malloc causing a SIGSEGV:Segmentation fault

Question

typedef struct Matrix
{
    double * matrix;
    int sizex;
    int sizey;
}Matrix;

int nn = 257;
Matrix * g = (Matrix *)malloc(sizeof(Matrix *));
g->matrix = malloc(sizeof(double) * nn * nn);
g->sizex = nn;
g->sizey = nn;

This code give an error when it gets to g->matrix = malloc(sizeof(double) * nn * nn); anyone see a problem with it ?

edit: found problem to be accessing unallocated memory in a place before the allocation shown, it was causing a SIGSEGV:Segmentation fault.

Answer 1

You need to pass malloc the sizeof the Matrix not sizeof pointer to the Matrix.

Change

Matrix * g = (Matrix *)malloc(sizeof(Matrix *));
                                           ^^ 

to

Matrix * g = (Matrix *)malloc(sizeof(Matrix));

Also you must always check the return value of malloc and make sure allocation succeed before you go and use the allocated memory.