On 10/3/2011 7:23 AM, Andrew Cooper wrote:
> Hello,
>
> I am using valgrind-3.6.0.SVN-Debian, and cant work out why I am getting
> Invalid read errors.
>
> The routine is one which is adding symbols to a std::map, indexed by
> their virtual address. As multiple symbols may share the same virtual
> address, I need to concatenate the symbol names in the already present
> symbol entry.
>
> I have the following (cut down, debugging) code which demonstrates the
> problem:
>
> Symbol * prev = prev_itt->second;
> char * prev_name = prev->name;
>
> size_t newsize = strlen(prev_name);
> newsize += strlen(sym->name);
> newsize += 2;
>
>
> prev->name = new char[newsize];
> //Should concat and delete prev_name, but for debugging
> leave like this
> delete [] prev->name;
>
> //For debugging, put this back, so no leaks
> prev->name = prev_name;
There is a bug here: prev_name is a copy of a pointer to the memory you
just deleted. prev->name now points to invalid memory. valgrind tries
to keep track of these but if the memory is not referenced for a long
time (and many new allocations), it may not be able to determine that
the reference is to just-freed memory (as opposed to as-yet unallocated
memory).
>
> With the new char[newsize] and delete [] prev->name commented out,
> valgrind is perfectly happy with the code (which is effectively a nop)
It is not a no-op unless those two lines are commented out; see above.
>
> However, with the new and delete present as above, valigrind complains with:
>
> ==10046== Invalid read of size 1
> ==10046== at 0x4C286E4: __GI_strlen (mc_replace_strmem.c:284)
> ==10046== by 0x40CB88: SymbolTable::insert(Symbol*) (symbol-table.cpp:37)
> ==10046== by 0x406536: parse_symbol_file(std::string) (main.cpp:44)
> ==10046== by 0x406DA4: main (main.cpp:70)
> ==10046== Address 0x7ae7d36 is 0 bytes after a block of size 6 alloc'd
> ==10046== at 0x4C27939: operator new[](unsigned long)
> (vg_replace_malloc.c:305)
> ==10046== by 0x40C969: Symbol::Symbol(unsigned long long, char, char
> const*) (symbol.cpp:9)
> ==10046== by 0x406526: parse_symbol_file(std::string) (main.cpp:44)
> ==10046== by 0x406DA4: main (main.cpp:70)
> ==10046==
> ==10046== Invalid read of size 1
> ==10046== at 0x4C286E4: __GI_strlen (mc_replace_strmem.c:284)
> ==10046== by 0x40CB94: SymbolTable::insert(Symbol*) (symbol-table.cpp:38)
> ==10046== by 0x406536: parse_symbol_file(std::string) (main.cpp:44)
> ==10046== by 0x406DA4: main (main.cpp:70)
> ==10046== Address 0x7ae7fe6 is 0 bytes after a block of size 6 alloc'd
> ==10046== at 0x4C27939: operator new[](unsigned long)
> (vg_replace_malloc.c:305)
> ==10046== by 0x40C969: Symbol::Symbol(unsigned long long, char, char
> const*) (symbol.cpp:9)
> ==10046== by 0x406526: parse_symbol_file(std::string) (main.cpp:44)
> ==10046== by 0x406DA4: main (main.cpp:70)
>
> Where the two errors are referring to the two strlen() calls when
> calculating newsize.
>
> Are these errors indicating a supposed bug in my code, or are they
> complaining about something in the __GI_strlen replaced code. If so,
> does this mean there is a bug in __GI_strlen ?
>
> Thanks in advance,
>
--
David Chapman [email protected]
Chapman Consulting -- San Jose, CA
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