Malloc Memory corruption in C

Question

I have a problem using malloc.

I have a function called jacobi_gpu wich is called many times :

int main(int argc, char* argv[]){

    /* ... */

    int totalrot=0;
    while(nrot>0){
        iter++;
        nrot=jacobi_gpu(a,q, tol, dimmat);
        totalrot+=nrot;

        printf("iter =%3d  nrot=%3d\n",iter, nrot);
    }

    /* ... */
}

The parameters a,q,tol and dimmat are correctly initialized. A and Q are 2 square matrices and dimmat is their dimension.

Here is my code :

int jacobi_gpu(double A[], double Q[], double tol, long int dim){
    int nrot, p, q, k, tid;
    double c, s;
    double *mc, *vc;

    printf("jacobi begins \n");

    mc   = (double *)malloc(2 * dim * sizeof(double));
    vc   = (double *)malloc(2 * dim * sizeof(double));

    if( mc == NULL || vc == NULL){
        fprintf(stderr, "pb allocation matricre\n");
        exit(1);
    }

    nrot = 0;

    for(k = 0; k < dim - 1; k++){
        eye(mc, dim);
        eye(vc, dim);

        for(tid = 0; tid < floor(dim /2); tid++){
            p = (tid + k)%(dim - 1);
            if(tid != 0)
                q = (dim - tid + k - 1)%(dim - 1);
            else
                q = dim - 1;

            //printf("p = %d | q = %d\n", p, q);
            if(fabs(A[p + q*dim]) > tol){

                nrot++;
                symschur2(A, dim, p, q, &c, &s);

                mc[2*tid] = p;        vc[2 * tid] = c;
                mc[2*tid + 1] = q;    vc[2*tid + 1] = -s;

                mc[2*tid + 2*(dim - 2*tid) - 2] = p;
                vc[2*tid + 2*(dim - 2*tid)   - 2 ] = s;

                mc[2*tid + 2*(dim - 2*tid) - 1] = q;
                vc[2 * tid + 2*(dim - 2*tid) - 1 ] = c;     
            }
        }

        affiche(mc,dim,2,"Matrice creuse");
        affiche(vc,dim,2,"Valeur creuse");

    }
    printf("end\n");
    free(mc);
    free(vc);
    return nrot;
}

My problem is in the malloc call on the mc variable :

*** glibc detected *** ./jacobi_gpu: double free or corruption (!prev): 0x00000000022944a0 ***
    *** glibc detected *** ./jacobi_gpu: malloc(): memory corruption: 0x0000000002294580 ***

Any advice?

[EDIT]

• The function eye initializes an identity matrix
• The function affiche displays the matrix with lines and columns. The first parameter is the matrix, the second is the number of lines and the third one is the number of column.

More explanation

The purpose of the matrix mc is to store the variables p and q. Those variables contains column indices. The purpose of the matrix vc is to store the values contained in those column. For instance, if the first line of the matrix mc is 0 and 5 ( p = 0, q = 5), that means that the values in the matrix vc will be in the column 0 and 5. If the matrix fifth line in the matrix mc is 2 3 ( p = 2, q = 3), that means that the values in the fifth line in vc will be in column 2 and 3.

Hope this time, i am more clear.

Thanks for your help

Answer 1

Identity matrices are always square, but mc is not. When you call eye(mc, dim) I suspect that eye treats mc like it is a dim by dim matrix when it is in fact a 2 by dim matrix, and writes to unallocated memory.

Answer 2

You are not allocating enough memory for a square matrix in your call to malloc(). The correct size would be dim squared, not just 2*dim.

This should do the trick:

mc   = (double *)malloc(dim * dim * sizeof(double));
vc   = (double *)malloc(dim * dim * sizeof(double));