在Linux中似乎没有正常的系统调用，python如何确定PID？

Question

When running the following command

strace -f python3 -c 'import os; print(os.getpid())'

I noticed that strace does not catch the call to the getpid (2) system call. I first considered that this was due to glibc caching the pid, but there shouldn't be a pid for libc to cache without at least a single real system call. Then I considered that maybe vdso was the culprit, but running a C program that makes this system call through libc shows a getpid call when straced. I finally gave up and looked up the source of the os.getpid python module, which apparently seems to be defined in Modules/posixmodule.c . To my surprise (and subsequent confusion), it makes a normal call to getpid !

So my question is: How does python determine the result of os.getpid ? and if such value is indeed obtained by a call to getpid , how is that call actually being made?

Answer 1

The way the vdso works is, among other things, mapping process-specific variables into userspace that the vdso functions know how to read. One of them is the current process ID, so gettimeofday doesn't need to make a syscall to access that information.

Now, specifically for getpid , it's not actually a VDSO call. In glibc before 2.25, the library would cache calls, and since part of the Python runtime calls getpid , there wouldn't be calls to it after the first. From 2.25 onward, the library doesn't cache the process ID, and so every getpid call results in a syscall.