//_tiddata的定义. CRT SRC\mtdll.h
struct _tiddata { unsigned long _tid; /* thread ID */
unsigned long _thandle; /* thread handle */
int _terrno; /* errno value */
unsigned long _tdoserrno; /* _doserrno value */
unsigned int _fpds; /* Floating Point data segment */
unsigned long _holdrand; /* rand() seed value */
char * _token; /* ptr to strtok() token */
#ifdef _WIN32
wchar_t * _wtoken; /* ptr to wcstok() token */
#endif /* _WIN32 */
unsigned char * _mtoken; /* ptr to _mbstok() token */
/* following pointers get malloc'd at runtime */
char * _errmsg; /* ptr to strerror()/_strerror() buff */
char * _namebuf0; /* ptr to tmpnam() buffer */
#ifdef _WIN32
wchar_t * _wnamebuf0; /* ptr to _wtmpnam() buffer */
#endif /* _WIN32 */
char * _namebuf1; /* ptr to tmpfile() buffer */
#ifdef _WIN32
wchar_t * _wnamebuf1; /* ptr to _wtmpfile() buffer */
#endif /* _WIN32 */
char * _asctimebuf; /* ptr to asctime() buffer */
#ifdef _WIN32
wchar_t * _wasctimebuf; /* ptr to _wasctime() buffer */
#endif /* _WIN32 */
void * _gmtimebuf; /* ptr to gmtime() structure */
char * _cvtbuf; /* ptr to ecvt()/fcvt buffer */
/* following fields are needed by _beginthread code */
void * _initaddr; /* initial user thread address */
void * _initarg; /* initial user thread argument */
/* following three fields are needed to support signal handling and
* runtime errors */
void * _pxcptacttab; /* ptr to exception-action table */
void * _tpxcptinfoptrs; /* ptr to exception info pointers */
int _tfpecode; /* float point exception code */
/* following field is needed by NLG routines */
unsigned long _NLG_dwCode;
/*
* Per-Thread data needed by C++ Exception Handling
*/
void * _terminate; /* terminate() routine */
void * _unexpected; /* unexpected() routine */
void * _translator; /* S.E. translator */
void * _curexception; /* current exception */
void * _curcontext; /* current exception context */
#if defined (_M_MRX000)
void * _pFrameInfoChain;
void * _pUnwindContext;
void * _pExitContext;
int _MipsPtdDelta;
int _MipsPtdEpsilon;
#elif defined (_M_PPC)
void * _pExitContext;
void * _pUnwindContext;
void * _pFrameInfoChain;
int _FrameInfo[6];
#endif /* defined (_M_PPC) */
};
typedef struct _tiddata * _ptiddata;
//_getptd _freeptd 的相关实现. CRT SRC\tidtable.c
/***
*tidtable.c - Access thread data table
*
* Copyright (c) 1989-1997, Microsoft Corporation. All rights reserved.
*
*Purpose:
* This module contains the following routines for multi-thread
* data support:
*
* _mtinit = Initialize the mthread data
* _getptd = get the pointer to the per-thread data structure for
* the current thread
* _freeptd = free up a per-thread data structure and its
* subordinate structures
* __threadid = return thread ID for the current thread
* __threadhandle = return pseudo-handle for the current thread
*
*******************************************************************************/
#if defined (_MT)
# if defined(_NTSUBSET_)
#include
#include
#include
#include
#include
#include
# endif /* _NTSUBSET_ */
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
unsigned long __tlsindex = 0xffffffff;
/****
*_mtinit() - Init multi-thread data bases
*
*Purpose:
* (1) Call _mtinitlocks to create/open all lock semaphores.
* (2) Allocate a TLS index to hold pointers to per-thread data
* structure.
*
* NOTES:
* (1) Only to be called ONCE at startup
* (2) Must be called BEFORE any mthread requests are made
*
*Entry:
*
*Exit:
* returns on success
* calls _amsg_exit on failure
*
*Uses:
*
*
*Exceptions:
*
*******************************************************************************/
int __cdecl _mtinit (
void
)
{ _ptiddata ptd;
/*
* Initialize the mthread lock data base
*/
_mtinitlocks();
/*
* Allocate a TLS index to maintain pointers to per-thread data
*/
if ( (__tlsindex = TlsAlloc()) == 0xffffffff )
return FALSE; /* fail to load DLL */
/*
* Create a per-thread data structure for this (i.e., the startup)
* thread.
*/
if ( ((ptd = _calloc_crt(1, sizeof(struct _tiddata))) == NULL) ||
!TlsSetValue(__tlsindex, (LPVOID)ptd) )
return FALSE; /* fail to load DLL */
/*
* Initialize the per-thread data
*/
_initptd(ptd);
ptd->_tid = GetCurrentThreadId();
ptd->_thandle = (unsigned long)(-1L);
return TRUE;
}
/****
*_mtterm() - Clean-up multi-thread data bases
*
*Purpose:
* (1) Call _mtdeletelocks to free up all lock semaphores.
* (2) Free up the TLS index used to hold pointers to
* per-thread data structure.
*
* NOTES:
* (1) Only to be called ONCE at termination
* (2) Must be called AFTER all mthread requests are made
*
*Entry:
*
*Exit:
* returns
*
*Uses:
*
*Exceptions:
*
*******************************************************************************/
void __cdecl _mtterm (
void
)
{ /*
* Clean up the mthread lock data base
*/
_mtdeletelocks();
/*
* Free up the TLS index
*
* (Set the variable __tlsindex back to the unused state (-1L).)
*/
if ( __tlsindex != 0xffffffff ) { TlsFree(__tlsindex);
__tlsindex = 0xffffffff;
}
}
/***
*void _initptd(_ptiddata ptd) - initialize a per-thread data structure
*
*Purpose:
* This routine handles all of the per-thread initialization
* which is common to _beginthread, _beginthreadex, _mtinit
* and _getptd.
*
*Entry:
* pointer to a per-thread data block
*
*Exit:
* the common fields in that block are initialized
*
*Exceptions:
*
*******************************************************************************/
void __cdecl _initptd (
_ptiddata ptd
)
{ ptd->_pxcptacttab = (void *)_XcptActTab;
ptd->_holdrand = 1L;
#ifdef _M_MRX000
/*
* MIPS per-thread data
*/
ptd->_MipsPtdDelta =
ptd->_MipsPtdEpsilon = -1L ;
#endif /* _M_MRX000 */
}
/***
*_ptiddata _getptd(void) - get per-thread data structure for the current thread
*
*Purpose:
*
*Entry:
* unsigned long tid
*
*Exit:
* success = pointer to _tiddata structure for the thread
* failure = fatal runtime exit
*
*Exceptions:
*
*******************************************************************************/
_ptiddata __cdecl _getptd (
void
)
{ _ptiddata ptd;
DWORD TL_LastError;
TL_LastError = GetLastError();
if ( (ptd = TlsGetValue(__tlsindex)) == NULL ) { /*
* no per-thread data structure for this thread. try to create
* one.
*/
if ( ((ptd = _calloc_crt(1, sizeof(struct _tiddata))) != NULL) &&
TlsSetValue(__tlsindex, (LPVOID)ptd) ) { /*
* Initialize of per-thread data
*/
_initptd(ptd);
ptd->_tid = GetCurrentThreadId();
ptd->_thandle = (unsigned long)(-1L);
}
else
_amsg_exit(_RT_THREAD); /* write message and die */
}
SetLastError(TL_LastError);
return(ptd);
}
/***
*void _freeptd(_ptiddata) - free up a per-thread data structure
*
*Purpose:
* Called from _endthread and from a DLL thread detach handler,
* this routine frees up the per-thread buffer associated with a
* thread that is going away. The tiddata structure itself is
* freed, but not until its subordinate buffers are freed.
*
*Entry:
* pointer to a per-thread data block (malloc-ed memory)
* If NULL, the pointer for the current thread is fetched.
*
*Exit:
*
*Exceptions:
*
*******************************************************************************/
void __cdecl _freeptd (
_ptiddata ptd
)
{ /*
* Do nothing unless per-thread data has been allocated for this module!
*/
if ( __tlsindex != 0xFFFFFFFF ) { /*
* if parameter "ptd" is NULL, get the per-thread data pointer
* Must NOT call _getptd because it will allocate one if none exists!
*/
if ( ! ptd )
ptd = TlsGetValue(__tlsindex );
/*
* Free up the _tiddata structure & its malloc-ed buffers.
*/
if ( ptd ) { if(ptd->_errmsg)
_free_crt((void *)ptd->_errmsg);
if(ptd->_namebuf0)
_free_crt((void *)ptd->_namebuf0);
if(ptd->_namebuf1)
_free_crt((void *)ptd->_namebuf1);
if(ptd->_asctimebuf)
_free_crt((void *)ptd->_asctimebuf);
if(ptd->_gmtimebuf)
_free_crt((void *)ptd->_gmtimebuf);
if(ptd->_cvtbuf)
_free_crt((void *)ptd->_cvtbuf);
if (ptd->_pxcptacttab != _XcptActTab)
_free_crt((void *)ptd->_pxcptacttab);
_free_crt((void *)ptd);
}
/*
* Zero out the one pointer to the per-thread data block
*/
TlsSetValue(__tlsindex, (LPVOID)0);
}
}
/***
*__threadid() - Returns current thread ID
*__threadhandle() - Returns "pseudo-handle" for current thread
*
*Purpose:
* The two function are simply do-nothing wrappers for the corresponding
* Win32 APIs (GetCurrentThreadId and GetCurrentThread, respectively).
*
*Entry:
* void
*
*Exit:
* thread ID value
*
*Exceptions:
*
*******************************************************************************/
_CRTIMP unsigned long __cdecl __threadid (
void
)
{ return( GetCurrentThreadId() );
}
_CRTIMP unsigned long __cdecl __threadhandle(
void
)
{ return( (unsigned long)GetCurrentThread() );
}
#endif /* defined (_MT) */
以下来自：，节选。
或许有人会说，我用CreateThread创建线程以后，也调用了C运行库函数，并且也使用ExitThread退出了，可是我的程序运行得好好的，既
没有因为CRT没有初始化而崩溃，也没有因为忘记调用_endthread而发生内存泄漏，这是为什么呢，让我们继续我们的CRT之旅。
假设我用CreateThread创建了一个线程，我调用strtok函数来进行字符串处理，这个函数肯定是需要某些额外的运行时支持的。strtok的源代码在strtok.c中。从代码可见，在多线程情况下，strtok的第一句有效代码就是_ptiddata ptd = _getptd()，它通过这个来获得当前的ptd。可是我们并没有通过_beginthread来创建ptd，那么一定是_getptd捣鬼了。打开tidtable.c，可以看到_getptd的实现，果然，它先尝试获得当前的ptd，如果不能，就重新创建一个，因此，后续的CRT调用就安全了。可是这块ptd最终又是谁释放的呢？打开dllcrt0.c，可以看到一个DllMain函数。在VC中，CRT既可以作为一个动态链接库和主程序链接，也可以作为一个静态库和主程序链接，这个在Project Setting->Code Generations里面可以选。当CRT作为DLL链接到主程序时，DllMain就是CRT DLL的入口。Windows的DllMain可以由四种原因调用：Process Attach/Process Detach/Thread Attach/Thread Detach，最后一个，也就是当线程函数退出后但是线程还没有销毁前，会在这个线程的上下文中用Thread Detach调用DllMain，这里，CRT做了一个_freeptd(NULL)，也就是说，如果有ptd，就free掉。所以说，恰巧没有发生内存泄漏是因为你用的是动态链接的CRT。
于是我们得出了一个更精确的结论，如果我没有使用那些会使用_getptd的CRT函数，使用CreateThread就是安全的。