7.3. Waiting for the Process to Exit

We have seen how to terminate a process and receive the exit code forcefully. Let’s wait for a process to exit normally instead of terminating it. We will use a command-line application for this instead of dealing with threads in a GUI.

7.3.1. Create a Command-Line Child Process

Copy file lab7.2.c  to lab7.3.c for this exercise.

  1. Create a function after main() to create a process and return the process ID. Put the function prototype after the include statements.

    // Function prototype
int create_process();

int main()
{
   . . .
}

/*
* Create a new windows process and returns the process ID
*/
int create_process()
{
    // create process
    // return process ID
}

  2. Paste the CreateProcess() Template Code in the function body and return the process id.

  3. Put a Sleep statement of 1500ms in the function to give the process time to start.

  4. Call create_process() in your code to start notepad and returns the PID

  5. Verify that notepad starts and then exits.

    1. You might need to increase the sleep value if you don’t see the notepad window

    Sent exit code '35' to PID 18356.

PID 18356 returned exit code '35'.

  6. Save this code to child_process.c, compile, and verify that it executes correctly.

    #include <stdio.h>
#include <windows.h>

int main()
{
    printf("\n\tChild process running...");

    for (int i = 0; i < 10; i++)
    {
        Sleep(500);
        printf("%d ", i);
    }

    printf("\n\tExiting child process normally.\n\n");

    return 0;
}

    Expected Output

    Child process running...0 1 2 3 4 5 6 7 8 9
Exiting child process normally.

  7. Replace the exe path to notepad in your code with child_process.exe.

  8. Verify that your output is similar to this.

    1. You can see that child_process was terminated before it completed counting. It didn’t reach the return 0 statement. Instead, it ended with an exit code of 35 that we sent in TerminateProcess().

    Expected Output

    Created PID '16412' from 'child_process.exe'.

    Child process running...0 1

Sent exit code '35' to PID 16412.
PID 16412 returned exit code '35'.

  9. Comment out the TerminateProcess() line to allow the child process time to finish its work. You will see something like:

    Expected Output

    Created PID '14568' from 'child_process.exe'.

    Child process running...0 1
PID 14568 returned exit code '259'.
PS C:\Users\Tobis\source\c\lab7> 2 3 4 5 6 7 8 9
    Exiting child process normally.

  10. The parent created the child process and then exited. The child process continued to run.

    1. Notice the exit code of 259, which is the GetExitCodeProcess code for STILL_ACTIVE.

    2. We can evaluate this code to determine if a process is still working.

7.3.2. Wait for the Process to Exit

In a previous lab, we saw how to wait for a process to exit using

WaitForSingleObject(process_info.hProcess, INFINITE);

Let’s do something similar by evaluating the exit code before exiting.

  1. Create a do while loop to evaluate the exit code every 1000ms. The loop should finish when the exit code is not equal to STILL_ACTIVE or 259.

    Note

    You must get the exit code during every loop iteration.

  2. Print the exit code for visual feedback.

    Expected Output

    Created PID '19468' from 'child_process.exe'.

    Child process running...0 1
PID 19468 returned exit code '259'.
2 3 PID 19468 returned exit code '259'.
4 5 PID 19468 returned exit code '259'.
6 7 PID 19468 returned exit code '259'.
8 9
    Exiting child process normally.

PID 19468 returned exit code '0'.

7.3.3. Send Termination Signal Ctrl C

Sometimes we don’t want to wait for a process to complete its task. Additionally, we don’t want to terminate it immediately. We can send a termination request to the process to start the exit procedure. We can then wait for a specified amount of time for the process to exit before calling TerminateProcess.

This is an in-depth and complex topic that we don’t have time to explore during this lab. You can read more about Terminating a Process in Windows on your own.

A simple solution for command-line applications is to send Ctrl+C, which then tries to terminate the application. The GenerateConsoleCtrlEvent function provides that functionality.

BOOL WINAPI GenerateConsoleCtrlEvent(
  _In_ DWORD dwCtrlEvent,
  _In_ DWORD dwProcessGroupId
);

Here is a working solution from Can I send a ctrl-C (SIGINT) to an application on Windows?

/*
* Sends Ctrl+C to a console process
*/
void send_control_c(int pid)
{
    AttachConsole(pid);                        // attach to process console
    SetConsoleCtrlHandler(NULL, TRUE);         // disable Control+C handling for our app
    GenerateConsoleCtrlEvent(CTRL_C_EVENT, 0); // generate Control+C event
}

You must first define _WIN32_WINNT as 0x0501 or later to use the AttachConsole function utilized in this example.

Note

The define statement must go above the #include <windows.h> derivative.

Let’s give it a try!

  1. Add #define _WIN32_WINNT 0x0501 above the include directives.

  2. Add the send_control_c(int) function to your code.

  3. Call the function to send Ctrl+C to the child process before your do while loop.

    • Your loop will continue until the child process exits.

  4. The child process should exit with an unsigned int value of -1073741510 or a hex value of c000013a, which is the Ctrl+C exit code.

  5. Change the %d to %x in your printf statement to see the hex value.

    printf("PID %d returned exit code '%d' (int) / '%x' (hex)'.\n",
   pid, returned_exit_code, returned_exit_code);

    Expected Output

    Created PID '10776' from 'child_process.exe'.

    Child process running...0 1
Sending Ctrl+C to PID 10776.
PID 10776 returned exit code '-1073741510' (int) / 'c000013a' (hex)'.