dbg_malloc

NAME
  malloc - debugging memory allocation routines

SYNOPSIS
  #include <stdlib.h>
  void *malloc(size_t size);
  void *realloc(void *ptr, size_t size);
  void *calloc(size_t size, size_t nel);
  void free(void);
  void *check_malloc(void *ptr);

DESCRIPTION
  These routines are replacements for the standard memory
  allocation routines for use in debugging an application that
  corrupts its memory arena.

  Applications corrupt the memory arena by writing to or
  reading from memory that has not been allocated, or by
  scribbling on the internal malloc(3C) data structures that
  precede and/or follow each allocated buffer.  Typically this
  corruption occurs before the application unexpectedly
  terminates. Tracking and locating the errant access at the
  time of corruption facilitates fixing the application.
  These routines are designed to return pointers to blocks
  that are aligned to cause the generation of a fault when
  accessing a memory location that is beyond the allocated
  block.  They are based on the virtual memory sub-system's
  denying  a process access to unmapped pages.

  Memory arena corruption can occur at either end of an
  allocated block.  The page fault on access behavior can only
  be enforced towards one end within a process at any one
  time. By default, these routines will fault on access of
  data beyond the end of the allocated block. This is the more
  typical cause of malloc arena corruption. Fault generation
  can be changed to occur upon access of data in front of the
  allocated block by setting the DEBUG_MALLOC environment
  variable, as described below.

  Another example of memory corruption occurs when accessing
  memory that has already been free'd. This version of free
  makes freed memory inaccessible.  All freed memory is
  released when the system can not allocate any new memory
  resources.  Accessing data within such a block after it has
  been freed but before it has been made accessible again will
  generate a fault.  The default behavior can be modified to
  immediately make freed pages available. See the DEBUG_MALLOC
  description below.




  ptr is NULL, the entire allocated memory arena is checked.
  check_malloc  returns 0 if the block pointed to by ptr, or
  in the case of NULL the entire arena, is intact.  Otherwise
  it returns a corruption address. If ptr is NULL and there
  are multiple corrupted blocks, check_malloc returns a
  corrupted address. This function is useful in determining if
  the memory arena has been corrupted at the non-fault
  generating end.  The environment variable DEBUG_MALLOC can
  then be reset to fault on access of the reverse end, and the
  fault generated by running the application again.

  f(CWdebug(1) should be used in conjunction with these
  routines to facilitate tracking down the instruction that
  accessed unavailable memory.  Setting DEBUG_MALLOC to show
  the information from any of the allocation routines includes
  the return address of the function that is calling the
  memory routines.  Using this information with debug makes it
  easier to isolate memory corruption.

  Control over the amount of debugging information presented
  is provided by setting and exporting the environment
  variable DEBUG_MALLOC.  The supported options are: M - trace
  return values from malloc R - trace entry and return values
  from realloc C - trace entry and return values from calloc F
  - trace entry to free H - return freed blocks to the memory
  pool immediately X - provide additional debugging/tracing
  information B - allocation routines will fault on access of
  data beyond the block (default) T - allocation routines will
  fault on access of data before the beginning of a block N -
  fill allocated space with a non-zero bit pattern

  If both B and T are specified, B will be used.

DIAGNOSTICS
  The allocation routines return NULL if the allocation can
  not be satisfied.

SEE ALSO
  mmap(2), munmap(2), mprotect(2), malloc(3C)