C Walkthrough

This section reprises the steps of the Python Walkthrough — evaluating an irradiance spectrum 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:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>

#include "cmsg.h"

#define LABEL_LEN 16
#define N_X_VEC 2
#define N_LAM 501
#define LAM_MIN 3800.
#define LAM_MAX 7000.

int main(int argc, char *argv[]) {

  SpecGrid specgrid;
  PhotGrid photgrid_U;
  PhotGrid photgrid_B;
  PhotGrid photgrid_V;
  
  char label[LABEL_LEN+1];

  double x_vec[N_X_VEC];

  double z;
  double R2_d2;

  double lam[N_LAM];
  double lam_c[N_LAM-1];
  double F[N_LAM-1];
  double F_obs[N_LAM-1];
  double F_U;
  double F_B;
  double F_V;
  double F_U_obs;
  double F_B_obs;
  double F_V_obs;
  double U;
  double B;
  double V;

  FILE *fptr;

  // Create the specgrid object

  load_specgrid("sg-demo.h5", &specgrid, NULL);

  // Set photospheric parameters to correspond to Sirius A

  for(int i=0; i < N_X_VEC; i++) {

    get_specgrid_axis_label(specgrid, i, label);

    if (strcmp(label, "log(g)") == 0) {
      x_vec[i] = 4.2774;
    }
    else if (strcmp(label, "Teff") == 0) {
      x_vec[i] = 9909.2;
    }
    else {
      printf("invalid label\n");
      exit(1);
    }
      
  }

  // Set the redshift

  z = -1.830E-5;

  // Set the dilution factor R2_d2 = R**2/d**2, where R is Sirius A's
  // radius and d its distance

  R2_d2 = 2.1351E-16;

  // Set up the wavelength abscissa

  for(int i=0; i < N_LAM; i++) {
    lam[i] = (LAM_MIN*(N_LAM-1-i) + LAM_MAX*i)/(N_LAM-1);
  }

  for(int i=0; i < N_LAM-1; i++) {
    lam_c[i] = 0.5*(lam[i] + lam[i+1]);
  }

  // Evaluate the irradiance

  interp_specgrid_flux(specgrid, N_LAM, N_X_VEC, x_vec, z, lam, F, NULL, NULL, NULL);

  for(int i=0; i < N_LAM-1; i++) {
    F_obs[i] = R2_d2*F[i];
  }

  // Write it to a file

  fptr = fopen("spectrum.dat", "w");

  for(int i=0; i < N_LAM-1; i++) {
    fprintf(fptr, "%.17e %.17e\n", lam_c[i], F_obs[i]);
  }

  fclose(fptr);

  // Create the photgrid objects

  load_photgrid_from_specgrid("sg-demo.h5", "pb-Generic-Johnson.U-Vega.h5", &photgrid_U, NULL);
  load_photgrid_from_specgrid("sg-demo.h5", "pb-Generic-Johnson.B-Vega.h5", &photgrid_B, NULL);
  load_photgrid_from_specgrid("sg-demo.h5", "pb-Generic-Johnson.V-Vega.h5", &photgrid_V, NULL);

  // Evaluate photometric irradiances

  interp_photgrid_flux(photgrid_U, N_X_VEC, x_vec, &F_U, NULL, NULL, NULL);
  interp_photgrid_flux(photgrid_B, N_X_VEC, x_vec, &F_B, NULL, NULL, NULL);
  interp_photgrid_flux(photgrid_V, N_X_VEC, x_vec, &F_V, NULL, NULL, NULL);

  F_U_obs = R2_d2*F_U;
  F_B_obs = R2_d2*F_B;
  F_V_obs = R2_d2*F_V;

  // Evaluate apparent magnitudes

  U = -2.5*log10(F_U_obs);
  B = -2.5*log10(F_B_obs);
  V = -2.5*log10(F_V_obs);

  printf("  V=  %24.17E\n", V);
  printf("U-B=  %24.17E\n", U-B);
  printf("B-V=  %24.17E\n", B-V);

  // Clean up

  unload_specgrid(specgrid);

  unload_photgrid(photgrid_U);
  unload_photgrid(photgrid_B);
  unload_photgrid(photgrid_V);

  // Finish

  exit(0);

}

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 dict datatypes, the photospheric parameters must be passed to MSG as a plain array (here, stored in the variable x_vec). A loop with strcmp() calls is used to make sure the correct values are stored in each array element.
Many of the calls to MSG routines (e.g., load_specgrid(), interp_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, run gcc to compile the program:

$ gcc -o c-walkthrough c-walkthrough.c `pkgconf --with-path=$MSG_DIR/lib/pkgconfig --cflags --libs cmsg`

The `pkgconf ...` clause (note the enclosing backticks) uses pkgconf to generate the compiler flags necessary to link the program against the appropriate libraries.

Running

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

$ ln -s $MSG_DIR/data/grids/sg-demo.h5
$ ln -s $MSG_DIR/data/passbands/pb-Generic-Johnson.U-Vega.h5
$ ln -s $MSG_DIR/data/passbands/pb-Generic-Johnson.B-Vega.h5
$ ln -s $MSG_DIR/data/passbands/pb-Generic-Johnson.V-Vega.h5

Then, execute the command

$ ./c-walkthrough

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