Python/thread.c

/* Thread package.
   This is intended to be usable independently from Python.
   The implementation for system foobar is in a file thread_foobar.h
   which is included by this file dependent on config settings.
   Stuff shared by all thread_*.h files is collected here. */

#include "Python.h"


#ifndef _POSIX_THREADS
/* This means pthreads are not implemented in libc headers, hence the macro
   not present in unistd.h. But they still can be implemented as an external
   library (e.g. gnu pth in pthread emulation) */
# ifdef HAVE_PTHREAD_H
#  include <pthread.h> /* _POSIX_THREADS */
# endif
#endif

#ifndef DONT_HAVE_STDIO_H
#include <stdio.h>
#endif

#include <stdlib.h>

#ifdef __sgi
#ifndef HAVE_PTHREAD_H /* XXX Need to check in configure.in */
#undef _POSIX_THREADS
#endif
#endif

#include "pythread.h"

#ifndef _POSIX_THREADS

#ifdef __sgi
#define SGI_THREADS
#endif

#ifdef HAVE_THREAD_H
#define SOLARIS_THREADS
#endif

#if defined(sun) && !defined(SOLARIS_THREADS)
#define SUN_LWP
#endif

#endif /* _POSIX_THREADS */


#ifdef Py_DEBUG
static int thread_debug = 0;
#define dprintf(args)	(void)((thread_debug & 1) && printf args)
#define d2printf(args)	((thread_debug & 8) && printf args)
#else
#define dprintf(args)
#define d2printf(args)
#endif

static int initialized;

static void PyThread__init_thread(void); /* Forward */

void PyThread_init_thread(void)
{
#ifdef Py_DEBUG
	char *p = getenv("THREADDEBUG");

	if (p) {
		if (*p)
			thread_debug = atoi(p);
		else
			thread_debug = 1;
	}
#endif /* Py_DEBUG */
	if (initialized)
		return;
	initialized = 1;
	dprintf(("PyThread_init_thread called\n"));
	PyThread__init_thread();
}

#ifdef SGI_THREADS
#include "thread_sgi.h"
#endif

#ifdef SOLARIS_THREADS
#include "thread_solaris.h"
#endif

#ifdef SUN_LWP
#include "thread_lwp.h"
#endif

#ifdef HAVE_PTH
#include "thread_pth.h"
#undef _POSIX_THREADS
#endif

#ifdef _POSIX_THREADS
#include "thread_pthread.h"
#endif

#ifdef C_THREADS
#include "thread_cthread.h"
#endif

#ifdef NT_THREADS
#include "thread_nt.h"
#endif

#ifdef OS2_THREADS
#include "thread_os2.h"
#endif

#ifdef BEOS_THREADS
#include "thread_beos.h"
#endif

#ifdef WINCE_THREADS
#include "thread_wince.h"
#endif

#ifdef PLAN9_THREADS
#include "thread_plan9.h"
#endif

#ifdef ATHEOS_THREADS
#include "thread_atheos.h"
#endif

/*
#ifdef FOOBAR_THREADS
#include "thread_foobar.h"
#endif
*/

#ifndef Py_HAVE_NATIVE_TLS
/* If the platform has not supplied a platform specific
   TLS implementation, provide our own.

   This code stolen from "thread_sgi.h", where it was the only
   implementation of an existing Python TLS API.
*/
/* ------------------------------------------------------------------------
Per-thread data ("key") support.

Use PyThread_create_key() to create a new key.  This is typically shared
across threads.

Use PyThread_set_key_value(thekey, value) to associate void* value with
thekey in the current thread.  Each thread has a distinct mapping of thekey
to a void* value.  Caution:  if the current thread already has a mapping
for thekey, value is ignored.

Use PyThread_get_key_value(thekey) to retrieve the void* value associated
with thekey in the current thread.  This returns NULL if no value is
associated with thekey in the current thread.

Use PyThread_delete_key_value(thekey) to forget the current thread's associated
value for thekey.  PyThread_delete_key(thekey) forgets the values associated
with thekey across *all* threads.

While some of these functions have error-return values, none set any
Python exception.

None of the functions does memory management on behalf of the void* values.
You need to allocate and deallocate them yourself.  If the void* values
happen to be PyObject*, these functions don't do refcount operations on
them either.

The GIL does not need to be held when calling these functions; they supply
their own locking.  This isn't true of PyThread_create_key(), though (see
next paragraph).

There's a hidden assumption that PyThread_create_key() will be called before
any of the other functions are called.  There's also a hidden assumption
that calls to PyThread_create_key() are serialized externally.
------------------------------------------------------------------------ */

/* A singly-linked list of struct key objects remembers all the key->value
 * associations.  File static keyhead heads the list.  keymutex is used
 * to enforce exclusion internally.
 */
struct key {
	/* Next record in the list, or NULL if this is the last record. */
	struct key *next;

	/* The thread id, according to PyThread_get_thread_ident(). */
	long id;

	/* The key and its associated value. */
	int key;
	void *value;
};

static struct key *keyhead = NULL;
static PyThread_type_lock keymutex = NULL;
static int nkeys = 0;  /* PyThread_create_key() hands out nkeys+1 next */

/* Internal helper.
 * If the current thread has a mapping for key, the appropriate struct key*
 * is returned.  NB:  value is ignored in this case!
 * If there is no mapping for key in the current thread, then:
 *     If value is NULL, NULL is returned.
 *     Else a mapping of key to value is created for the current thread,
 *     and a pointer to a new struct key* is returned; except that if
 *     malloc() can't find room for a new struct key*, NULL is returned.
 * So when value==NULL, this acts like a pure lookup routine, and when
 * value!=NULL, this acts like dict.setdefault(), returning an existing
 * mapping if one exists, else creating a new mapping.
 *
 * Caution:  this used to be too clever, trying to hold keymutex only
 * around the "p->next = keyhead; keyhead = p" pair.  That allowed
 * another thread to mutate the list, via key deletion, concurrent with
 * find_key() crawling over the list.  Hilarity ensued.  For example, when
 * the for-loop here does "p = p->next", p could end up pointing at a
 * record that PyThread_delete_key_value() was concurrently free()'ing.
 * That could lead to anything, from failing to find a key that exists, to
 * segfaults.  Now we lock the whole routine.
 */
static struct key *
find_key(int key, void *value)
{
	struct key *p;
	long id = PyThread_get_thread_ident();

	PyThread_acquire_lock(keymutex, 1);
	for (p = keyhead; p != NULL; p = p->next) {
		if (p->id == id && p->key == key)
			goto Done;
	}
	if (value == NULL) {
		assert(p == NULL);
		goto Done;
	}
	p = (struct key *)malloc(sizeof(struct key));
	if (p != NULL) {
		p->id = id;
		p->key = key;
		p->value = value;
		p->next = keyhead;
		keyhead = p;
	}
 Done:
	PyThread_release_lock(keymutex);
	return p;
}

/* Return a new key.  This must be called before any other functions in
 * this family, and callers must arrange to serialize calls to this
 * function.  No violations are detected.
 */
int
PyThread_create_key(void)
{
	/* All parts of this function are wrong if it's called by multiple
	 * threads simultaneously.
	 */
	if (keymutex == NULL)
		keymutex = PyThread_allocate_lock();
	return ++nkeys;
}

/* Forget the associations for key across *all* threads. */
void
PyThread_delete_key(int key)
{
	struct key *p, **q;

	PyThread_acquire_lock(keymutex, 1);
	q = &keyhead;
	while ((p = *q) != NULL) {
		if (p->key == key) {
			*q = p->next;
			free((void *)p);
			/* NB This does *not* free p->value! */
		}
		else
			q = &p->next;
	}
	PyThread_release_lock(keymutex);
}

/* Confusing:  If the current thread has an association for key,
 * value is ignored, and 0 is returned.  Else an attempt is made to create
 * an association of key to value for the current thread.  0 is returned
 * if that succeeds, but -1 is returned if there's not enough memory
 * to create the association.  value must not be NULL.
 */
int
PyThread_set_key_value(int key, void *value)
{
	struct key *p;

	assert(value != NULL);
	p = find_key(key, value);
	if (p == NULL)
		return -1;
	else
		return 0;
}

/* Retrieve the value associated with key in the current thread, or NULL
 * if the current thread doesn't have an association for key.
 */
void *
PyThread_get_key_value(int key)
{
	struct key *p = find_key(key, NULL);

	if (p == NULL)
		return NULL;
	else
		return p->value;
}

/* Forget the current thread's association for key, if any. */
void
PyThread_delete_key_value(int key)
{
	long id = PyThread_get_thread_ident();
	struct key *p, **q;

	PyThread_acquire_lock(keymutex, 1);
	q = &keyhead;
	while ((p = *q) != NULL) {
		if (p->key == key && p->id == id) {
			*q = p->next;
			free((void *)p);
			/* NB This does *not* free p->value! */
			break;
		}
		else
			q = &p->next;
	}
	PyThread_release_lock(keymutex);
}

#endif /* Py_HAVE_NATIVE_TLS */