Need help on assigning output arguments in C language

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Tero on 13 Jun 2018
Answered: Tero on 13 Jun 2018
Hi all,
I have a simple function written in C, which I've been using as a MEX. In the earlier versions of Matlab, it has been working ok, but the current release (and couple of earlier ones) crashes eventually when attempting to run the code. By compiling the C into a MEX in a current release, and then running it, gives out the error "one or more output arguments not assigned...". So, a simple problem it seems, but I'm not that familiar with C, so any help is much appreciated.
Below is a screenshot showing how I'm able to run the old MEX in a current release couple of times until Matlab eventually crashes:
The function "generateRandSumc" returns a vector of random integers, where the length of the vector is the first argument (15 in the example), and the sum of the vector is the second argument (48).
Below is a second screenshot that shows how the current release can detect the missing output arguments after the MEX compilation:
And finally, here is the C-code: (sorry for the not-so-good-structuring)
#include "mex.h"
#include "matrix.h"
/* The gateway function */
void mexFunction(int nlhs, mxArray *plhs[],
int nrhs, const mxArray *prhs[])
// random number sequence picked from 1:k, of size n
void randpermc(mwSize N, mwSize k, double *p)
// N is the max range of the results in the array
// k is the length of the array
// i.e., select k values in the range 1:N
mwIndex i;
int randNo;
int *haveRand; // pointer to haveRand
haveRand=malloc(sizeof(int) * N); // allocate memory to haveRand
for(i=0; i<=N; i++)
haveRand[i] = 0; // initialize array.
for(i=0; i<k; i++)
randNo = rand()%N +1;
} while ( haveRand[randNo] == 1 );
haveRand[randNo] = 1;
int cmp(const void *x, const void *y)
double xx = *(double*)x, yy = *(double*)y;
if (xx < yy) return -1;
if (xx > yy) return 1;
return 0;
// random number sequence picked from 1:n, of size k
void generateRandSumc(mwSize n, mwSize summ, double *p)
int i;
double pLength = sizeof(p)/sizeof(p[0]);
double *q;
q=malloc(sizeof(double) * n); // allocate memory
qsort(q, n-1 ,sizeof(double), cmp);
for(i=0; i<n; i++)
if ( i == 0 ) {
p[i]=q[i]-1; }
else if ( i == n-1 ) {
p[i]=n+summ-q[i-1]-1 ; } // add endpoint
else {
p[i]=q[i]-q[i-1]-1 ;
I hope this makes some sense
Thanks, Tero
Guillaume on 13 Jun 2018
In addition to the problems pointed out by Jan, I'm fairly certain that:
double pLength = sizeof(p)/sizeof(p[0]);
is wrong. That syntax only works when p is an array, not a pointer. There is no way to retrieve the size of the original array from within generateRandSumc. It needs to passed as an argument. Thankfully, pLength is never used so the incorrect calculation doesn't matter. As it is pLength will always be 1 on a 64-bit machine.
If mex generateRandSumc.c compiles succesfully, there's no way that it's compiling the code posted above. It must be compiling a different file in another location. One that is at least valid C.
As Steven says, why bother with the mex code if you've got working matlab code. At least, the matlab code is reliable and does not have the bias exhibited by the C code.

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Accepted Answer

James Tursa
James Tursa on 13 Jun 2018
Edited: James Tursa on 13 Jun 2018
Defining functions inside of functions is an extension to the language. In your posted code, that means the functions compile OK with MinGW (i.e., GCC) but throw errors in other compilers such as MSVC. Given this, the mexFunction in your posted code has no body (other than the function definitions), so no plhs[] variables are produced. This generates an error at the caller level when trying to assign an output since no output was produced by the mex function. See this discussion:
Your posted code is basically equivalent to this do nothing function:
#include "mex.h"
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]){}

More Answers (1)

Tero on 13 Jun 2018
Thanks all for taking the time.
It seems the code has more than enough issues to solve, which means I will follow Steven's argument and continue using the m-function. Having the function in C does not gain me benefit that would overcome the work needed here.
Thanks again, Tero


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