Storing struct in file - problem when new item needed in struct

[Powerponken]

Hi,

I have received an application that stores some properties in a file. The existing struct looks like this:

struct TData 
{
	UINT	uSizeIncludingStrings;
	// copy of Telnet data struct
	UINT	uSize;
	// basic properties:
	TCHAR	szHost[MAXHOSTLEN];  //defined in Sshconfig
	UINT	iPortNr;
	TCHAR	szTermType[MAXTERMTYPE];
	// login properties:
	BOOL	bTelnetLogin;
	TCHAR	szUserName[MAXUSERNAME];
	BOOL	bDetectUserName;
	BOOL	bDetectPassWord;
	TCHAR	szDetectUserName[MAXDTCTLEN];
	TCHAR	szDetectPassWord[MAXDTCTLEN];
	// advanced options:
	BOOL	bReplNull;
	TCHAR	cNullChar;
	BOOL	bFilterCR;
	BOOL	bAddLF;
	BOOL	bTransRel;
	BOOL	bFilterXonXoff;
	BOOL	bDetectDomain;
	TCHAR	szDetectDomain[MAXDTCTLEN];
	TCHAR szDomain[MAXDOMAIN];

	char		*ciphers;
	char		*macs;
	int		authentication; };

The two char pointers ciphers and macs seem to be something that would not work because storing a pointer will not store anything else but the pointer itself and not the value but this is not the case here. When I open the property file I can read the values that were clicked in the checkboxes in the application. Some values seem to be human readable and some not in the rest of the file. The values ciphers and macs seem to be "enclosed" in some kind of block starting with STR_HEAD_VER_5 and ending with END_STR. This block also seem to be dynamically sized since there can be different numbers of ciphers and macs inside this block.

The problem is now that there is a need for adding another item in the struct and store it in the file. I was going to add a char* at the end as for ciphers and do the coding like the one handling ciphers and macs. It looks like this afterwards:

struct TData 
{
	UINT	uSizeIncludingStrings;
	// copy of Telnet data struct
	UINT	uSize;
	// basic properties:
	TCHAR	szHost[MAXHOSTLEN];  //defined in Sshconfig
	UINT	iPortNr;
	TCHAR	szTermType[MAXTERMTYPE];
	// login properties:
	BOOL	bTelnetLogin;
	TCHAR	szUserName[MAXUSERNAME];
	BOOL	bDetectUserName;
	BOOL	bDetectPassWord;
	TCHAR	szDetectUserName[MAXDTCTLEN];
	TCHAR	szDetectPassWord[MAXDTCTLEN];
	// advanced options:
	BOOL	bReplNull;
	TCHAR	cNullChar;
	BOOL	bFilterCR;
	BOOL	bAddLF;
	BOOL	bTransRel;
	BOOL	bFilterXonXoff;
	BOOL	bDetectDomain;
	TCHAR	szDetectDomain[MAXDTCTLEN];
	TCHAR szDomain[MAXDOMAIN];

	char		*ciphers;
	char		*macs;
	int		authentication;
         char             *key;
 };

So far so good but the problem showed up when upgrading from the old application using the old struct in the property files. When starting the upgraded application with the new struct declared in the program some properties were not read and you need to enter them again and save and then it looks fine.

When debugging the new code reading an old property file I could see that there was a check done using sizeof(TData) to set the pointer in the file. Due to the increased size when adding the key item in the struct it steps to far. The check compares that the pointer points to the string STR_HEAD_VER_5(which is hardcoded in the code) in the file before reading the data within this block and then END_STR when finished. If the pointer does not point to the STR_HEAD_VER_5-string the reading is interrupted and an error message saying that the file is corrupt is received. When debugging I could see that the pointer pointed to HEAD_VER_5 instead which is not STR_HEAD_VER_5 and thus the reading is interrupted and some properties are not read.

My questions are now:

- Is there some common used technique, method, way to handle a problem like this so the operator does not need to enter the values again and can use the old property files and when saving the properties in the upgraded version and then get the new property file?

Regards,
Kalle