1. Consider a file of deposit records of the form:

type deposit = record
bname : char(22);
account# : char(10);
balance : real;
end

• If we assume that each character occupies one byte, an integer occupies 4 bytes, and a real 8 bytes, our deposit record is 40 bytes long.
• The simplest approach is to use the first 40 bytes for the first record, the next 40 bytes for the second,and so on.
• However, there are two problems with this approach.
• It is diffcult to delete a record from this structure.
• Space occupied must somehow be deleted, or we need to mark deleted records so that they can be ignored.
• Unless block size is a multiple of 40, some records will cross block boundaries.
• It would then require two block accesses to read or write such a record.

2. When a record is deleted, we could move all successive records up one (Figure 10.7), which may require moving a lot of records.

• We could instead move the last record into the "hole" created by the deleted record (Figure 10.8).
• This changes the order the records are in.
• It turns out to be undesirable to move records to occupy freed space, as moving requires block accesses.
• Also, insertions tend to be more frequent than deletions.
• It is acceptable to leave the space open and wait for a subsequent insertion.
• This leads to a need for additional structure in our file design.

3. So one solution is:

• At the beginning of a file, allocate some bytes as a file header.
• This header for now need only be used to store the address of the first record whose contents are deleted.
• This first record can then store the address of the second available record, and so on (Figure 10.9).
• To insert a new record, we use the record pointed to by the header, and change the header pointer to
  the next available record.
• If no deleted records exist we add our new record to the end of the file.

4. Note: Use of pointers requires careful programming. If a record pointed to is moved or deleted, and that pointer is not corrected, the pointer becomes a dangling pointer. Records pointed to are called pinned.

5. Fixed-length file insertions and deletions are relatively simple because \one size fits all". For variable length,this is not the case.