二级指针删除单向链表
最近在看《Python源码剖析》中关于python多线程机制一章节，python维护着一个线程状态对象链表
thread.c文件代码如下：
/* 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 */
其中static struct key *find_key(int key, void *value) 查找指定的结点，如果不存在则创建结点并头插法插入链表。int PyThread_set_key_value(int key, void *value)插入新结点
PyThread_release_lock(keymutex) 和PyThread_acquire_lock(keymutex, 1) 是python提供的线程同步锁，这里不做介绍。
void PyThread_delete_key(int key) 删除指定结点，使用提到的二级指针删除法。通过VS2013工具读取到各个指针的地址分配变化来理解指针操作。
为了调试，修改了python的状态对象结点链表的结点定义：
单向链表指针删除教科书版本：需要3个指针，prev，curr和next指针来完成删除操作。遍历每个结点做判断，
1. 保存当前结点的下一结点；
2.检测当前结点，如果是目标结点，需判断目标结点是头结点和非头结点的情况。对于非头结点，让prev指针的next指针指向当前结点的下一个结点，释放当前结点，完成删除。
3.如果不是目标结点，前一个结点指向当前结点，然后当前指点指向下一个结点.之后循环回到1继续遍历。。。。。。
二级指针形式：
二级指针删除法如下进行：
1.二级指针指向head头结点（80行）
2.获取当前结点做判断（81行）
2.1 遍历到第一个结点，p保存了*q指针内容，也就是头结点，如果是目标结点，则转到3。如果不是目标结点则调到4。
2.2 如果是非头结点，p保存*q，q为上一轮结点的next的地址，则*q为此地址的指向的内容值，即当前结点为next，如果是目标结点，则转到3。如果不是目标结点则调到4。
3. 删除结点 *q = p->next;（83行）// *q指向当前结点的next，也就是修改*q的内容其内容值为下一个结点的地址，完成删除。
4. 遍历下一个结点，q = &p->next（89行）;// q保存当前结点的next指针的地址，所以下一次循环，*q就取得这个地址的内容=next
随机生成几个结点插入之后，链表和地址分布如下：（使用VS2013 community查看）
由图知道，head值为0x005dcd08，是一个地址值，其中的id值9，是第一个结点，而&head是取头结点的地址0x01198130。这些地址值是动态分配的，所以不是连续的，每一次调试，值也不是固定的。
各个指针变量地址分配如下：
item Address ： Value
head 0x01198130 ： 0x005dcd08
q 0x0030fe14 ： 0x01198130
*q 0x01198130 : 0x005dcd08
**q 0x005dcd08 : 0x005dccc0
p 0x0030fe10 : 0x005dcd08
二级指针包含了3中表现形式，
q，类型node**，指向指针的指针，在这个list里，它的地址0x0030fe14，而地址里的内容是0x01198130 ，也就是头结点的地址。
*q，类型node*，指向结点指针，它的地址0x01198130 ，而地址里的内容是0x005dcd08，
**q ，类型node，，指针所指向的值，它的地址0x005dcd08，也就是*q的地址里的内容
下图是删除倒数第二个结点的操作示意图：
重新build了工程，地址变化了，但是原理不便。对双指针q的直观了解：
删除前后，链表的地址变化（重新编译，分配的地址有变，但是结构不变）
全部代码：
#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(); }
http://coolshell.cn/articles/8990.html
http://wordaligned.org/articles/two-star-programming
双联表操作
http://www.cnblogs.com/clover-toeic/p/3793131.html
