calling a c function with calllib doesn't work with pointers
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I want to call a external C library I developped from matlab (ubuntu 18.04, matlab R2020a). I make a first test where everything was ok, with a simple addition function, but when I try things a little harder with pointers, that doesn't work anymore.
The easy test going right
1) First I create files addition.h and addition.c
addition.h
#include <stdio.h>
#include <stdlib.h>
double addition( double a, double b) ;
addition.c
#include <stdio.h>
#include <stdlib.h>
#include "addition.h"
double addition( double a, double b)
{
return a + b ;
}
2) Create a .so library with gcc
gcc -c addition.c -o addition.o
gcc -o addition.so -shared addition.o
3) call it from matlab
loadlibrary("addition","addition.h")
sortie = calllib("addition","addition", 2, 3)
And sortie is set to 5, everything is OK.
Now I try to put some spices by summing two arrays, and everything went wrong. Here is the C file.
#include <stdio.h>
#include <stdlib.h>
#include "addition.h"
double* addition( double* a, double* b, int n) // n is the length of a and b
{
double* sortie = malloc( n * sizeof(double) ) ;
for (int i = 0 ; i<n ; i++ )
{
sortie[i] = a[i] + b[i] ;
}
return sortie ;
}
I try to call it from matlab using calllib by two ways, no one worked.
First try
loadlibrary("addition","addition.h")
[s1,s2,s3] = calllib("addition","addition", [4,5,6], [1,2,3], int32(3))
Second try
a = libpointer("doublePtr",[4,5,6])
b = libpointer("doublePtr",[1,2,3])
[s1,s2,s3] = calllib("addition","addition", [4,5,6], [1,2,3], int32(3))
Event the signature of the function I got from libfunctions is quite weird, I don't see why I should have three outputs.
Could it have something to see with the version of gcc? I notice, reading posts about another approch (mex file) that we can have a version problem with gcc
3 Comments
Mohammad Sami
on 5 Jun 2020
I am not sure if you can free the pointers created in C in Matlab. You would have to write another function to free the memory in C. The delete the pointer in Matlab.
Accepted Answer
Mohammad Sami
on 4 Jun 2020
It will not return you 3 different values, it will return you pointer to an array containing 3 double values.
Also you need to pass in your two double arrays as pointers.
a = libpointer('doublePtr',[4 5 6]);
b = libpointer('doublePtr',[1 2 3]);
n = int32(3);
s = calllib("addition","addition", a, b, n);
s.setdatatype('doublePtr',double(n),1);
out = s.Value;
More Answers (1)
James Tursa
on 5 Jun 2020
Edited: James Tursa
on 5 Jun 2020
A basic general outline of freeing the memory would be:
double *sortie = NULL; // top level variable
void free_sortie(void)
{
if( sortie ) {
free(sortie);
sortie = NULL;
}
double* addition( double* a, double* b, int n) // n is the length of a and b
{
if( sortie ) free_sortie();
sortie = malloc( n * sizeof(double) ) ;
for (int i = 0 ; i<n ; i++ )
{
sortie[i] = a[i] + b[i] ;
}
return sortie ;
}
You call addition( ) to do the addition, then you immediately follow that up with a call to free_sortie( ) to free the memory.
However, even this isn't really foolproof, since there is no mechanism to automatically free the memory if the dll is unloaded from memory. It relies on the user doing it manually. Also, there is no check if malloc( ) fails and returns a NULL.
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