Dealing with warnings in c compilations - c

Question

I have the following warnings during the compilation:
solver.c:24: warning: passing argument 2 of ‘mtrx_multiple’ from incompatible pointer type
mat.h:5: note: expected ‘double *’ but argument is of type ‘double **’
solver.c:30: warning: assignment makes pointer from integer without a cast
solver.c:39: warning: assignment makes pointer from integer without a cast
/tmp/ccmU9zRf.o: In function `vec_norm':
math.c:(.text+0x331): undefined reference to `sqrt'
collect2: ld returned 1 exit status
the lines are:
solver.c
double *cg_solve( sparse_mat_t A, double *b, double *x ) {
double *a;
double **r;
double *be;
double **p;
double **x0;
x0[0] = vec_copy(x, size);
...
line 24: r[0] = vec_subtraction( b, mtrx_multiple(A, x0), size );
line 30: x0[k+1] = vec_addition( x0[k], vec_numb_multiple(a[k], p[k], size), size );
line 39: p[k+1] = vec_addition( r[k+1], vec_numb_multiple(be[k], p[k], size), size );
}
math.h
line 5: double *mtrx_multiple (sparse_mat_t A, double *c);
The function that are used there: (math.c)
double *vec_subtraction (double *a, double *b, int n) {
double *result = malloc(sizeof(double)*n);
int i;
for(i=0; i<n; i++)
result[i] = a[i]-b[i];
return result;
}
double *vec_addition (double *a, double *b, int n) {
double *result = malloc(sizeof(double)*n);
int i;
for(i=0; i<n; i++)
result[i] = a[i]+b[i];
return result;
}
double *vec_numb_multiple (double a, double *b, int n) {
double *result = malloc(sizeof(double)*n);
int i;
for(i=0; i<n; i++)
result[i] = a*b[i];
return result;
}
double *mtrx_multiple (sparse_mat_t A, double *c) {
double *result;
int i, j;
result = malloc((A.size) * sizeof *result);
printf("c.n: %d \n", A.size);
for (i = 0; i < A.size; i++) {
int v = 0;
for (j = A.ia[i]; j < A.ia[i + 1]; j++) {
v += A.a[j] * c[A.ja[j]];
}
result[i] = v;
}
return result;
}
double vec_norm (double *a, int n){
double result;
int i;
for(i=0; i<n; i++)
result = result + ( a[i] * a[i] );
result = sqrt(result);
return result;
}
double *vec_copy (double *a, int n) {
double *result;
int i;
for(i=0; i<n; i++)
result[i] = a[i];
return result;
}
I will be grateful for any help.
EDIT
I found the solution to the x0 problem, thanks Ben. Now what left is:
solver.c:30: warning: assignment makes pointer from integer without a cast
solver.c:39: warning: assignment makes pointer from integer without a cast
/tmp/ccL4uSoH.o: In function 'vec_norm':
math.c:(.text+0x331): undefined reference to 'sqrt'
collect2: ld returned 1 exit status

Answer 1

Based on what you've posted, I'm going to guess that you don't have a declaration for vec_numb_multiple in scope before you call it, and the compiler is implicitly typing it to return int; that would lead to the warnings on lines 30 and 39.
The undefined reference to sqrt() means you aren't linking in the standard math library; I'm assuming you're using gcc, so you would need to add -lm to the command line.
It's a really bad idea to use a standard library file name for your own code (math.h, math.c).

Answer 2

replace (line 24)
r[0] = vec_subtraction( b, mtrx_multiple(A, x0), size );
with
r[0] = vec_subtraction( b, mtrx_multiple(A, x0[0]), size );
You said you whant to multiply a matrix (A I guess) with a vector, so the second argument must be a vector. x0 is a pointer to pointers which can be see as a 2D array of doubles, it means a single cell of x0 is an array of doubles (ie. what you could call a vector). This is why you want to pass x0[0], not just x0 which is : many arrays.
see John's aswer for the rest.