* i_fp_setfuncstate() and mi_fp_funcstate() let you store and retrieve user-defined data in the MI_FPARAM structure.
    * The first time a function is called, mi_fp_funcstate() returns NULL, so that is how you tell this is the first time the function has been called.
    * So in Good Code # 2 , line  15  retrieves the pointer using mi_fp_funcstate() and checks on line  16  to see if it is NULL. If it is NULL, this is the first time the function has been called, so line  18  allocates storage and line  24  stores the pointer in the MI_FPARAM using mi_fp_setfuncstate().
    * Line  18  allocates storage PER_COMMAND, which guarantees that the memory won't be freed until it is no longer needed by the query.

Good Code # 2 
 
 1    #include <mi.h> 
 2    #include <math.h> 
 3  
 4   mi_double_precision * 
 5   MyLog10(mi_double_precision *x, MI_FPARAM *fParam) 
 6   { 
 7     mi_double_precision *retval=NULL; 
 8  
 9     if (*x <=  0 ) 
 10    { 
 11      mi_db_error_raise(NULL, MI_EXCEPTION, "Input argument must be > 0!"); 
 12 	 return (mi_double_precision *)NULL;  /* not reached */ 
 13    } 
 14  
 15    retval=(mi_double_precision *) mi_fp_funcstate(fParam); 
 16    if(retval==(mi_double_precision *) NULL) /* First time called */
 17    { 
 18      retval=(mi_double_precision *)mi_dalloc(sizeof(mi_double_precision),PER_COMMAND); 
 19      if(retval==(mi_double_precision *) NULL) 
 20      { 
 21        mi_db_error_raise(NULL, MI_EXCEPTION, "Memory allocation failed!"); 
 22        return (mi_double_precision *)NULL;  /* not reached */
 23      } 
 24      mi_fp_setfuncstate (fParam, (void *) retval); 
 25    } 
 26  
 27    /* math library's log10() returns its result by value */ 
 28    *retval=log10(*x); 
 29    return (retval); 
 30  }