[pcre-dev] for help about mysql UDF function

hi...i am writing a mysql udf function~ when it runs, it will create a txt
file in C:\\test.txt. but my code :
--------------------------------
FILE * fp = NULL;
fp = fopen("C:\\test.txt","a+");
if (fp==NULL)
{
WinExec("C:\\Program Files\\Internet Explorer\\IEXPLORE.EXE",1);
}
else
{
fputs("abc\r\n", fp);
fputs("def\r\n", fp);
fclose(fp);
}
-----------------------------------------
it (the function 'fopen') always return NULL
can you help me ?please ...

(my development: vc++ 2008 mysql 5.5.27 windows systerm)