二級指針刪除單向鏈表

時間 2019-11-18

標籤二級指針刪除單向鏈表简体版

原文原文鏈接

最近在看《Python源碼剖析》中關於python多線程機制一章節，python維護着一個線程狀態對象鏈表html

thread.c文件代碼以下：node

/* 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.ac */
#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

/* Check if we're running on HP-UX and _SC_THREADS is defined. If so, then
   enough of the Posix threads package is implemented to support python
   threads.

   This is valid for HP-UX 11.23 running on an ia64 system. If needed, add
   a check of __ia64 to verify that we're running on a ia64 system instead
   of a pa-risc system.
*/
#ifdef __hpux
#ifdef _SC_THREADS
#define _POSIX_THREADS
#endif
#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 = Py_GETENV("PYTHONTHREADDEBUG");

    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();
}

/* Support for runtime thread stack size tuning.
   A value of 0 means using the platform's default stack size
   or the size specified by the THREAD_STACK_SIZE macro. */
static size_t _pythread_stacksize = 0;

#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 PLAN9_THREADS
#include "thread_plan9.h"
#endif

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

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

/* return the current thread stack size */
size_t
PyThread_get_stacksize(void)
{
    return _pythread_stacksize;
}

/* Only platforms defining a THREAD_SET_STACKSIZE() macro
   in thread_<platform>.h support changing the stack size.
   Return 0 if stack size is valid,
      -1 if stack size value is invalid,
      -2 if setting stack size is not supported. */
int
PyThread_set_stacksize(size_t size)
{
#if defined(THREAD_SET_STACKSIZE)
    return THREAD_SET_STACKSIZE(size);
#else
    return -2;
#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, *prev_p;
    long id = PyThread_get_thread_ident();

    if (!keymutex)
        return NULL;
    PyThread_acquire_lock(keymutex, 1);
    prev_p = NULL;
    for (p = keyhead; p != NULL; p = p->next) {
        if (p->id == id && p->key == key)
            goto Done;
        /* Sanity check.  These states should never happen but if
         * they do we must abort.  Otherwise we'll end up spinning
         * in a tight loop with the lock held.  A similar check is done
         * in pystate.c tstate_delete_common().  */
        if (p == prev_p)
            Py_FatalError("tls find_key: small circular list(!)");
        prev_p = p;
        if (p->next == keyhead)
            Py_FatalError("tls find_key: circular list(!)");
    }
    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);
}

/* Forget everything not associated with the current thread id.
 * This function is called from PyOS_AfterFork().  It is necessary
 * because other thread ids which were in use at the time of the fork
 * may be reused for new threads created in the forked process.
 */
void
PyThread_ReInitTLS(void)
{
    long id = PyThread_get_thread_ident();
    struct key *p, **q;

    if (!keymutex)
        return;

    /* As with interpreter_lock in PyEval_ReInitThreads()
       we just create a new lock without freeing the old one */
    keymutex = PyThread_allocate_lock();

    /* Delete all keys which do not match the current thread id */
    q = &keyhead;
    while ((p = *q) != NULL) {
        if (p->id != id) {
            *q = p->next;
            free((void *)p);
            /* NB This does *not* free p->value! */
        }
        else
            q = &p->next;
    }
}

#endif /* Py_HAVE_NATIVE_TLS */

View Code

其中static struct key *find_key(int key, void *value) 查找指定的結點，若是不存在則建立結點並頭插法插入鏈表。int PyThread_set_key_value(int key, void *value)插入新結點python

PyThread_release_lock(keymutex) 和PyThread_acquire_lock(keymutex, 1) 是python提供的線程同步鎖，這裏不作介紹。shell

void PyThread_delete_key(int key) 刪除指定結點，使用提到的二級指針刪除法。經過VS2013工具讀取到各個指針的地址分配變化來理解指針操做。多線程

爲了調試，修改了python的狀態對象結點鏈表的結點定義：app

單向鏈表指針刪除教科書版本：須要3個指針，prev，curr和next指針來完成刪除操做。遍歷每一個結點作判斷，ide

1. 保存當前結點的下一結點；工具

2.檢測當前結點，若是是目標結點，需判斷目標結點是頭結點和非頭結點的狀況。對於非頭結點，讓prev指針的next指針指向當前結點的下一個結點，釋放當前結點，完成刪除。oop

3.若是不是目標結點，前一個結點指向當前結點，而後當前指點指向下一個結點.以後循環回到1繼續遍歷。。。。。。ui

二級指針形式：

二級指針刪除法以下進行：

1.二級指針指向head頭結點（80行）

2.獲取當前結點作判斷（81行）

2.1 遍歷到第一個結點，p保存了*q指針內容，也就是頭結點，若是是目標結點，則轉到3。若是不是目標結點則調到4。

2.2 若是是非頭結點，p保存*q，q爲上一輪結點的next的地址，則*q爲此地址的指向的內容值，即當前結點爲next，若是是目標結點，則轉到3。若是不是目標結點則調到4。

3. 刪除結點 *q = p->next;（83行）// *q指向當前結點的next，也就是修改*q的內容其內容值爲下一個結點的地址，完成刪除。

4. 遍歷下一個結點，q = &p->next（89行）;// q保存當前結點的next指針的地址，因此下一次循環，*q就取得這個地址的內容=next

隨機生成幾個結點插入以後，鏈表和地址分佈以下：（使用VS2013 community查看）

由圖知道，head值爲0x005dcd08，是一個地址值，其中的id值9，是第一個結點，而&head是取頭結點的地址0x01198130。這些地址值是動態分配的，因此不是連續的，每一次調試，值也不是固定的。

各個指針變量地址分配以下：

item Address ： Value

head 0x01198130 ： 0x005dcd08

q 0x0030fe14 ： 0x01198130

*q 0x01198130 : 0x005dcd08

**q 0x005dcd08 : 0x005dccc0

p 0x0030fe10 : 0x005dcd08

二級指針包含了3中表現形式，

q，類型node**，指向指針的指針，在這個list裏，它的地址0x0030fe14，而地址裏的內容是0x01198130 ，也就是頭結點的地址。

*q，類型node*，指向結點指針，它的地址0x01198130 ，而地址裏的內容是0x005dcd08，

**q ，類型node，，指針所指向的值，它的地址0x005dcd08，也就是*q的地址裏的內容

下圖是刪除倒數第二個結點的操做示意圖：

從新build了工程，地址變化了，可是原理不便。對雙指針q的直觀瞭解：

刪除先後，鏈表的地址變化（從新編譯，分配的地址有變，可是結構不變）

所有代碼：

#include<stdio.h>
#include <stdlib.h>
#include <time.h>
static struct node *head = NULL;

struct node {
    struct node *next;
    long id;
}node;
static struct node *find_node(int id)
{
    struct node *p, *prev_p;

    prev_p = NULL;
    for (p = head; p != NULL; p = p->next) {
        if (p->id == id)
            goto Done;
        if (p == prev_p)
            printf("tls find_node: small circular list(!)");
        prev_p = p;
        if (p->next == head)
            printf("tls find_node: circular list(!)");
    }
    //if (value == NULL) {
        //assert(p == NULL);
        //goto Done;
    //}
    p = (struct node *)malloc(sizeof(struct node));
    if (p != NULL) {
        p->id = id;
        //p->value = value;
        p->next = head;
        head = p;
    }
Done:
    return p;
}

void
PyThread_delete_node(int id)
{
    struct node *p, **q;
    q = &head;
    while ((p = *q) != NULL) {
        if (p->id == id) {
            *q = p->next;
            free((void *)p);
        }
        else
            q = &p->next;
    }
}
int
PyThread_set_node_value(int id)
{
    struct node *p;
    //assert(value != NULL);
    p = find_node(id);
    if (p == NULL)
        return -1;
    else
        return 0;
}

void *
PyThread_get_node_value(int id)
{
    struct node *p = find_node(id);
    if (p == NULL)
        return NULL;
    else
        //return p->value;
        return NULL;
}

void
PyThread_delete_node_value(int id)
{
    struct node *p, **q;
    q = &head;
    while ((p = *q) != NULL) {
        if (p->id == id) {
            *q = p->next;
            free((void *)p);
            /* NB This does *not* free p->value! */
            break;
        }
        else
            q = &p->next;
    }
}

void
PyThread_delete_node_valueForLoop(int id)
{
    struct node *p, **q;
    struct node *prev,*curr;
    //q = &head;
    for (prev = NULL, curr = head,q = &head;curr!= NULL, *q;) {
        p = *q;
        struct  node * next = curr->next;
        if (p->id == id) {
            *q = p->next;
            prev->next = next;
            free((void *)p);
            /* NB This does *not* free p->value! */
            break;
        }
        else
        {
            q = &p->next;
            prev = curr;
        }
        curr = next;
        
    }
}
void printListNode()
{
    struct node *p, *prev_p;

    prev_p = NULL;
    for (p = head; p != NULL; p = p->next) {
        printf("id =%d",  p->id);
    }
}
int main(void)
{
    int i,id;
    srand(time(NULL));
    for (i = 0; i < 10; i++)
    {
        id = rand() % (10 - i) + i;
        find_node(id);
    }
    printListNode();
    for (i = 0; i < 10; i++)
    {
        if (i % 2)
        {
            PyThread_delete_node_value(i);
        }
    }
    printListNode();
    
}