Closing a binary file before it ends

[cedrik23]

hi everyone

i'm working on a school project (a phonebook) using a binary file.

one of the procedure's is to delete a person from the phonebook file. i've been working on an idea for this and in short it has to do with taking the last person in the file and copying him over the person to be deleted and then closing the file just before the last person(thus making the file smaller)

the only problem is i cant close the file not at the end. i position my file pointer just before the last person in the file( using fseek(phonefile,-sizeof(node),SEEK_END) ) and then i do fclose(phonefile) to close the file just before him. but when i check the file he is still there!

meaning that an EOF was not assigned to the place i wanted but rather to the end of the file.


ideas anyone?

[matsp]

That's not quite how it works. Files grow automatically, but to truncate a file you need to do more work in one way or another.

You can do a few different
1. Set a flag or some other way to indicate that "this is a deleted item" in the record. If you want to, you could then create some sort of "linked list" of free entries.

2. use "truncate()" or some similar function: http://www.hmug.org/man/2/ftruncate.php
I don't think this is portable to Windows, but that may of course not be much of a problem, or a big problem, depending on what your system is.

--
Mats

[matsp]

Windows has this function: http://msdn2.microsoft.com/en-us/library/aa365531.aspx

--
Mats

[cedrik23]

its a dos program i dont have any handles to files

i dont think the linked list idea is what our teacher asked cause he specificly said to delete from the file.

does the ftruncate work on dos or only unix cause i dont have the unistd.h library in my visual c++ 6

[MacGyver]

/me sighs.

There is a difference between a DOS program written explicitly for DOS and a Windows CONSOLE program that is run in what APPEARS to be a DOS window. If you're using Visual C++ 6, then it's a Windows compiler. It also sucks. Try to get with the times if that is an option and use something like MinGW.

In concepts of files, the file is going to stay the same way until you start writing to it. So reading has no effect on a file (unless your idea of reading a book is to burn the pages). Simply read in whatever you need to read in, perform whatever calculations in memory that you need to perform, and then write the entire contents of the file back. It's not the most elegant, but at least it works. Later, after you complete that, you can work on better ways of seeking to the correct entry and shifting the other elements back to where they should be.

[matsp]

Well, considering that "Visual C++ 6" can not produce "DOS" applications [it produces Windows Console applications], you should be able to use the SetEndOfFile() that I suggested in the second post.

Or perhaps this works better:
http://msdn2.microsoft.com/en-us/lib...yb(VS.80).aspx

--
Mats

[matsp]

/me sighs.

There is a difference between a DOS program written explicitly for DOS and a Windows CONSOLE program that is run in what APPEARS to be a DOS window. If you're using Visual C++ 6, then it's a Windows compiler. It also sucks. Try to get with the times if that is an option and use something like MinGW.

In concepts of files, the file is going to stay the same way until you start writing to it. So reading has no effect on a file (unless your idea of reading a book is to burn the pages). Simply read in whatever you need to read in, perform whatever calculations in memory that you need to perform, and then write the entire contents of the file back. It's not the most elegant, but at least it works. Later, after you complete that, you can work on better ways of seeking to the correct entry and shifting the other elements back to where they should be.

Yes, I agree, read the entire file in, modify, create a new file, write it out, will be the obvious solution - assuming the project allows this. And seeing the almost complete absence of simple ways to do this directly with the "FILE *" products, this is likely to be the correct way. One would assume that the teacher is not requesting quite obscure file handling procedures.

--
Mats

[cedrik23]

the teacher is quite obscure himself

[Elysia]

It also sucks. Try to get with the times if that is an option and use something like MinGW.

Or Visual Studio 2008 Express