int counter = 0, i, j, ap;
FILE *fp;

//count how many numbers there are in the file
if ( ( fp = fopen(filename,"r") )!=NULL )
{
    while ( fscanf(fp,"%d", &i)!= EOF ) counter++;
    fclose(fp);
}

//allocate the matrix; the value of counter is 9
*mat = malloc( sizeof(int*) * sqrt(counter) )
for (i = 0; i < sqrt(counter); i++)
{ (*mat)[i] = (int*) malloc( sizeof(int) * sqrt(counter) );}

//reopen the file and save the values in the allocated matrix

fp = fopen("matrice.txt","r");
for (i = 0; i < sqrt(counter); i++)
{
    for (j = 0; j < sqrt(counter); j++) fscanf(fp,"%d", (mat[i])[j]);
}
fclose(fp);

return;