Modules/posixmodule.c (part 5)

Source:

cpython 3.14 @ ab2d84fe1023/Modules/posixmodule.c ↗

This annotation covers process management system calls. See the earlier posix parts for os.open, os.read, os.stat, and os.scandir.

Map

Lines	Symbol	Role
1-80	os.fork	Create a child process
81-160	os.execve	Replace process image
161-260	os.waitpid	Wait for a child process
261-360	os.pipe	Create a pipe pair
361-500	os.dup2	Duplicate a file descriptor

Reading

os.fork

// CPython: Modules/posixmodule.c:4280 posix_fork
static PyObject *
posix_fork(PyObject *self, PyObject *noargs)
{
    pid_t pid;
    PyOS_BeforeFork();
    pid = fork();
    if (pid == 0) {
        /* Child */
        PyOS_AfterFork_Child();
    } else {
        /* Parent */
        PyOS_AfterFork_Parent();
    }
    return PyLong_FromPid(pid);
}

PyOS_BeforeFork acquires all GIL-level locks (import lock, etc.) before forking, preventing the child from inheriting a deadlocked state. PyOS_AfterFork_Child reinitializes the GIL, threading structures, and random seed.

os.execve

// CPython: Modules/posixmodule.c:4480 posix_execve
static PyObject *
posix_execve(PyObject *self, PyObject *args)
{
    /* execve(path, args, env) — replace this process with a new program */
    const char *path;
    char **argvlist, **envlist;
    /* Build C argv and envp arrays from Python lists/dicts */
    ...
    execve(path, argvlist, envlist);
    /* execve only returns on error */
    PyErr_SetFromErrnoWithFilenameObject(PyExc_OSError, opath);
    return NULL;
}

os.execve converts the Python args list and env dict to C string arrays, then calls execve. On success, this process is replaced and Python never resumes. os.execv omits the environment.

os.pipe

// CPython: Modules/posixmodule.c:4680 posix_pipe
static PyObject *
posix_pipe(PyObject *self, PyObject *noargs)
{
    int fds[2];
    if (pipe(fds) != 0) {
        return PyErr_SetFromErrno(PyExc_OSError);
    }
    return Py_BuildValue("(ii)", fds[0], fds[1]);
}

r, w = os.pipe() creates a unidirectional pipe. Data written to w can be read from r. Used by subprocess.Popen to connect stdin/stdout/stderr between parent and child processes.

gopy notes

os.fork is module/os.Fork in module/os/module.go. It calls syscall.Fork(). Go's goroutine scheduler requires careful handling: runtime.LockOSThread is called before fork, and after fork in the child, the goroutine runtime is re-initialized. os.execve calls syscall.Exec. os.pipe calls os.Pipe().