Memo Problems

Not all memo-related problems trigger error messages such as the corrupt next free block pointer triggers the "Memo file missing/invalid" message. In fact, most memo problems are not problems in the memo file at all, but are problems in the memo field pointers of the DBF.

Memo field pointers are not revealed to you by dBASE, Fox or Clipper. All you see (in BROWSE for example) is the word "Memo" or "memo" depending on whether or not memo text has been defined for the field. But underlying these displays are memo field pointers (or a lack thereof). A memo field pointer is a number that identifies the location of related memo text in the memo file. Specifically, the pointer value times the memo blocksize yields the offset where the memo text starts in the memo file. Well almost. This statement is really true only for dBASE III+, FoxBase and Clipper files. In dBASE 4/5 and FoxPro the memo file contains an 8-byte block header at the calculated offset. This block header is followed by the memo text.

There are two types of corruption to which memo field pointers are susceptible: illegal pointers and wrong pointers. An illegal pointer identifies an invalid location such as a location beyond the end of the memo file or (in dBASE 4/5 or FoxPro) a location where there is no block header. The effects of an illegal pointer can be very serious when you attempt to PACK a file, COPY a file or COPY an affected record. A wrong pointer may point to a legitimate place in the memo file, but it is the wrong place. The only consequence is that you bring up the record for Sally Brown and see the memo text for Harry Smith's record instead. Although it is not possible for software to detect wrong pointers (since the software has no way of knowing to which record a particular block of memo text should relate), software can detect illegal pointers.

dSALVAGE can detect illegal pointers and allows the user to either correct or remove them. dSALVAGE also allows the user to correct or remove wrong pointers once the user has determined their existence by working with the file.

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