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Inputs and Outputs
Of course, a function like hello.c with no inputs or outputs is not very useful. To understand inputs
and outputs, let’s take a closer look at the line
void mexFunction(int nlhs, mxArray
*
plhs[], int nrhs, const mxArray
*
prhs[])
Here “mxArray” is a type for representing a Matlab variable, and the arguments are:
C/MEX Meaning M-code equivalent
nlhs Number of output variables nargout
plhs Array of mxArray pointers to the output variables varargout
nrhs Number of input variables nargin
prhs Array of mxArray pointers to the input variables varargin
These MEX variables are analogous to the M-code variables nargout, varargout, nargin, and varargin.
The naming “lhs” is an abbreviation for left-hand side (output variables) and “rhs” is an abbreviation
for right-hand side (input variables).
For example, suppose the MEX-function is called as
[X,Y] = mymexfun(A,B,C)
Then nlhs = 2 and plhs[0] and plhs[1] are pointers (type mxArray*) pointing respectively to X and Y.
Similarly, the inputs are given by rlhs = 3 with prhs[0], prhs[1], and prhs[2] pointing respectively
to A, B, and C.
The output variables are initially unassigned; it is the responsibility of the MEX-function to create
them. If nlhs = 0, the MEX-function is still allowed return one output variable, in which case plhs[0]
represents the ans variable.
The following code demonstrates a MEX-function with inputs and outputs.
normalizecols.c
/
*
NORMALIZECOLS.C Normalize the columns of a matrix
Syntax: B = normalizecols(A)
or B = normalizecols(A,p)
The columns of matrix A are normalized so that norm(B(:,n),p) = 1.
*
/
#include <math.h>
#include "mex.h"
#define IS REAL 2D FULL DOUBLE(P) (!mxIsComplex(P) && \
mxGetNumberOfDimensions(P) == 2 && !mxIsSparse(P) && mxIsDouble(P))
#define IS REAL SCALAR(P) (IS REAL 2D FULL DOUBLE(P) && mxGetNumberOfElements(P) == 1)
void mexFunction(int nlhs, mxArray
*
plhs[], int nrhs, const mxArray
*
prhs[])
{
/
*
Macros for the ouput and input arguments
*
/
#define B OUT plhs[0]
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