In python with pywin32, win32job, the CreateJobObject function, how do I pass NULL for the name parameter?

Question

I'm trying to use the win32job API that is part of the PyWin32 package. I want to do this:

win32job.CreateJobObject(None, NULL)

I want to pass NULL as the second parameter as documented here:

https://msdn.microsoft.com/en-us/library/windows/desktop/ms682409(v=vs.85).aspx

If lpName is NULL, the job is created without a name.

How do I pass in NULL?

Here's what I hoped would work, but doesn't:

win32job.CreateJobObject(None, None)

Error:

TypeError: None is not a valid string in this context

---

(Side question... if anyone knows how to view JobObjects associated with a process in Windows, that would be helpful.)

Answer 1

You can use win32job.CreateJobObject(None, ""). Although it isn't specified on [MS.Docs]: CreateJobObjectA function, the empty string acts just like NULL.

According to the link above:

If the function succeeds, the return value is a handle to the job object. The handle has the JOB_OBJECT_ALL_ACCESS access right. If the object existed before the function call, the function returns a handle to the existing job object and GetLastError returns ERROR_ALREADY_EXISTS.

I wrote a small C program for demonstrating purposes:

#include <Windows.h>
#include <stdio.h>

#define EMPTY_TEXT ""
#define DUMMY0_TEXT "dummy0"
#define DUMMY1_TEXT "dummy1"

#define DIM 12

int main() {
    char* names[DIM] = { NULL, NULL, EMPTY_TEXT, EMPTY_TEXT, DUMMY0_TEXT,
                         DUMMY0_TEXT, EMPTY_TEXT, DUMMY0_TEXT, DUMMY1_TEXT,
                         NULL, DUMMY0_TEXT, DUMMY1_TEXT };
    HANDLE jobs[DIM] = { NULL };
    for (int i = 0; i < DIM; i++) {
        jobs[i] = CreateJobObjectA(NULL, names[i]);
        printf("%02d [%6s] CreateJobObject: %08X - GetLastError: %d\n", i, names[i], (long)jobs[i], GetLastError());
    }
    for (int i = 0; i < DIM; i++)
        CloseHandle(jobs[i]);
    return 0;
}

Output (built and ran with VStudio 2015 Community Edition):

00 [(null)] CreateJobObject: 000000D8 - GetLastError: 0
01 [(null)] CreateJobObject: 000000E0 - GetLastError: 0
02 [      ] CreateJobObject: 00000088 - GetLastError: 0
03 [      ] CreateJobObject: 000000F0 - GetLastError: 0
04 [dummy0] CreateJobObject: 000000F4 - GetLastError: 0
05 [dummy0] CreateJobObject: 000000F8 - GetLastError: 183
06 [      ] CreateJobObject: 000000E8 - GetLastError: 0
07 [dummy0] CreateJobObject: 000000FC - GetLastError: 183
08 [dummy1] CreateJobObject: 00000100 - GetLastError: 0
09 [(null)] CreateJobObject: 000000DC - GetLastError: 0
10 [dummy0] CreateJobObject: 000000E4 - GetLastError: 183
11 [dummy1] CreateJobObject: 00000104 - GetLastError: 183

You can see that for "dummy0" and "dummy", every time (excepting the 1^st) when creating a new object, the function does return a new HANDLE, but fails (GetLastError returns 183 (which is ERROR_ALREADY_EXISTS)). This doesn't happen for NULL and the empty string names (from here I understand that a new object is created with every call instead of incrementing an existing object's reference).

Python "translation" (code.py):

#!/usr/bin/env python3

import win32job
import win32api

DUMMY0 = "dummy00"
DUMMY1 = "dummy11"

JOB_NAMES = ["", "", DUMMY0, DUMMY0, "", DUMMY1, DUMMY0, "", DUMMY1]

if __name__ == "__main__":
    handles = list()
    for i, name in enumerate(JOB_NAMES):
        h = win32job.CreateJobObject(None, name)
        print("{} [{:10}] {} - GetLastError: {}".format(i, name, h, win32api.GetLastError()))
        handles.append(h)

    for h in handles:
        win32api.CloseHandle(h)

Output (same result as in C's case - which is natural since Python functions only wrap the lower level C ones):

(py35x64_test) e:\Work\Dev\StackOverflow\q046800142>"c:\Work\Dev\VEnvs\py35x64_test\Scripts\python.exe" code.py
0 [          ] <PyHANDLE:300> - GetLastError: 0
1 [          ] <PyHANDLE:308> - GetLastError: 0
2 [dummy00   ] <PyHANDLE:580> - GetLastError: 0
3 [dummy00   ] <PyHANDLE:584> - GetLastError: 183
4 [          ] <PyHANDLE:588> - GetLastError: 0
5 [dummy11   ] <PyHANDLE:592> - GetLastError: 0
6 [dummy00   ] <PyHANDLE:596> - GetLastError: 183
7 [          ] <PyHANDLE:600> - GetLastError: 0
8 [dummy11   ] <PyHANDLE:604> - GetLastError: 183

Regarding the "side" question: unfortunately, I'm not familiar with that topic.