Wrting to a file

Posted on 1998-12-02
Last Modified: 2012-05-04
I want to insert data into a file and was wondering if there is an easier way other than reading all the data into a linked list of structures , inserting new data into the list then rewriting all the data back out again.
Am using BC++5.02 and OWL.
Question by:dooley090698

Expert Comment

ID: 1179097
You only need to read the stuff after the insertion point...For a simple sequential file format, there's no magic solution to the insertion problem.

Actually a better idea would be to implement the link list (or other complex data structures) on disk...instead of pointers to the next element, use file offsets.


Author Comment

ID: 1179098
Thanks for quick response.
I was afraid there weren't any easy solutions!
Problem being its a comercial software package so I can't change file layouts.
Just have to suffer  I guesse.

LVL 22

Expert Comment

ID: 1179099
>> You only need to read the stuff after the insertion point
Don't you mean "before"?  i.e. read the part of the file that is after the insertion point into a buffer.  write out the new data at the insertion point.  Write the data saved in the buffer after the new data.

Note you don't necessarily need to use linked list etc to do this.  If the format is simple enough, this is just a matter of reading and writting arrays of bytes.
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Expert Comment

ID: 1179100
Well it depends on the file format, but I meant _after_ :

1. Skip the start of the file,
2. save the current file position into a variable
3. Read the end part of the file into a buffer
4. Seek back to saved file position from 2
5. Write the new data to insert
6. Write the buffer

What's the best data structure to mimic the file in memory obvious depends on the format.    Actually I find that for a lot of text based formats (configuration files, etc.) maps work pretty well - but that actually requires you parse the file - which may or may not be necessary depending on what you do.

If you know the file is fairly small (esp. in using Unix or Win32) just allocate/declare a big buffer, read the whole thing into memory, then rewrite it how you want, a char array is a simple quick'n'dirty way to do this.

I won't mention the program, but I do know of one commercial Unix program (I did not write it) which begins

int main( int argc, char * argv[] )
char buffer[1500000] ;

LVL 22

Expert Comment

ID: 1179101
Right, I swore there was a "don"t in there.  Like "you don't need to read...."

Accepted Solution

corneil earned 100 total points
ID: 1179102
If the order of the data in the file is not that important but you need quick access you should consider building an structure that will act like a table of contents so that you can access data anywhere in the file quickly and add new data to the end. As the TOC grows you add new blocks to the end with a pointer to the new block in the previous TOC block. This way you will not need to rewrite the file every time you add data. You can sort and rewrite your TOC but because the size does not change you will only overwrite existing TOC blocks.
LVL 22

Expert Comment

ID: 1179103
corneil, please read the question history before answering.  (look who's talking--I read it--just wrong.)  dooley says "Problem being its a comercial software package so I can't change file layouts."

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