why is this function to write a matrix on a file not working? It's in C

Hallo everyone! this is my first post so if I make any mistakes plase let me know..
I am writing this program in which I generate a hilbert matrix and create it's inverse. then I manipulate these 2 matrices and obtain other ones, that I shall call M,M1, M2, etc.. at a certain point I want to save one of these matrices, say M, on a file and to do so, I have to write and independet function that is called from the main.. this is what I wrote but it's not working!! can anyone help?!

/* function to save matrix on file */

Code:

void savematrix(double n, double matrix[][MaxDim])
{
     
     int i,j;
     FIlE *f1;
     f1=fopen("matrix.txt","w");
     if (f1=NULL)
     {
            printf("file could not be opened");
                 exit(-1);
                 }
     
     for (i=0;i<n;i++)
     {
         for (j=0;j<n;j++)
         {
             fprintf(f1,"%5.2f ", matrix[i][j]);
             }
             fprintf(f1,"\n");
}
fclose(f1);
}

ok.. I found the error:
it shold have been:

if (f1==NULL)