[lwip-users] [lwip] Proposal for magic numbers in LwIP

[Paul Sheer]

I propose that all structures which are dynamically allocated
begin with a magic number. Before the structure is free'd this
number should be set to zero. My own practice is:

  s = nalloc();
  memset (s, 0, sizeof (*s));
  s->magic = THE_MAGIC_OF_S;

     ....

  s->magic = 0;
  free (s);

now before every occurance of the use of s, should come the line:

  magic (s, THE_MAGIC_OF_S);
  if (s->some_member == x) { ...

where magic() is defined as

#define magic(e,m)                                                      \
do {                                                                    \
  if ((e)->magic != (m)) {                                      \
    __die (__FILE__, __FUNCTION__, __LINE__, \
"magic: (%s)->magic != %p(%s): magic failed for pointer %p", \
    #e, (void *) (m), #m, (void *) (e));        \
  }                                                                     \
} while (0)

which gives you a lot of useful info.
(__die() is PaulOS specific and does print a traceback.)

this should really catch any problems.

i'll send a patch soon where I use magic
everywhere in pcb structs, pbufs etc.

additionally you can add a traceback array member to the
struct that gets filled before the struct is free'd.
if it is free'd again it is easy to see how it
was free'd the first time. - but thats another story.

best

-paul


Paul Sheer Consulting IT Services . . . Tel . . . +27 21 761 7224
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