chmod101
asked on
Binary file writing corrupted
I have a small program that stores information to a binary file that uses the following record as its format:
type Tdata = string[255];
type
problem = record
item1 : string[3];
item2 : string[3];
item3 : string[3];
info : Array [0..9] of Tdata;
numItems : integer;
end;
The program allows the user to either open an existing binary file to edit/update, or to create a new file to store entries in.
The array of Tdata called info is used to store string data of length > 255. When the user inputs data into a memo box, the string is broken down into 255-character chunks and placed in info[x].
My problem is that if a user creates a new file and then writes data out to it, the binary file will have data in it from the file that was open before the user created the new file if the following condition occurs:
The previously opened file had text of length > 255 stored in the info array, thus meaning more than info[0] has data in it.
When the user creates a new file, the program closes the previous file if one is open and clears out all of the data structures and variables holding data at the time, so the new file starts off with a completely blank slate. I have stepped through the program multiple times watching every variable that stores data on the records in the files, and they are all cleared out properly before the new file is even opened. The odd thing is that if the user uses the program to view the record, they will not see the data from the old file. The program only reads in the data that it is supposed to, but if I open up the binary file in notepad, I can see the text there from the previous file.
This may not be clear at all, but I hope you see what I am getting at. I just want to know where that unwanted text from the previous file is coming from, and how to get rid of it.
chmod101
type Tdata = string[255];
type
problem = record
item1 : string[3];
item2 : string[3];
item3 : string[3];
info : Array [0..9] of Tdata;
numItems : integer;
end;
The program allows the user to either open an existing binary file to edit/update, or to create a new file to store entries in.
The array of Tdata called info is used to store string data of length > 255. When the user inputs data into a memo box, the string is broken down into 255-character chunks and placed in info[x].
My problem is that if a user creates a new file and then writes data out to it, the binary file will have data in it from the file that was open before the user created the new file if the following condition occurs:
The previously opened file had text of length > 255 stored in the info array, thus meaning more than info[0] has data in it.
When the user creates a new file, the program closes the previous file if one is open and clears out all of the data structures and variables holding data at the time, so the new file starts off with a completely blank slate. I have stepped through the program multiple times watching every variable that stores data on the records in the files, and they are all cleared out properly before the new file is even opened. The odd thing is that if the user uses the program to view the record, they will not see the data from the old file. The program only reads in the data that it is supposed to, but if I open up the binary file in notepad, I can see the text there from the previous file.
This may not be clear at all, but I hope you see what I am getting at. I just want to know where that unwanted text from the previous file is coming from, and how to get rid of it.
chmod101
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I explaned whz simply setting variable to null won't help. If zou set it to null nothing in the memorz changes bzt first byte stating that new string length iz 0. You have to fill in memory with #0's (or whatewer you want).
If you have
var
a : string[3],
to clear it out you have to do the following:
a := #0#0#0;
a := '';
Then you will have clear memory.
This is necessary because writing record to field
does not store values but whole memory block
reserved for the record.
You may take a look at FillCHar() function in Delphi's help. I am not sure about right name, my help is somehow corrupted so I cannotopen it :( It fills memory block with specified character. That will allow you to fill record's memory space with #0 before storing values to it.
Pedja
If you have
var
a : string[3],
to clear it out you have to do the following:
a := #0#0#0;
a := '';
Then you will have clear memory.
This is necessary because writing record to field
does not store values but whole memory block
reserved for the record.
You may take a look at FillCHar() function in Delphi's help. I am not sure about right name, my help is somehow corrupted so I cannotopen it :( It fills memory block with specified character. That will allow you to fill record's memory space with #0 before storing values to it.
Pedja
ASKER
Thanks pedia, my basic assumption was that when the debugger showed me that the string was empty, it really was empty. Thanks for the help.
chmod101
chmod101
ASKER
thank you for the comment, but as I said in the question, I am already clearing out all of the data structures before opening another file - that includes setting all of the string variables to null.
Any other thoughts?
chmod101