[lwip-devel] [bug #21077] lwip_stats.mem.used is inaccurate, sometimes negative

[Mike Kleshov]

Follow-up Comment #1, bug #21077 (project lwip):

Having spent some time analyzing the code in mem.c, I think I found where the
problem comes from.

In the function mem_malloc(), lwip_stats.mem.used is updated as follows (line
470):
lwip_stats.mem.used += (size + SIZEOF_STRUCT_MEM);

But a few lines before that (see line 459), a block is reused whose size is
larger than the requested size. Note the comment:
* also can't move mem->next directly behind mem, since mem->next
* will always be used at this point!

Taking this into account, in the mentioned special case lwip_stats.mem.used
should be updated using mem->next:
lwip_stats.mem.used += mem->next - ((u8_t *)mem - ram); /* suggested fix */

By the way, mem->next is used to update it in mem_free():
lwip_stats.mem.used -= mem->next - ((u8_t *)mem - ram);

A quick test confirms that the fix works, lwip_stats.mem.used seems to behave
normal.
Can anyone comment on this? I can commit the changes if there are no
objections.

- Mike

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