engOutputBuffer (C)

Specify buffer for MATLAB output

C Syntax

#include "engine.h"
int engOutputBuffer(Engine *ep, char *p, int n);

Description

engOutputBuffer defines a character buffer for engEvalString to return any output that ordinarily appears on the screen. The function returns 1 if you pass it a NULL engine pointer. Otherwise, returns 0.

The default behavior of engEvalString is to discard any standard output caused by the command it is executing. A call to engOutputBuffer with a buffer of nonzero length tells any subsequent calls to engEvalString to save output in the character buffer pointed to by p.

To turn off output buffering in C, use:

engOutputBuffer(ep, NULL, 0);

Note

The buffer returned by engEvalString is not NULL terminated.

Note

matlab::engine::StreamBuffer in the MATLAB^® Engine API for C++ is recommended over engOutputBuffer. The MATLAB Engine API for C++ includes modern C++ features for writing engine applications. For more information, see Call MATLAB from C++. There are no plans to remove engOutputBuffer or the Engine API for C.

Input Arguments

ep — Pointer to engine

Engine *

Pointer to engine, specified as Engine *.

p — Pointer to character buffer

char *

Pointer to character buffer, specified as char *.

n — Length of buffer

int

Length of buffer, specified as int.

Examples

Display MATLAB Engine Function Output from C on Windows

This C code shows how to capture and display MATLAB output in a MATLAB C engine function on a Windows^® system.

 engwindemo.c

/*
 *	engwindemo.c
 *
 *	This program illustrates how to call MATLAB
 *	Engine functions from a C program on Windows.
 *
 * Copyright 1984-2017 The MathWorks, Inc.
 */
#include <windows.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "engine.h"
#include <matrix.h>
#define BUFSIZE 256

static double Areal[6] = { 1, 2, 3, 4, 5, 6 };

int PASCAL WinMain (HINSTANCE hInstance,
                    HINSTANCE hPrevInstance,
                    LPSTR     lpszCmdLine,
                    int       nCmdShow)

{
	Engine *ep;
	mxArray *T = NULL, *a = NULL, *d = NULL;
	char buffer[BUFSIZE+1];
	double *Dreal, *Dimag;
	double time[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

	/*
	 * Start MATLAB engine
	 */
	if (!(ep = engOpen(NULL))) {
		MessageBox ((HWND)NULL, (LPSTR)"Can't start MATLAB engine",
			(LPSTR) "Engwindemo.c", MB_OK);
		exit(-1);
	}

	/*
	 * PART I
	 *
	 * For the first half of this demonstration, we will send data
	 * to MATLAB, analyze the data, and plot the result.
	 */

	/*
	 * Create a variable from our data
	 */
	T = mxCreateDoubleMatrix(1, 10, mxREAL);
	/*
	 * Copy data from time to matrix T
	 */
	#if MX_HAS_INTERLEAVED_COMPLEX
		memcpy(mxGetDoubles(T), time, 10*sizeof(double));
	#else
		memcpy(mxGetPr(T), time, 10*sizeof(double));
	#endif
	/*
	 * Place the variable T into the MATLAB workspace
	 */
	engPutVariable(ep, "T", T);

	/*
	 * Evaluate a function of time, distance = (1/2)g.*t.^2
	 * (g is the acceleration due to gravity)
	 */
	engEvalString(ep, "D = .5.*(-9.8).*T.^2;");

	/*
	 * Plot the result
	 */
	engEvalString(ep, "plot(T,D);");
	engEvalString(ep, "title('Position vs. Time for a falling object');");
	engEvalString(ep, "xlabel('Time (seconds)');");
	engEvalString(ep, "ylabel('Position (meters)');");

	/*
	 * PART II
	 *
	 * Create another mxArray, put it into MATLAB,
	 * and calculate its eigen values.
	 * 
	 */

	a = mxCreateDoubleMatrix(3, 2, mxREAL);
	/*
	 * Copy data from Areal to matrix a
	 */
	#if MX_HAS_INTERLEAVED_COMPLEX
		memcpy(mxGetDoubles(a), Areal, 6*sizeof(double));
	#else
		memcpy(mxGetPr(a), Areal, 6*sizeof(double));
	#endif
	engPutVariable(ep, "A", a);

	/*
	 * Calculate the eigen value
	 */
	engEvalString(ep, "d = eig(A*A')");

	/*
	 * Use engOutputBuffer to capture MATLAB output. Ensure first that
	 * the buffer is always NULL terminated.
	 */
	buffer[BUFSIZE] = '\0';
	engOutputBuffer(ep, buffer, BUFSIZE);

	/*
	 * the evaluate string returns the result into the
	 * output buffer.
	 */
	engEvalString(ep, "whos");
	MessageBox ((HWND)NULL, (LPSTR)buffer, (LPSTR) "MATLAB - whos", MB_OK);

	/*
	 * Get the eigen value mxArray
	 */
	d = engGetVariable(ep, "d");
	engClose(ep);

	if (d == NULL) {
		MessageBox ((HWND)NULL, (LPSTR)"Get Array Failed", (LPSTR)"Engwindemo.c", MB_OK);
	}
	else {
		/*
		 * Using interleaved complex representation we can access complex number
		 * with a single pointer instead of using two separate pointers.
		 */
		#if MX_HAS_INTERLEAVED_COMPLEX
			if (mxIsComplex(d)) {
				sprintf(buffer,"Eigenval 2: %g+%gi",mxGetComplexDoubles(d)[1].real,mxGetComplexDoubles(d)[1].imag);
			}
			else {
				sprintf(buffer,"Eigenval 2: %g",mxGetDoubles(d)[1]);
			}
		#else
			Dreal = mxGetPr(d);
			Dimag = mxGetPi(d);
			if (Dimag) {
				sprintf(buffer,"Eigenval 2: %g+%gi",Dreal[1],Dimag[1]);
			}
			else {
				sprintf(buffer,"Eigenval 2: %g",Dreal[1]);
			}
		#endif
		MessageBox ((HWND)NULL, (LPSTR)buffer, (LPSTR)"Engwindemo.c", MB_OK);
		mxDestroyArray(d);
	} 

	/*
	 * We're done! Free memory, close MATLAB engine and exit.
	 */
	mxDestroyArray(T);
	mxDestroyArray(a);

	return(0);
}

/* LocalWords:  eigen eng Eigenval gi
 */

Display MATLAB Engine Function Output from C on Linux or macOS

This C code shows how to capture and display MATLAB output in a MATLAB C engine function on a Linux^® or macOS system.

 engdemo.c

/*
 *	engdemo.c
 *
 *	A simple program to illustrate how to call MATLAB
 *	Engine functions from a C program.
 *
 * Copyright 1984-2016 The MathWorks, Inc.
 * All rights reserved
 */
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "engine.h"
#define  BUFSIZE 256

int main()

{
	Engine *ep;
	mxArray *T = NULL, *result = NULL;
	char buffer[BUFSIZE+1];
	double time[10] = { 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 };

	/*
	 * Call engOpen with a NULL string. This starts a MATLAB process 
     * on the current host using the command "matlab".
	 */
	if (!(ep = engOpen(""))) {
		fprintf(stderr, "\nCan't start MATLAB engine\n");
		return EXIT_FAILURE;
	}

	/*
	 * PART I
	 *
	 * For the first half of this demonstration, send data
	 * to MATLAB, analyze the data, and plot the result.
	 */

	/* 
	 * Create a variable for the data
	 */
	T = mxCreateDoubleMatrix(1, 10, mxREAL);
	memcpy((void *)mxGetPr(T), (void *)time, sizeof(time));
	/*
	 * Place the variable T into the MATLAB workspace
	 */
	engPutVariable(ep, "T", T);

	/*
	 * Evaluate a function of time, distance = (1/2)g.*t.^2
	 * (g is the acceleration due to gravity)
	 */
	engEvalString(ep, "D = .5.*(-9.8).*T.^2;");

	/*
	 * Plot the result
	 */
	engEvalString(ep, "plot(T,D);");
	engEvalString(ep, "title('Position vs. Time for a falling object');");
	engEvalString(ep, "xlabel('Time (seconds)');");
	engEvalString(ep, "ylabel('Position (meters)');");

	/*
	 * use fgetc() to pause long enough to be
	 * able to see the plot
	 */
	printf("Hit return to continue\n\n");
	fgetc(stdin);
	/*
	 * We're done for Part I! Free memory, close MATLAB figure.
	 */
	printf("Done for Part I.\n");
	mxDestroyArray(T);
	engEvalString(ep, "close;");


	/*
	 * PART II
	 *
	 * For the second half of this demonstration, we will request
	 * a MATLAB string, which should define a variable X.  MATLAB
	 * will evaluate the string and create the variable.  We
	 * will then recover the variable, and determine its type.
	 */
	  
	/*
	 * Use engOutputBuffer to capture MATLAB output, so we can
	 * echo it back.  Ensure first that the buffer is always NULL
	 * terminated.
	 */

	buffer[BUFSIZE] = '\0';
	engOutputBuffer(ep, buffer, BUFSIZE);
	while (result == NULL) {
	    char str[BUFSIZE+1];
	    char *input = NULL;
	    /*
	     * Get a string input from the user
	     */
	    printf("Enter a MATLAB command to evaluate.  This command should\n");
	    printf("create a variable X.  This program will then determine\n");
	    printf("what kind of variable you created.\n");
	    printf("For example: X = 1:5\n");
	    printf(">> ");

	    input = fgets(str, BUFSIZE, stdin);
	  
	    /*
	     * Evaluate input with engEvalString
	     */
	    engEvalString(ep, str);
	    
	    /*
	     * Echo the output from the command.  
	     */
	    printf("%s", buffer);
	    
	    /*
	     * Get result of computation
	     */
	    printf("\nRetrieving X...\n");
	    if ((result = engGetVariable(ep,"X")) == NULL)
	      printf("Oops! You didn't create a variable X.\n\n");
	    else {
		printf("X is class %s\t\n", mxGetClassName(result));
	    }
	}

	/*
	 * We're done! Free memory, close MATLAB engine and exit.
	 */
	printf("Done!\n");
	mxDestroyArray(result);
	engClose(ep);
	
	return EXIT_SUCCESS;
}

Version History

Introduced before R2006a