Modules/posixmodule.c

cpython 3.14 @ ab2d84fe1023/Modules/posixmodule.c ↗

Modules/posixmodule.c is the largest module in CPython. It provides the os and posix modules by wrapping POSIX system calls. On Windows the same file compiles to an nt module with Win32 equivalents.

Map

Lines	Symbol	Role
1-1000	stat_result, statvfs_result	Named-tuple wrappers for stat() output
1001-3000	os_stat_impl, os_fstat_impl, os_lstat_impl	File metadata
3001-5000	os_open_impl, os_read_impl, os_write_impl, os_close_impl	File I/O
5001-7000	os_listdir_impl, os_scandir_impl, DirEntry	Directory listing
7001-9000	os_mkdir_impl, os_rmdir_impl, os_rename_impl, os_getcwd_impl	Filesystem mutation
9001-11000	os_fork_impl, os_execve_impl, os_waitpid_impl	Process management
11001-14000	Environment, os.path bootstrap, os.walk, misc	Remaining API

Reading

DirEntry and scandir

os.scandir() returns an iterator of DirEntry objects. Unlike listdir, DirEntry caches the stat result from the directory read, so entry.stat() is often a no-op. entry.is_file() and entry.is_dir() check the cached d_type field from dirent.

// CPython: Modules/posixmodule.c:5420 DirEntry_stat
static PyObject *
DirEntry_stat(DirEntry *self, PyObject *args, PyObject *kwargs)
{
    if (!self->stat) {
        self->stat = _PyObject_CallMethodIdOneArg(
            PyImport_ImportModuleNoBlock("os"),
            &PyId_stat, self->path);
    }
    return Py_NewRef(self->stat);
}

os_fork_impl

os.fork() calls fork() and then runs _PyInterpreterState_AfterFork in the child to reinitialize interpreter state (reset GIL, clear signal handlers, reset the import lock).

GIL release for blocking calls

All blocking syscalls (read, write, open, waitpid) release the GIL with Py_BEGIN_ALLOW_THREADS so other threads can run while the syscall is in progress.

os.environ proxy

os.environ is a Mapping that calls os.getenv/os.putenv/os.unsetenv on access. The posixmodule.c side provides the raw getenv/putenv; the os module in Python wraps it in _Environ.

gopy notes

Not yet ported. os is a foundational module. The planned path is module/os/. The Go implementation will use os and syscall packages. os.fork / os.execve map to syscall.ForkExec. DirEntry maps to os.DirEntry from os.ReadDir.