Re: appending to a file
mohi wrote:
On Jul 25, 8:34 pm, Victor Bazarov <v.Abaza...@comAcast.net> wrote:
mohi wrote:
hello everyone,
i have a program where i write a structure of 1020B to a file a number
of times and in the next run of the program i have to again write
those structures but by taking some count from the last structure
written on the file
so i write it like:
fstream file("xyz",ios::out|ios::in|ios::ate);
long pos=file.tellg();
pos-=sizeof(struct);
file.putg(pos,0);//position at pos
file.read(reinterpret_cast<char *>(&struct),sizeof(struct));
use struct.cout;///////
the problem is that after the read struct has a count which is of the
second last struct written
can some one help me why this gets two structs back in the file.
How do you know that it is what's happening? Can you print the
position? Can you examine the file at that position? Can you tell
exactly how far from the end the file gets positioned? What's the file
size? Do you know with certainty that the "last struct" actually does
get written to the file?
Direct access database can be tricky. To debug it you need to be able
to look at the file at the byte level, at the record level, and
understand how your data get written (or even if they get written).
V
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thanks for repling.....
this is actually an implementation of bst(binary search tree) written
on a file(with pointers as file position pointers)
now i display the structure after its written to the file and it has a
count field thats incremented for each write
Two things: don't quote signatures, and don't repeat what you already
told us.
now when run for the first time say the last output says that the
present count is 10(say)...that is a record with count=10 is written
to the file,
Do you see it in the file? Do you have the tools (other than your own
code) which would read the file and allow you to inspect its content?
now in the next run where the file is opened as ios::ate ,and after
substracting one sizeof(strut) from the present file pointer
it reads a structure that has a count=9,though it shud have been 10.
now thats the problem....
Your code consists of two large parts. First is *writing* to the file.
Make sure it works. When you know that writing works, you can debug the
*reading* of the file. If you don't know *for sure* that either is OK,
you cannot rely on one to debug the other.
Forget reading from the file for now. Find a tool that would allow you
to dump the contents of the file in, say, hexadecimal form, and examine
them carefully, making sure that what you say happens actually *does*
happen. Otherwise it's just your imagination at work.
Now, if you want some kind of theoretical advice on why your program
(which I've never seen) does what you think it does (that I have doubts
about anyway), then you have it: your reading does not work because your
writing does not work. If you want *concrete* help with your program,
*post your code*.
V
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Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask