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[netCDF #XKJ-645277]: Type punning in swap4b()

Hi Larry,

Thanks for the bug report; I've squashed similar ones in the past year, and 
will take a look at this for the upcoming release!

Have a great day,

-Ward

> When I call nc_get_vara_double() to read floats from a dataset I get
> zeros rather than the values in the dataset.
> 
> I tracked it down to a typecast in the function swap4b() in
> libsrc/ncx.m4:205.  Here's the line of code:
> *(float *)op = *(float *)(&tempOut);
> This does type punning, accessing an unsigned value (tempOut) via a cast
> to a float*, which the C standard says results in undefined behavior.
> When I compile with GCC's -Wall, the compiler points out the bug in the
> source:
> 
> ncx.m4:205: warning: dereferencing type-punned pointer will break
> strict-aliasing rules
> 
> There are also four lines in ncdump/ncdump.c that cause the same warning.
> 
> I can work around the problem by building the output of libsrc/ncx.m4
> with the GCC option -fno-strict-aliasing.  Compiled with no special
> options, the function ends like this and results in zeros:
> mov    -0x4(%rsp),%eax
> or     %edx,%ecx
> mov    %ecx,-0x4(%rsp)
> mov    %eax,(%rdi)
> retq
> Complied with -fno-strict-aliasing, it ends like this and works:
> or     %edx,%ecx
> mov    %ecx,-0x4(%rsp)
> mov    -0x4(%rsp),%eax
> mov    %eax,(%rdi)
> retq
> 
> I'm building netCDF 4.4.1 with GCC 4.9.2 for Linux.
> 
> -Larry
> 
> 


Ticket Details
===================
Ticket ID: XKJ-645277
Department: Support netCDF
Priority: Normal
Status: Closed
===================
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