[LLVMbugs] [Bug 13669] New: std::uncaught_exception should not call abort() if __cxa_uncaught_exception is unavailable

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13669

             Bug #: 13669
           Summary: std::uncaught_exception should not call abort() if
                    __cxa_uncaught_exception is unavailable
           Product: libc++
           Version: unspecified
          Platform: All
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
        AssignedTo: hhinnant at apple.com
        ReportedBy: oneill+llvmbugs at cs.hmc.edu
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Created attachment 9097
  --> http://llvm.org/bugs/attachment.cgi?id=9097
Return false rather than abort.

On Linux, I found libc++ was crashing in basic_ostream<_CharT,
_Traits>::sentry::~sentry().  I tracked down the source of the problem to the
fact that this function calls std::uncaught_exception(), and apparently
std::uncaught_exception() calls abort().

This seems to be wrong on two counts.  With the GNU C++ abi on Linux there
apparently *is* a way to find out if there are uncaught exceptions; with the
GNU ABI, it appears that

    return __cxa_get_globals()->uncaughtExceptions != 0;

would do the trick, but for that to work, you need the necessary declarations
for the GNU version of ABI.  An alternative would be to consider
std::uncaught_exception a GNU ABI function (since these days it's part of the
source for libsupc++) and just have std::__1::uncaught_exception call
std::uncaught_exception.

A workable bandaid for the problem would be to just use

    return false;

This solution (while wrong) is far less wrong than unconditionally aborting,
since the number of programs that abort now drops to a tiny fraction of those
that would have aborted before.

An even better solution would be to have coherent instructions for building
libc++abi and libc++ together on Linux.

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