[Win7 vs2010]
Q： C库和系统api之间是什么关系？
A： 如下图，简单示意：
可以看出，C库一部分是使用系统api实现自身功能(比如文件操作)，另一部分并不会直接依赖系统api，单独实现功能(比如字符串处理)。另外，对于驱动模块，按照不同的理解，也可以放入操作系统内部或者操作系统下层；如果把操作系统看成隐形的CPU和内存的驱动，那么它也可以看成和常规意义的硬件驱动是平级的。而，C库，从理论上来说，没必要和驱动有依赖关系。当然，访问操作系统的方式不仅仅是它提供的api，也是可以通过其它方式来访问。
Q：c库和多线程到底什么关系？
A： 多线程在操作系统上的运用导致了很多库，包括之前的单线程版本的c库，也必须做出相应修改，才能保证运行不会出现问题。例如，如下是vs2010附带的fgets.c中部分源代码：
_TSCHAR * __cdecl _fgetts (_TSCHAR *string,int count,FILE *str)
{REG1 FILE *stream;REG2 _TSCHAR *pointer = string;_TSCHAR *retval = string;int ch;_VALIDATE_RETURN(( string != NULL ) || ( count == 0 ), EINVAL, NULL);_VALIDATE_RETURN(( count >= 0 ), EINVAL, NULL);_VALIDATE_RETURN(( str != NULL ), EINVAL, NULL);if (count == 0){return NULL;}/* The C Standard states the input buffer should remainunchanged if EOF is encountered immediately. Hence wedo not blank out the input buffer here *//* Init stream pointer */stream = str;_lock_str(stream);__try {
#ifndef _UNICODE_VALIDATE_STREAM_ANSI_SETRET(stream, EINVAL, retval, NULL);
#endif /* _UNICODE */if (retval!=NULL){while (--count){if ((ch = _fgettc_nolock(stream)) == _TEOF){if (pointer == string) {retval=NULL;goto done;}break;}if ((*pointer++ = (_TSCHAR)ch) == _T('\n'))break;}*pointer = _T('\0');}/* Common return */
done: ;}__finally {_unlock_str(stream);}return(retval);
}#define _lock_str(s) _lock_file(s)void __cdecl _lock_file (FILE *pf)
{/** The way the FILE (pointed to by pf) is locked depends on whether* it is part of _iob[] or not*/if ( (pf >= _iob) && (pf <= (&_iob[_IOB_ENTRIES-1])) ){/** FILE lies in _iob[] so the lock lies in _locktable[].*/_lock( _STREAM_LOCKS + (int)(pf - _iob) );/* We set _IOLOCKED to indicate we locked the stream */pf->_flag |= _IOLOCKED;}else/** Not part of _iob[]. Therefore, *pf is a _FILEX and the* lock field of the struct is an initialized critical* section.*/EnterCriticalSection( &(((_FILEX *)pf)->lock) );
}对于_lock函数：
void __cdecl _lock (int locknum)
{/** Create/open the lock, if necessary*/if ( _locktable[locknum].lock == NULL ) {if ( !_mtinitlocknum(locknum) )_amsg_exit( _RT_LOCK );}/** Enter the critical section.*/EnterCriticalSection( _locktable[locknum].lock );
}可以看出，不管加锁的方式如何，它实际上还是会调用系统提供的原始api来进行排他访问，从而避免多线程访问共享资源可能导致读或者写出错的问题。
Q： 那么如何设置将使用单线程版本的c库或者多线程版本的c库？
A： 如下图，是vs2010设置使用多线程版本c库的截图：
按照微软的说法，从vs2005开始，单线程版本的C库就已经被移除，所以可以不用担心使用单线程版本C库导致问题了。如果使用的是VC6，依然可以设置使用单线程版本C库。如果使用IDE工具没找到，可以使用命令行工具寻找相关选项：
当然，grep需要cygwin的支持。
Q： printf函数，它可以处理变参，内部会如何处理呢？
A： 参数入栈的原则很好地支撑了变参的处理。也就是说，当确定了参数中一个的地址，那么其他参数的地址是随着这个地址按照类型大小变动即可。如下：
#define va_start _crt_va_start#define va_end _crt_va_end_crt_va_start和_crt_va_end的声明如下：
#define _crt_va_start(ap,v) ( ap = (va_list)_ADDRESSOF(v) + _INTSIZEOF(v) )#define _crt_va_end(ap) ( ap = (va_list)0 )#ifdef __cplusplus
#define _ADDRESSOF(v) ( &reinterpret_cast<const char &>(v) )
#else
#define _ADDRESSOF(v) ( &(v) )
#endif#define _INTSIZEOF(n) ( (sizeof(n) + sizeof(int) - 1) & ~(sizeof(int) - 1) )
Q： printf函数内部究竟调用了什么操作系统API?
A： 从表面分析，应该会调用系统控制台输出的API; 如下是用ida工具对于如下代码hello.c生成的可执行文件的分析：
#include <stdio.h>
#include <wchar.h>int main()
{char ch = 'a';wchar_t wch = (wchar_t)ch;printf("%C\n", wch);return 0;
}
可以确切看到内部调用的系统API名称。当然，使用hook WriteConsoleW函数的方式同样可以得出结论。
Q： 经常看到关于文件输入输出的符号stdin, stdout, stderr，它们究竟是什么？
A： 它们是FILE *类型的变量。如下定义：
#define stdin (&__iob_func()[0])
#define stdout (&__iob_func()[1])
#define stderr (&__iob_func()[2])/** FILE descriptors; preset for stdin/out/err (note that the __tmpnum field* is not initialized)*/
FILE _iob[_IOB_ENTRIES] = {/* _ptr, _cnt, _base, _flag, _file, _charbuf, _bufsiz *//* stdin (_iob[0]) */{ _bufin, 0, _bufin, _IOREAD | _IOYOURBUF, 0, 0, _INTERNAL_BUFSIZ },/* stdout (_iob[1]) */{ NULL, 0, NULL, _IOWRT, 1, 0, 0 },/* stderr (_iob[3]) */{ NULL, 0, NULL, _IOWRT, 2, 0, 0 },};/* These functions are for enabling STATIC_CPPLIB functionality */
_CRTIMP FILE * __cdecl __iob_func(void)
{return _iob;
}对于FILE *和file handle, 可以使用fileno和fdopen来互相获取对应的数值。
另外，上面的代码是从_file.c头文件中摘录，stderr (_iob[3])应该是它的错误，应该为stderr (_iob[2]).
Q： 对于字符串处理函数strtok, 它是如何保存中间状态的？
A： 单线程版本的strtok函数，可以通过static变量保存中间处理的位置信息，使得后来的调用可以继续工作。对于多线程版本的strtok函数，这样就不行了。实际上，它是采用了TLS的方式来保存这些中间数据。如下：
struct _tiddata {unsigned long _tid; /* thread ID */uintptr_t _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 */wchar_t * _wtoken; /* ptr to wcstok() token */unsigned char * _mtoken; /* ptr to _mbstok() token *//* following pointers get malloc'd at runtime */char * _errmsg; /* ptr to strerror()/_strerror() buff */wchar_t * _werrmsg; /* ptr to _wcserror()/__wcserror() buff */char * _namebuf0; /* ptr to tmpnam() buffer */wchar_t * _wnamebuf0; /* ptr to _wtmpnam() buffer */char * _namebuf1; /* ptr to tmpfile() buffer */wchar_t * _wnamebuf1; /* ptr to _wtmpfile() buffer */char * _asctimebuf; /* ptr to asctime() buffer */wchar_t * _wasctimebuf; /* ptr to _wasctime() buffer */void * _gmtimebuf; /* ptr to gmtime() structure */char * _cvtbuf; /* ptr to ecvt()/fcvt buffer */unsigned char _con_ch_buf[MB_LEN_MAX];/* ptr to putch() buffer */unsigned short _ch_buf_used; /* if the _con_ch_buf is used *//* 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 *//* pointer to the copy of the multibyte character information used by* the thread */pthreadmbcinfo ptmbcinfo;/* pointer to the copy of the locale informaton used by the thead */pthreadlocinfo ptlocinfo;int _ownlocale; /* if 1, this thread owns its own locale *//* 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 * _purecall; /* called when pure virtual happens */void * _curexception; /* current exception */void * _curcontext; /* current exception context */int _ProcessingThrow; /* for uncaught_exception */void * _curexcspec; /* for handling exceptions thrown from std::unexpected */
#if defined (_M_IA64) || defined (_M_AMD64)void * _pExitContext;void * _pUnwindContext;void * _pFrameInfoChain;unsigned __int64 _ImageBase;
#if defined (_M_IA64)unsigned __int64 _TargetGp;
#endif /* defined (_M_IA64) */unsigned __int64 _ThrowImageBase;void * _pForeignException;
#elif defined (_M_IX86)void * _pFrameInfoChain;
#endif /* defined (_M_IX86) */_setloc_struct _setloc_data;void * _reserved1; /* nothing */void * _reserved2; /* nothing */void * _reserved3; /* nothing */
#ifdef _M_IX86void * _reserved4; /* nothing */void * _reserved5; /* nothing */
#endif /* _M_IX86 */int _cxxReThrow; /* Set to True if it's a rethrown C++ Exception */unsigned long __initDomain; /* initial domain used by _beginthread[ex] for managed function */
};typedef struct _tiddata * _ptiddata;#ifdef _SECURE_VERSION
#define _TOKEN *context
#else /* _SECURE_VERSION */
#define _TOKEN ptd->_token
#endif /* _SECURE_VERSION */#ifdef _SECURE_VERSION
char * __cdecl strtok_s (char * string,const char * control,char ** context)
#else /* _SECURE_VERSION */
char * __cdecl strtok (char * string,const char * control)
#endif /* _SECURE_VERSION */
{unsigned char *str;const unsigned char *ctrl = control;unsigned char map[32];int count;#ifdef _SECURE_VERSION/* validation section */_VALIDATE_RETURN(context != NULL, EINVAL, NULL);_VALIDATE_RETURN(string != NULL || *context != NULL, EINVAL, NULL);_VALIDATE_RETURN(control != NULL, EINVAL, NULL);/* no static storage is needed for the secure version */#else /* _SECURE_VERSION */_ptiddata ptd = _getptd();#endif /* _SECURE_VERSION *//* Clear control map */for (count = 0; count < 32; count++)map[count] = 0;/* Set bits in delimiter table */do {map[*ctrl >> 3] |= (1 << (*ctrl & 7));} while (*ctrl++);/* Initialize str *//* If string is NULL, set str to the saved* pointer (i.e., continue breaking tokens out of the string* from the last strtok call) */if (string)str = string;elsestr = _TOKEN;/* Find beginning of token (skip over leading delimiters). Note that* there is no token iff this loop sets str to point to the terminal* null (*str == '\0') */while ( (map[*str >> 3] & (1 << (*str & 7))) && *str )str++;string = str;/* Find the end of the token. If it is not the end of the string,* put a null there. */for ( ; *str ; str++ )if ( map[*str >> 3] & (1 << (*str & 7)) ) {*str++ = '\0';break;}/* Update nextoken (or the corresponding field in the per-thread data* structure */_TOKEN = str;/* Determine if a token has been found. */if ( string == str )return NULL;elsereturn string;
}如果希望得到tiddata是如何初始化的，查看_beginthreadex函数的源代码(部分)：
/** Allocate and initialize a per-thread data structure for the to-* be-created thread.*/if ( (ptd = (_ptiddata)_calloc_crt(1, sizeof(struct _tiddata))) == NULL )goto error_return;/** Initialize the per-thread data*/_initptd(ptd, _getptd()->ptlocinfo);ptd->_initaddr = (void *) initialcode;ptd->_initarg = argument;ptd->_thandle = (uintptr_t)(-1);Q： 断言判断的代码应该怎么写？
A： 它的核心就在于如何输出错误信息和结束程序。
#define assert(_Expression) (void)( (!!(_Expression)) || (_wassert(_CRT_WIDE(#_Expression), _CRT_WIDE(__FILE__), __LINE__), 0) )在_wassert中将会组装错误信息并输出，并进入结束程序状态。
xichen
2012-5-29 16:54:53
