[netCDF #NQH-437723]: nc_test failure on aix7 power7 cluster

Hello Wolfgang,

Thank you very much for the bug report, and thank you even more for the 
diagnosis/fix.  We do not currently have access to an AIX environment or IBM's 
XL C compiler, so debugging these issues would have been very difficult.  I 
will incorporate an explicit NULL check after malloc is called, as you suggest, 
and will look for similar issues.  I'll also make a note of the 
-D_LINUX_SOURCE_COMPAT flag for the XL C compiler.

Thanks again, have a great day,

-Ward


> Dear NetCDF team,
> 
> I recently needed to build NetCDF 4.3.3.1 on a POWER6 machine with AIX 6.1 
> and IBM’s XL C compiler V11.1. I encountered two issues and found a simple 
> solution for both of these. As POWER/AIX is among the more exotic platforms, 
> I thought I’d send you this to share with other users, if you find it useful.
> 
> The issues were:
> 
> 1.      Compilation failure due to diverging function return type definitions 
> between prototypes and body (type “OCerror” vs type “int” in oc.h, ochttp.c, 
> and ocrc.c in the “oc2” directory)
> 
> 2.      Test failures in the “nc_test” suite (in the “nc_test” directory) due 
> to reported malloc failures
> It seems that both issues were found previously by other users (e.g. “[netCDF 
> #NQH-437723]: nc_test failure on aix7 power7 cluster for the second issue”).
> 
> The problems stem from differences in the behaviour of the XL C compiler 
> compared to, e.g., the GCC compiler. They can be easily handled without 
> applying any code changes by using a XL C pre-processor macro that asks for 
> “Linux-compatible” compiler behaviour:
> CPPFLAGS = -D_LINUX_SOURCE_COMPAT
> The NetCDF package can now be built and tested without problems.
> 
> Malloc failures:
> I investigated the malloc failures in the “nc_test” suite a bit more, they 
> stem from malloc(0) requests. The code generated by the XL C compiler returns 
> a NULL pointer for malloc(0), where GCC allocates memory.
> 
> This situation appears under the following conditions:
> 
> -        Data is written to a variable in a NetCDF file with NetCDF4 format 
> using one of the “nc_put_var_xxx” functions
> 
> -        The variable has at least one unlimited dimension with current 
> length 0
> 
> -        The program attempts to write numerical data with a different type 
> than the variable definition, triggering automatic conversion
> 
> This results in a “malloc(0)” request for a memory buffer in line 660 of 
> source file libsrc4/nc4hdf.c. Unlike GCC, the XL C malloc returns a NULL 
> pointer, causing NetCDF to report a malloc failure. It seems that the C 
> standard allows both NULL pointers and memory allocation for a malloc(0) 
> request. It could therefore be useful to handle this ambiguity explicitly in 
> NetCDF, to avoid the error message.
> 
> I hope that you find this useful. Please let me know if you have any 
> questions.
> 
> Best regards,
> 
> Wolfgang
> 
> Dr Wolfgang Hayek
> HPC Scientific Programmer
> 
> +64-4-386-0810 | +64-27-801-4625 | 301 Evans Bay Parade, Greta Point, 
> Wellington | www.niwa.co.nz<http://www.niwa.co.nz>
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> 


Ticket Details
===================
Ticket ID: NQH-437723
Department: Support netCDF
Priority: Normal
Status: Closed