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[PATCH] Fix backwards compatibility problem in libstdc++


It seems the
changes broke libstdc++ backwards compatibility, some GCC 3.4.x compiled
programs and/or libraries misbehave with GCC 4.0.x and later
The problem is if some 3.4.x program/library has an inlined copy of
_S_construct or _M_clone (that means it initializes _M_length and
_M_refdata's last char, but not _M_refcount) and calls out of line
_S_create (which in GCC 3.4.x used to clear _M_refcount, but after
the above patch it no longer does), then _M_refcount is uninitialized.
When running 3.4.x code against 3.4.x libstdc++ or 4.[01].x code against
4.[01].x libstdc++ there is no problem, in the first case _M_refcount
is initialized in _S_create, in the latter in _S_construct/_M_clone that
call it.
I'm afraid the only safe fix is now to clear _M_refcount in _S_create
in addition to clearing it in its callers (well, it is just one int/long
store, so it shouldn't slow things that much).

Ok for trunk/4.1/4.0?

2006-10-13  Jakub Jelinek  <>

	* include/bits/basic_string.tcc (_Rep::_S_create): Call
	_M_set_sharable() for backwards compatibility.

--- libstdc++-v3/include/bits/basic_string.tcc.jj	2006-10-05 00:28:47.000000000 +0200
+++ libstdc++-v3/include/bits/basic_string.tcc	2006-10-13 17:50:53.000000000 +0200
@@ -588,6 +588,14 @@ _GLIBCXX_BEGIN_NAMESPACE(std)
       void* __place = _Raw_bytes_alloc(__alloc).allocate(__size);
       _Rep *__p = new (__place) _Rep;
       __p->_M_capacity = __capacity;
+      // ABI compatibility - 3.4.x set in _S_create both
+      // _M_refcount and _M_length.  All callers of _S_create
+      // in basic_string.tcc then set just _M_length.
+      // In 4.0.x and later both _M_refcount and _M_length
+      // are initialized in the callers, unfortunately we can
+      // have 3.4.x compiled code with _S_create callers inlined
+      // calling 4.0.x+ _S_create.
+      __p->_M_set_sharable();
       return __p;


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