sprintf during ISR causes crash? - crash

Question

I am trying to format a string and then print it out to an LCD during an ISR. The ISR functions correctly with sprintf() commented out; but when it is introduced the program crashes during the ISR.
void __ISR(_CHANGE_NOTICE_VECTOR, IPL1) CNIntHandler(void) {
//more of my code later
LATBINV = LEDC; //invert LEDC to a 1
hw_msDelay(20);
int buttons = read_buttons();
decode_buttons(buttons, &g_step_mode, &g_step_dir, &g_motor_delay);
LATBINV = LEDC; //reinvert LEDC to 0
mCNClearIntFlag(); //clear the CN flag
}
The sprintf() occurs within the decode_buttons() fucntion:
void decode_buttons(int f_buttons, int *f_step_mode, int *f_step_dir, int *f_motor_delay) {
char f_mode_str;
char f_dir_str;
int f_RPM;
char f_LCD_str;
switch(f_buttons) {
case 0x0000: //none pressed
*f_step_mode = HS;
f_mode_str = "HALF";
*f_step_dir = CW;
f_dir_str = "CW";
*f_motor_delay = 20; // semipermanent value
f_RPM = 15;
break;
case 0x0040: //BTN 1 pressed
*f_step_mode = FS;
f_mode_str = "FULL";
*f_step_dir = CW;
f_dir_str = "CW";
*f_motor_delay = 40;
f_RPM = 15;
break;
case 0x0080: // BTN 2 pressed
*f_step_mode = HS;
f_mode_str = "HALF";
*f_step_dir = CCW;
f_dir_str = "CCW";
*f_motor_delay = 30;
f_RPM = 10;
break;
case 0x00C0: //Both pressed
*f_step_mode = FS;
f_mode_str = "FULL";
*f_step_dir = CCW;
f_dir_str = "CCW";
*f_motor_delay = 24;
f_RPM = 25;
break;
default:
break;
}
sprintf(f_LCD_str, "%s %s %d", f_dir_str, f_mode_str, &f_RPM);
LCD_puts(f_LCD_str); //output string to LCD
return;
}
I'm working on a PIC32 Cerebot board, and the goal is to update an LCD with the current mode, direction, and RPM of a stepper motor. I'll clarify what I can and post anything needed.

Answer 1

sprintf composes a string and stores it in an array that must be big enough: http://www.cplusplus.com/reference/cstdio/sprintf/
In your code, the output buffer you pass to sprintf is declared as
char f_LCD_str;
which is not an array but a single character. sprintf will inevitable overflow.
A good declaration would be
char f_LCD_str[30];
(same remark for your other char arrays: f_mode_str and f_dir_str)
It is always safer to use snprintf in order to avoid overflow risks. In which case the call would be:
sprintf(f_LCD_str, 30, "%s %s %d", f_dir_str, f_mode_str, f_RPM);
note: &f_RPM will print the address of the variable, where f_RPM will print out the variable value, depending on what you want.