Visit Jeremy's Blog.
Go Back > Forums > Non-*NIX Forums > Programming
User Name
Programming This forum is for all programming questions.
The question does not have to be directly related to Linux and any language is fair game.


  Search this Thread
Old 09-25-2004, 09:00 AM   #1
LQ Newbie
Registered: Sep 2004
Location: malaysia
Posts: 7

Rep: Reputation: 0
c++ file processing -- how to remove a record from a file

i am writing a c++ program to add, view, modify, and delete records from a ".dat" file.

for example, "student.dat" file containing fields: stud_no, name, etc.
i use struct student {} to store information of a student, then write to the file.

what are the codes should i use to allow user to remove a particular record? (i.e: remove the record with stud_no: 12345)

thank you if anyone willing to help me.
Old 09-25-2004, 11:48 AM   #2
Senior Member
Registered: Aug 2002
Location: Groningen, The Netherlands
Distribution: Debian
Posts: 2,536

Rep: Reputation: 111Reputation: 111
Just removing a block of bytes from a file is not possible. One solution to this is:
  • Rename the file to student.OLD
  • In a loop: copy the structs one-by-one to a new student.dat file, while skipping the one to remove (with of something like: "if (stud.stud_no == 12345) continue")
  • Remove student.OLD.

A faster but more complex option may be to "memory map" the file with the "mmap()" system-call. The mmap() function loads the file into memory, so you can read/write to the structs as if it were an array.

The fastest way will probably be to use mmap(), and not actually remove the struct, but to mark it as removed by setting stud_no of the struct-to-remove to 0 (or -1 if that suits you better). This way the struct is not really removed from the file, and the file will keep the same size. Also you'll need the rest of your program to detect the stud_no that are 0.

Maybe it is also possible (I'm not quite sure about this) to really remove the struct by using mmap(). E.g. when you found the (mmapped) array-index of the struct you want to remove to be 3, then copy the structs remaining in the file (index 4 and up) to fill the gap. Something like:
 for (i=0; i < arraylen-1; ++i) {
    mmapped_student[ i ] = mmapped_student[ i+1 ]
Then, when you're done removing structs, munmap() the file with the length being n times sizeof(struct student) less than the original length of the file. (n being the number of removed structs).

To read more on mmap() and munmap(), see "man mmap", and the book "Advanced Linux Programming" (by New Riders publishing), chapter 5. This book is downloadable freely from (one PDF file per chapter)
Old 09-25-2004, 01:21 PM   #3
Registered: Mar 2004
Location: Massachusetts
Distribution: Debian
Posts: 557

Rep: Reputation: 43
If the order of the structs in the file doesn't matter, you could just copy the last struct in the file over the deleted one, then truncate after the second-to-last struct. See fseek(3) and friends.
Old 09-26-2004, 12:03 AM   #4
LQ Newbie
Registered: Sep 2004
Location: malaysia
Posts: 7

Original Poster
Rep: Reputation: 0
modify a record in student.dat

Thank you very much.

Another question:
what are the codes to allow user to modify a record, for example, modify the record with stud_no 1234 to stud_no 2323? why when i modify a record using the following codes, that record(1234) does not changed. but it will insert the information(2323) as a new record at the end of student.dat?

My coding is like the follow:

void main()
cout<<"Enter the stud_no of the record you want to modify: ";

void modify(int stuno)
student s;
fstream file;"student.dat", ios::app|ios:ut|ios::in);

int count = findloc(stuno);

cout<<"\nDo you want to modify this record? (Y/N) ";

int location = (count - 1 ) * sizeof(s);

if(tolower(choice) == 'y')
cout<<"Enter stud_no:";
cout<<"Enter name: ";
file.write((char *) &s, sizeof(s));

int findloc(int stuno)
student s;
int n, endposition, count = 0;
fstream file;"student.dat", ios::app|ios:ut|ios::in);
file.seekg(0, ios::end);
endposition = file.tellg();
n = endposition / sizeof(s);
file.seekg(0); *) &s, sizeof(s));
while(file.eof() == 0)

if(s.stud_no == stuno)
break; *) &s, sizeof(s));

if(count - 1 == n)
cout<<"There is no record with stud_no ("<<stuno<<")"<<endl<<endl;


return count;

Last edited by sharonyiisl; 09-26-2004 at 12:34 AM.
Old 09-26-2004, 03:54 AM   #5
Registered: Jun 2001
Location: UK
Distribution: Gentoo, RHEL, Fedora, Centos
Posts: 43,417

Rep: Reputation: 1984Reputation: 1984Reputation: 1984Reputation: 1984Reputation: 1984Reputation: 1984Reputation: 1984Reputation: 1984Reputation: 1984Reputation: 1984Reputation: 1984
I am having a bit of difficulty believing that this is not just your homework... not least because it's abuot students. if this is work assigned to you as part of an educational program, please do not ask for help about it here.


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Unable to record or play a wav file using ALSA stephenwalter Programming 1 08-18-2005 02:08 PM
input text file processing the gnu way? zero79 Programming 3 03-04-2005 07:41 PM
What program can record of download file ? jerrytw Linux - Networking 1 01-11-2005 03:27 AM
problems reading in fixed-length record file naijaguy Programming 1 08-24-2004 02:34 PM
record sound onto a file biophysics Linux - Software 0 10-06-2003 04:57 PM > Forums > Non-*NIX Forums > Programming

All times are GMT -5. The time now is 02:37 PM.

Main Menu
Write for LQ is looking for people interested in writing Editorials, Articles, Reviews, and more. If you'd like to contribute content, let us know.
Main Menu
RSS1  Latest Threads
RSS1  LQ News
Twitter: @linuxquestions
Open Source Consulting | Domain Registration