Re: Access Violation problem - C / C++ / MFC Discussion Boards

p_26-Jul-07 18:23

26-Jul-07 18:23

please tell me
what this problem means in connection of vc++ and mysql
inhandled exception myprojectname.exe(LIBMYSQL.DLL) 0xC0000005: Access Violation

please help me to solve this problem

thanks Rose | [Rose]

Re: Access Violation problem

Jonathan [Darka]26-Jul-07 20:54

Jonathan [Darka]

26-Jul-07 20:54

It means you have a bug in your code, why not try the debugger ?

Jonathan Wilkes
Darka[Xanya.net]

Re: Access Violation problem

p_26-Jul-07 21:08

26-Jul-07 21:08

thanks for reply sir
i use debugger and debugger shows this error in msgbox
and my code is as sir :
int i;
MYSQL *myDB;
MYSQL_RES *res;
MYSQL_ROW row;
CString user,pass,aqp;
if ((myDB = mysql_init((MYSQL*) 0)) &&
mysql_real_connect(myDB,"localhost","root",NULL,NULL,3306,NULL,NULL))

{
if ( mysql_select_db( myDB,"gourav") < 0 )
{
MessageBox("Can't select the database !\nTry later.");
goto exit_here;
}
else
{
MessageBox("Can't connect !\nPlease try later.") ;
goto exit_here;
}
}
GetDlgItemText(IDC_USER,user);
GetDlgItemText(IDC_PASS,pass);
if(mysql_query(myDB,"select * from login"))
res=mysql_store_result(myDB);
row = mysql_fetch_row(res);
MessageBox(row[0]);

exit_here:
mysql_close( myDB);

please sir help me to solve this one
thanks again

Re: Access Violation problem

Jonathan [Darka]26-Jul-07 21:20

Jonathan [Darka]

26-Jul-07 21:20

No problem, does the debugger indicate which line might be cusing the access violation ?

Try changing the code a little as your error handling could be improved, I rewrote it like this, but obviously have not tested it, so give it a go and see if that improves anything.

int i;
MYSQL *myDB = NULL;
MYSQL_RES *res = NULL;
MYSQL_ROW row;
CString user,pass,aqp;

if((myDB = mysql_init((MYSQL*) 0)) &&
mysql_real_connect(myDB,"localhost","root",NULL,NULL,3306,NULL,NULL))
{
    if(mysql_select_db( myDB,"gourav"))
    {
        GetDlgItemText(IDC_USER,user);
        GetDlgItemText(IDC_PASS,pass);

        if(mysql_query(myDB,"select * from login"))
        {
            res=mysql_store_result(myDB);
            row = mysql_fetch_row(res);
            MessageBox(row[0]);
        }
    }
    else
        MessageBox("Can't connect !\nPlease try later.") ;

    mysql_close(myDB);
}

regards,

Jonathan Wilkes
Darka[Xanya.net]

Re: Access Violation problem

p_26-Jul-07 22:16

26-Jul-07 22:16

thanks sir
sir i try this one but it does not work
as same error at line

row = mysql_fetch_row(res);
before this same error at same line
thanks

Re: Access Violation problem

Jonathan [Darka]26-Jul-07 22:19

Jonathan [Darka]

26-Jul-07 22:19

What is the value of 'res' when this line is executed, you'll need to strp through the code in your debugger.

res=mysql_store_result(myDB);

Jonathan Wilkes
Darka[Xanya.net]

Re: Access Violation problem

p_26-Jul-07 22:31

26-Jul-07 22:31

sir after debugging :
myDB shows this value 0x00ba4008
and res shows this 0x00000000
thanks.

Re: Access Violation problem

Jonathan [Darka]26-Jul-07 23:07

Jonathan [Darka]

26-Jul-07 23:07

I suspect the call is failing, have you tried adding mysql_error() to see if you get an error.

Also, change this part of the code:

if(mysql_query(myDB,"select * from login"))
{
    res=mysql_store_result(myDB);
    row = mysql_fetch_row(res);
    MessageBox(row[0]);
}

to this:

<pre
if(mysql_query(mydb,"select *="" from="" login"))
{
="" res="mysql_store_result(myDB);
" if(res)
="" {
="" row="mysql_fetch_row(res);
" if(row="" !="FALSE)
" messagebox(row[0]);
="" }
}<="" pre="">

Jonathan Wilkes
Darka[Xanya.net]

Re: Access Violation problem

p_26-Jul-07 23:33

26-Jul-07 23:33

yes sir i think u r right call is failing
becauze when debugger comes on if(res) it will show login window again
and this will stop debugger
but i dont know sir why this if(mysql_query(myDB,"select * from login"))
is not fetching result

[sir i use mysql 6.0 and VC++ 6.0(is there any problem Sir)]

Re: Access Violation problem

Jonathan [Darka]26-Jul-07 23:37

Jonathan [Darka]

26-Jul-07 23:37

Are there any records in the database table ?

Jonathan Wilkes
Darka[Xanya.net]

Re: Access Violation problem

p_26-Jul-07 23:47

26-Jul-07 23:47

yes sir
i have database gourav , a table login and
this table has three fields user , password and id
and there are three records in this table

Re: Access Violation problem

Jonathan [Darka]27-Jul-07 0:02

Jonathan [Darka]

27-Jul-07 0:02

What is the return value from the mysql_query(myDB,"select * from login")) function ?

Change it to this for a test:

int i = mysql_query(myDB,"select * from login");

What is the value of i ?

Jonathan Wilkes
Darka[Xanya.net]

Re: Access Violation problem

p_27-Jul-07 0:09

27-Jul-07 0:09

sir return value for
int i = mysql_query(myDB,"select * from login");
and i shows value 1 when use debugger

Re: Access Violation problem

Jonathan [Darka]27-Jul-07 0:24

Jonathan [Darka]

27-Jul-07 0:24

Add the try/catch block that Shouvik suggested, it should help diagnose the problem.

Jonathan Wilkes
Darka[Xanya.net]

Re: Access Violation problem

p_27-Jul-07 0:40

27-Jul-07 0:40

thanks a lot sir
u have devoted a lot of time for me
thanks a lot

Re: Access Violation problem

Jonathan [Darka]27-Jul-07 0:41

Jonathan [Darka]

27-Jul-07 0:41

No problem, just trying to help a fellow coder.

Jonathan Wilkes
Darka[Xanya.net]

Re: Access Violation problem

Shouvik Das27-Jul-07 0:06

Shouvik Das

27-Jul-07 0:06

Surround the code for retrieving with a try and catch. then get the error reason by catching the Exception object throught the method what();

I think Jonathan is right. The recordset value is NULL and hence throws an Acess Violation Error

There are only two kinds of people who are really fascinating-people who know absolutely everything, and people who know absolutely nothing.
Oscar Wilde (1854-1900)
Regards...
Shouvik

Re: Access Violation problem

p_27-Jul-07 0:35

27-Jul-07 0:35

sir thnaks
but i dont know about the what(); function
so can u help me sir
thanks

Re: Access Violation problem

Shouvik Das27-Jul-07 0:45

Shouvik Das

27-Jul-07 0:45

Actually I'm not well versed with ODBC section in VC 6.0 So I m just givin u a generalized concept.
Whenever you surround any code with try catch you throw an exception which is the Object of CException Class.

try
{
//Ur record retrieval code goes here

//On error u write a logic to throw an exception object

}
catch(CException e)
{
 MessageBox(e.what()); // will contain the error message
}

So this way you can atleast trace what error are u getting.

There are only two kinds of people who are really fascinating-people who know absolutely everything, and people who know absolutely nothing.
Oscar Wilde (1854-1900)
Regards...
Shouvik

Re: Access Violation problem

p_27-Jul-07 1:06

27-Jul-07 1:06

sir what() is not working with this
it will give error
'what' : is not a member of 'CException'
please sir solve this
thanks

Re: Access Violation problem

Jonathan [Darka]27-Jul-07 1:07

Jonathan [Darka]

27-Jul-07 1:07

Change the code to this:

int i;
MYSQL *myDB = NULL;
MYSQL_RES *res = NULL;
MYSQL_ROW row;
CString user,pass,aqp;

try
{
    if((myDB = mysql_init((MYSQL*) 0)) &&
    mysql_real_connect(myDB,"localhost","root",NULL,NULL,3306,NULL,NULL))
    {
        if(mysql_select_db( myDB,"gourav"))
        {
            GetDlgItemText(IDC_USER,user);
            GetDlgItemText(IDC_PASS,pass);

            if(mysql_query(myDB,"select * from login"))
            {
                res=mysql_store_result(myDB);
                if(res)
                {
                    row = mysql_fetch_row(res);
                    if(row != FALSE)
                        MessageBox(row[0]);
                }
            }
        }
        else
            MessageBox("Can't connect !\nPlease try later.") ;

        mysql_close(myDB);
}
catch(CException& e)
{
    MessageBox(e.what()); // will contain the error message
}

do you get a messagebox with a description of the error in it ?

Jonathan Wilkes
Darka[Xanya.net]

Re: Access Violation problem

p_27-Jul-07 1:18

27-Jul-07 1:18

no messagebox sir
because what() is not working with this
it will give error
'what' : is not a member of 'CException'
thanks

Re: Access Violation problem

Jonathan [Darka]27-Jul-07 1:25

Jonathan [Darka]

27-Jul-07 1:25

Change what() to ReportError()

Jonathan Wilkes
Darka[Xanya.net]

Re: Access Violation problem

p_27-Jul-07 1:40

27-Jul-07 1:40

hi try this
catch(CException& e)
{
MessageBox(e.ReportError());
}
but gives this error
'MessageBoxA' : cannot convert parameter 1 from 'int' to 'const char *'

Re: Access Violation problem

Shouvik Das27-Jul-07 1:43

Shouvik Das

27-Jul-07 1:43

you will find in my previous post how to get the error into the char err[255].

now pass this as

MessageBox(err);

I hope this works for you fine now

There are only two kinds of people who are really fascinating-people who know absolutely everything, and people who know absolutely nothing.
Oscar Wilde (1854-1900)
Regards...
Shouvik

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