cleaning up process " << m_iProcessHandle << endl;
// If killing...
if (bKill)
{
// Kill the process
kill(m_iProcessHandle, SIGKILL);
}
// Diagnostics
cout << "Waiting for process " << m_iProcessHandle << " to finish" << endl;
// Wait for the process to finish
int iStatus = 0;
int iResult = waitpid(m_iProcessHandle, &iStatus, 0);
// Diagnostics
cout << "waitpid: status=" << iStatus << " result=" << iResult << endl;
// Reset the process-handle
m_iProcessHandle = 0;
// Call post-stop with success
_postStop(true);
// Diagnostics
cout << "Process cleaned" << endl;
}

Answer 1

Until the parent process calls one of the wait() functions to get the exit status of a child, the child stays around as a zombie process. If you run ps during this time, you'll see that the process is still there in the Z state. So kill() returns 0 because the process exists.
If you don't need to get the child's status, see How can I prevent zombie child processes? for how you can make the child disappear immediately when it exits.