G95: just say no to second underscore

[Mark Hadfield]

When code is being generated by a C compiler to be called by Fortran 
programs, as in the netCDF F77 interface, we have to worry about the 
Fortran compiler's name mangling. The G95 documentation describes its 
default behaviour thus: "G95 follows the f2c convention of adding an 
underscore to public names, or two underscores if the name contains an 
underscore." The other commonly used compiler that follows this 
convention is G77 (but its Gnu successor Gfortran just uses a single 
underscore).

The f2c one/two-underscore behaviour is rather difficult to handle, as I 
found out when trying to build the MITgcm ocean model, see here:

http://forge.csail.mit.edu/pipermail/mitgcm-support/2006-October/004374.html

So I have decided to become 100% second-underscore-free. To do this I 
had to rebuild netCDF. Here is the necessary configure statement (used 
successfully with 3.6.2-beta4 on Linux, Cygwin and MSYS+MinGW):

CC=gcc FC=g95 CPPFLAGS=-DpgiFortran FFLAGS=-fno-second-underscore 
FCFLAGS=-fno-second-underscore ./configure

If you get it wrong, "make" completes OK, but "make test" fails when 
testing the Fortran code.

Why am I sharing this with the group? Partly for posterity (AKA Google 
searches). But I have three questions/comments:

1. Perhaps this could be made the default? It would break G77 
compatibility and cause a bit of confusion in the short term, but in the 
long term I think we would all be better off.

2. I have to set both FFLAGS and FCFLAGS? Why? Surely one would do.

3. I note that the configure script reports (for example): "checking for 
C-equivalent to Fortran routine SUB... sub_". I haven't looked at the 
configure code, but I assume it is trying different C names (sub, sub_, 
SUB, etc) until it gets the right one. The subroutine/function names in 
the netCDF-3 F77 interface all have underscores. So why not do the test 
with a name that includes an underscore? Then the f2c name-mangling 
could be handled automagically?

-- 
Mark Hadfield          "Kei puwaha te tai nei, Hoea tahi tatou"
m.hadfield at niwa.co.nz
National Institute for Water and Atmospheric Research (NIWA)



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