Next: Working with Matrices and Arrays in Mex-Files, Up: Mex-Files [Contents][Index]
The basic command to build a mex-file is either mkoctfile --mex
or
mex
. The first command can be used either from within Octave or from
the command line. To avoid issues with MATLAB’s own mex
command,
the use of the command mex
is limited to within Octave. Compiled
mex-files have the extension .mex.
status
= mex (…) ¶Compile source code written in C, C++, or Fortran, to a MEX file.
status is the return status of the mkoctfile
function.
If the compilation fails, and the output argument is not requested, an error is raised. If the programmer requests status, however, Octave will merely issue a warning and it is the programmer’s responsibility to verify the command was successful.
This is equivalent to mkoctfile --mex [-options] file
.
Consider the following short example:
#include "mex.h" void mexFunction (int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) { mexPrintf ("Hello, World!\n"); mexPrintf ("I have %d inputs and %d outputs\n", nrhs, nlhs); /* Return empty matrices for any outputs */ int i; for (i = 0; i < nlhs; i++) plhs[i] = mxCreateDoubleMatrix (0, 0, mxREAL); }
The first line #include "mex.h"
makes available all of the definitions
necessary for a mex-file. One important difference between Octave and
MATLAB is that the header file "matrix.h"
is implicitly included
through the inclusion of "mex.h"
. This is necessary to avoid a
conflict with the Octave file "Matrix.h"
for operating systems and
compilers that don’t distinguish between filenames in upper and lower case.
The entry point into the mex-file is defined by mexFunction
. The
function takes four arguments:
Note that the function name definition is not explicitly included in
mexFunction
and so there can only be a single mexFunction
entry
point per file. Instead, the name of the function as seen in Octave is
determined by the name of the mex-file itself minus the extension. If the
above function is in the file myhello.c, it can be compiled with
mkoctfile --mex myhello.c
which creates a file myhello.mex. The function can then be run from Octave as
myhello (1,2,3) ⇒ Hello, World! ⇒ I have 3 inputs and 0 outputs
It should be noted that the mex-file contains no help string. To document mex-files, there should exist an m-file in the same directory as the mex-file itself. Taking the above as an example, there would need to be a file myhello.m which might contain the text
%MYHELLO Simple test of the functionality of a mex-file.
In this case, the function that will be executed within Octave will be given by the mex-file, while the help string will come from the m-file. This can also be useful to allow a sample implementation of the mex-file within the Octave language itself for testing purposes.
Although there cannot be multiple entry points in a single mex-file, one can
use the mexFunctionName
function to determine what name the mex-file was
called with. This can be used to alter the behavior of the mex-file based on
the function name. For example, if
#include "mex.h" void mexFunction (int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) { const char *nm; nm = mexFunctionName (); mexPrintf ("You called function: %s\n", nm); if (strcmp (nm, "myfunc") == 0) mexPrintf ("This is the principal function\n", nm); return; }
is in the file myfunc.c, and is compiled with
mkoctfile --mex myfunc.c ln -s myfunc.mex myfunc2.mex
then as can be seen by
myfunc () ⇒ You called function: myfunc This is the principal function myfunc2 () ⇒ You called function: myfunc2
the behavior of the mex-file can be altered depending on the function’s name.
Although the user should only include mex.h in their code, Octave declares additional functions, typedefs, etc., available to the user to write mex-files in the headers mexproto.h and mxarray.h.
Next: Working with Matrices and Arrays in Mex-Files, Up: Mex-Files [Contents][Index]