# Calling NAG Library Routines from Octave - Example 1

## 3.1. Example 1

### A simple example - the ln Γ(x) function routine nag_log_gamma (s14abc)

Here we show how to call a NAG C Library function with only one return value: the ln Γ'(x) logarithm gamma function routine nag_log_gamma (s14abc)

1. ### Function prototype from the NAG C Library Manual

According to the C Library Manual, the prototype for function s14abc looks like this:

  #include <nag.h>
#include <nags.h>

double s14abc(double x, NagError *fail);


The function takes two arguments. The first one is of type double and is the argument x of the log gamma function ln Γ (x). The second argument to s14abc is the error handling argument fail which is of type NagError. See the NAG C Library Manual for more information about the NagError type. To keep things simple we are not going to pass the contents of the NagError structure back to Octave. Instead, we will return only integer value of fail.code.

2. ### C++ function

Here is the source code of our C++ function nag_lgamma.cc:

#include <octave/oct.h>
#define Complex NagComplex
#define MatrixType NagMatrixType
#include <nag.h>
#include <nags.h>

DEFUN_DLD (nag_lgamma, args, ,
"Calls nag_log_gamma (s14abc), which returns\n\
the value of the logarithm of the Gamma function, ln Gamma(x)\n\
via the function name.\n")
{
octave_value_list retval;
// Retrieve input arguments from args
double x = args(0).double_value();
// Declare local variables
double y;
NagError fail;
INIT_FAIL(fail);

// Call NAG routine like in C
y = nag_log_gamma(x,&fail);

// Assign output arguments to retval
retval(0) = y;
retval(1) = fail.code;

return retval;
}


• For clarity, the input arguments type checking and error message printing have been omitted in the article text. However, they have been added to the file you can download from this site.
• Complex and MatrixType types are defined in Octave and NAG header files. We are not using these types in this example, so we can simply rename one of their definitions.
• The third DEFUN_DLD argument (nargout) is not used, so it is omitted in order to avoid the warning from gcc about an unused function parameter.
3. ### Compiling into Oct-File

To compile the C++ function into oct-files, we use the mkoctfile script supplied with Octave:

  % mkoctfile nag_lgamma.cc -L/opt/NAG/cll6a08dg/lib -lnagc_nag -I/opt/NAG/cll6a08dg/include


where:

• the -L switch tells the C compiler where to look for the NAG C Library installed on your system;
• -lnagc_nag is the name of the NAG C Library;
• the -I switch should be followed by the location of NAG C Library header files.
4. ### Calling the function

Assuming that all has gone well, we can call the function as if it was part of Octave itself, i.e. either from the Octave command line or from within an Octave program. An example call may be:

octave:1> [y,ifail]=nag_lgamma(1.25)
y = -0.098272
ifail = 0


Tip: If you get an error message in Octave saying that libnagc_nag.so cannot be found, you may need to set an environment variable to tell the system where to look. The environment variable name is operating-system dependent. On Linux machines it is named LD_LIBRARY_PATH, and is a colon-separated list of directories to be searched for libnagc_nag.so. For example, if you use the C shell, the command

  % setenv LD_LIBRARY_PATH /opt/NAG/cll6a08dg/lib:\${LD_LIBRARY_PATH}


will ensure that directory /opt/NAG/cll6a08dg/lib gets searched.