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20040511: linking a C

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  • Subject: 20040511: linking a C
  • Date: Wed, 12 May 2004 10:54:24 -0600
>From: Glen Kaukola <address@hidden>
>Organization: UCR
>Keywords: 200405111925.i4BJPZtK000957 netCDF link

Glen,

>I'm having trouble getting a simple C program to link.  Right now, I'm 
>only calling two NetCDF functions, nc_open and nc_close, but the linker 
>always tells gives me an undefined reference error for both of those 
>functions.
>
>The link command I'm running looks exactly like this:
>gcc -L/home/aqm/glen/Projects/netcdf-3.5.1/src/libsrc -lnetcdf 
>test_ncf.o -o test_ncf.x

Assuming that you have a valid netCDF library in the directory:

/home/aqm/glen/Projects/netcdf-3.5.1/src/libsrc

your gcc command line should be OK.  Given this, the first things
I would check are:

- you have a file libnetcdf.a in the 
/home/aqm/glen/Projects/netcdf-3.5.1/src/libsrc
  directory

- the library is non-zero in length

- the library was built on the system you are trying to use it on

After doing the above, you can use the 'nm' command to list out the
entry points in libnetcdf.a to make sure that it has the nc_open
and nc_close routines you need:

nm libnetcdf.a | grep nc_open

>Also, we've just upgraded all our systems to Fedora Core 1, so maybe 
>that has something to do with it?

If you did not rebuild your netCDF library after the upgrade, it may
be part of the problem.  There is no problem using netCDF 3.5.1 under
Fedora Core 1; we use it routinely here at the UPC.

>Any insight into this problem would be greatly appriciated.

If you did not rebuild libnetcdf.a, please do so and retry your link.

>Thanks,

No worries.

Cheers,

Tom Yoksas
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>From address@hidden  Wed May 12 16:15:07 2004

Actually, I figured out what the problem is.

If my link step is set up like this then it doesn't work:

gcc -L/home/aqm/glen/Projects/netcdf-3.5.1/src/libsrc -lnetcdf test_ncf.o -o 
test_ncf.x

But if I change it around so it looks like this, then it works:
gcc test_ncf.o -o test_ncf.x -L/home/aqm/glen/Projects/netcdf-3.5.1/src/libsrc 
-lnetcdf

It seems my thinking about how linkers work was all backwards.  It needs to 
find symbols in the object file first so it doesn't first scan through a 
library and determine that there's nothing in the library it needs.  At least 
that's how I understand it.


Glen