glibc Heap Consistency Checking

Question

1. According to posts from 2008 (I can't find it right now), glibc heap check doesn't work in multithreaded environment. Is it still situation now in 2010?
2. Does heap check enabled by default? (gcc 4.1.2)? I don't set MALLOC_CHECK_, don't aware of calling mcheck(), but still sometimes receive double free glibc error with backtrace. Maybe it's enabled by some compilation flag?

Answer 1

By default, without using malloc_check_ or mcheck(), glibc does some little checks that doesn't hurt the performance, like calling twice free() on the same memory chunk. That's why you are getting some of these messages, but you won't have all messages provided by the malloc substitute api you can get by using MALLOC_CHECK_ (which are doing far more tests, but far more cpu intensive too). You can check this by triggering an error, and testing it with and without malloc_check_. For example, for a simple double-free(), i get "double free or corruption (top)" or "free(): invalid pointer" errors depending whenever I set MALLOC_CHECK_ or not.

To answer the 1/ question, mcheck relies on malloc hooks since they exists (like 15 years), and those are not intended to be thread safe.

Sources: glibc/malloc/malloc.c, http://sourceware.org/bugzilla/show_bug.cgi?id=9939