How "os.system()" Deals with Signals
by yaobin.wen
I encountered a problem the other day in the software platform that I am developing these days. The problem can be briefly described as three facts:
- I am writing a Python script that will run infinitely, but is listening to the SIGINT to break the loop and terminate.
- This Python script calls some underlying shell commands by using the
os.system()in order to finish its tasks. - This Python script runs in the background so the user cannot terminate it by pressing the Ctrl + C. Instead, the user needs to use
kill -2 <pid>to do so.
The problem was: Sometimes the os.system() calls a command that runs long time. During this time, executing kill -2 <pid> does not kill the target process successfully.
After searching on the Internet, I realized that this was caused by the fact that Python’s os.system(), which will further calls the C’s system() function, ignores the SIGINT signal. In os.system() function, the command that is called is spawned as a new process, but this new process will listen and handle the signals. As a result, when the script is running in the foreground, pressing Ctrl + C will send a SIGINT to all the processes in the process group so they can be terminated as expected. However, kill sends the specified signal only to the specified process, not the entire group. Because the target process, in my case, is the one that calls the os.system() and it ignores the SIGINT, it cannot be terminated as expected.
Here are the notes that I made about this problem.
First, Python’s os.system() calls C’s system() function. This can be seen from the source code of os.system():
#ifdef HAVE_SYSTEM
PyDoc_STRVAR(posix_system__doc__,
"system(command) -> exit_status\n\n\
Execute the command (a string) in a subshell.");
static PyObject *
posix_system(PyObject *self, PyObject *args)
{
char *command;
long sts;
if (!PyArg_ParseTuple(args, "s:system", &command))
return NULL;
Py_BEGIN_ALLOW_THREADS
sts = system(command);
Py_END_ALLOW_THREADS
return PyInt_FromLong(sts);
}
#endif
(This question shows where to find the source code.)
In glibc, C’s system() function is an alias of do_system() function which is implemented with the calls to fork(), execl() and waitpid(), as shown in its source code.
The source code shows that the SIGINT and SIGQUIT are both ignored:
Line 58: A signal actionsais defined.Line 64:sa’s handler is assigned toSIG_IGN.Line 68 ~ 83: Ignore theSIGINTandSIGQUIT.
The lines in Line 115 ~ 119 spawn a new child process. On the child side, the normal handling of SIGINT and SIGQUIT is restored, and then __execve() is called to run the specified program.
However, on the parent side, which is also the side which initiated the do_system() call, the handling of these two signals are not restored. Shortly, it calls the __waitpid() to wait for the completion of the just-spawned child process.
After the child process is completed, the normal signal handling is restored in the parent process, as shown in Line 157 ~ 171.