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Memory Violation Trapping Disclosure Number: IPCOM000118045D
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 8 page(s) / 187K

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Kagan, O: AUTHOR


Memory Violations are difficult to trace and fix, and may "live" in software for a long time, reducing software performance, especially in statistical programs.

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Memory Violation Trapping

      Memory Violations are difficult to trace and fix, and may
"live" in software for a long time, reducing software performance,
especially in statistical programs.

      The described method permits the provision of simple software
which taps memory violations, reducing debugging time, and increasing
software performance.  The software referred to hereafter as Memory
Violation Trapping (MVT) provides the following essential elements:
  1.  All allocations and frees are performed using the MVT
       software.  The MVT uses the regular allocation system
       but adds the following elements
  2.  MVT marks each allocation in an Allocation Table (AT),
       including all information required for tracing the
       violation and giving the programmer all information,
       such as variable name, size, location, etc..
  3.  MVT adds for each allocated memory area a Safety Prefix
       and Suffix Padding Areas (PSSA).
  4.  MVT initializes the PSSA padding to a Specific Value (SV)
       (or to a specific sequence of values).
  5.  MVT supplies optional reports of the Allocation Table (AT).
  6.  When free is being executed for an allocated area, MVT
       erases it from AT while checking its PSSA.  If the PSSA
       does not have the SV value, a violation has occurred and
       an error message is printed.
  7.  MVT traces double free of a variable.
  8.  MVT, when freeing a variable, sets its pointer to
       NULL to avoid further access to the freed areas.

      The PSSA area provides prefix and suffix padding for each
allocated variable which is initialized by MVT to a SV (not zero, for
trace violation when initializing zeros to a variable) "travels" with
the allocated variable through all program stages, but is never
touched by the program.  When freeing this variable, MVT checks that
this padding is set to its original setting (SV) value.  In addition,
the use of the AT for handling the allocated variables list enables
duplication of free allocations to be avoided and provides
information on violation when trapped.

      This enables MVT to trap violations between program variables,
and report the exact location of violations in code.  The Operating
system traps violations only when memory page (memory segments) are
affected.  "Small" violations (within program memory page) are
usually un-trapped.

The following main.c program contains three types of violations:
  1.  Suffix violation (setting of bigger area than allocated
  2.  Prefix violation (setting negative index on array).
  3.  Double free violation (which may free undesired areas).

      All of the above may cause unexpected errors in a
program.  This main.c was compiled using the IBM*-C2 compiler.  When
executed, no violation was detected by the system (program finished
without segmentation fault).