c-walkthrough.rst

C Walkthrough

This section reprises the steps of the walkthroughs-python --- evaluating spectra and photometric colors for Sirius A --- but now using the MSG C interface.

Preparation

In your working directory, create a new file c-walkthrough.c with the following source code:

c-walkthrough.c

A few brief comments on the code:

• The #include "cmsg.h" statement includes the header definitions for the MSG C interface.
• Because C doesn't have :pydict datatypes, the photospheric parameters must be passed to MSG as a plain array (here, stored in the variable :cx_vec). A loop with :cstrcmp() calls is used to make sure the correct values are stored in each array element.
• Many of the calls to MSG routines (e.g., :cload_specgrid, :cinterp_specgrid_flux) contain NULL trailing arguments; these correspond to omitted optional arguments.

Compiling

The next step is to compile the demo program. Make sure the MSG_DIR environment variable is set, as described in the quick-start chapter. Then, enter the following on the command line:

bash

gcc -o c-walkthrough c-walkthrough.c -I$MSG_DIR/include $MSG_DIR/scripts/cmsg_link

The -I$MSG_DIR/include option tells the compiler where to find the header file, while the $MSG_DIR/scripts/cmsg_link clause (note the enclosing backticks) runs a link script that returns the compiler flags necessary to link the program against the appropriate libraries.

Running

To run the code, first create a symbolic link to the demo grid:

bash

ln -s $MSG_DIR/data/grids/sg-demo.h5

Then, execute the command

bash

./c-walkthrough

The code will create a file spectrum.dat containing the flux spectrum for Sirius A (as an ASCII table), and print out the apparent V-band magnitude and colors of the star.