慎用USES_CONVERSION
慎用USES_CONVERSION
USES_CONVERSION是ATL中的一个宏定义。用于编码转换(用的比较多的是CString向LPCWSTR转换)。在ATL下使用要包含头文件#include "atlconv.h"
使用USES_CONVERSION一定要小心,它们从堆栈上分配内存,直到调用它的函数返回,该内存不会被释放。如果在一个循环中,这个宏被反复调用几万次,将不可避免的产生stackoverflow。
在一个函数的循环体中使用A2W等字符转换宏可能引起栈溢出。
#include
<
atlconv.h
>
void fn()
{
while ( true )
{
{
USES_CONVERSION;
DoSomething(A2W( " SomeString " ));
}
}
}
void fn()
{
while ( true )
{
{
USES_CONVERSION;
DoSomething(A2W( " SomeString " ));
}
}
}
让我们来分析以上的转换宏
#define
A2W(lpa) (\
((_lpa = lpa) == NULL) ? NULL : (\
_convert = (lstrlenA(_lpa) + 1 ),\
ATLA2WHELPER((LPWSTR) alloca(_convert * 2 ), _lpa, _convert)))
#define ATLA2WHELPER AtlA2WHelper
inline LPWSTR WINAPI AtlA2WHelper(LPWSTR lpw, LPCSTR lpa, int nChars, UINT acp)
{
ATLASSERT(lpa != NULL);
ATLASSERT(lpw != NULL);
// verify that no illegal character present
// since lpw was allocated based on the size of lpa
// don't worry about the number of chars
lpw[ 0 ] = ' \0 ' ;
MultiByteToWideChar(acp, 0 , lpa, - 1 , lpw, nChars);
return lpw;
}
((_lpa = lpa) == NULL) ? NULL : (\
_convert = (lstrlenA(_lpa) + 1 ),\
ATLA2WHELPER((LPWSTR) alloca(_convert * 2 ), _lpa, _convert)))
#define ATLA2WHELPER AtlA2WHelper
inline LPWSTR WINAPI AtlA2WHelper(LPWSTR lpw, LPCSTR lpa, int nChars, UINT acp)
{
ATLASSERT(lpa != NULL);
ATLASSERT(lpw != NULL);
// verify that no illegal character present
// since lpw was allocated based on the size of lpa
// don't worry about the number of chars
lpw[ 0 ] = ' \0 ' ;
MultiByteToWideChar(acp, 0 , lpa, - 1 , lpw, nChars);
return lpw;
}
关键的地方在 alloca 内存分配内存上。
#define
alloca _alloca
_alloca
Allocates memory on the stack.
Remarks
_alloca allocates size bytes from the program stack. The allocated space is automatically freed when the calling function
exits. Therefore, do not pass the pointer value returned by _alloca as an argument to free.
问题就在这里,分配的内存是在函数的栈中分配的。而VC编译器默认的栈内存空间是2M。当在一个函数中循环调用它时就会不断的分配栈中的内存。
以上问题的解决办法:
1、自己写字符转换函数,不要偷懒
Function that safely converts a
'
WCHAR
'
String to
'
LPSTR
'
:
char * ConvertLPWSTRToLPSTR (LPWSTR lpwszStrIn)
{
LPSTR pszOut = NULL;
if (lpwszStrIn != NULL)
{
int nInputStrLen = wcslen (lpwszStrIn);
// Double NULL Termination
int nOutputStrLen = WideCharToMultiByte (CP_ACP, 0 , lpwszStrIn, nInputStrLen, NULL, 0 , 0 , 0 ) + 2 ;
pszOut = new char [nOutputStrLen];
if (pszOut)
{
memset (pszOut, 0x00 , nOutputStrLen);
WideCharToMultiByte(CP_ACP, 0 , lpwszStrIn, nInputStrLen, pszOut, nOutputStrLen, 0 , 0 );
}
}
return pszOut;
}
char * ConvertLPWSTRToLPSTR (LPWSTR lpwszStrIn)
{
LPSTR pszOut = NULL;
if (lpwszStrIn != NULL)
{
int nInputStrLen = wcslen (lpwszStrIn);
// Double NULL Termination
int nOutputStrLen = WideCharToMultiByte (CP_ACP, 0 , lpwszStrIn, nInputStrLen, NULL, 0 , 0 , 0 ) + 2 ;
pszOut = new char [nOutputStrLen];
if (pszOut)
{
memset (pszOut, 0x00 , nOutputStrLen);
WideCharToMultiByte(CP_ACP, 0 , lpwszStrIn, nInputStrLen, pszOut, nOutputStrLen, 0 , 0 );
}
}
return pszOut;
}
等等一个一个的实现。
2、把字符转换部分放到一个函数中处理。
void
fn2()
{
USES_CONVERSION;
DoSomething(A2W( " SomeString " ));
}
void fn()
{
while ( true )
{
fn2();
}
}
{
USES_CONVERSION;
DoSomething(A2W( " SomeString " ));
}
void fn()
{
while ( true )
{
fn2();
}
}
如果不知道这点问题,在使用后崩溃时很难查出崩溃原因的。
转自: http://www.cnblogs.com/carekee/articles/1935789.html