Joystick Code error of undeclared variables

Question

I am attempting to write a code to read a joystick axis values, which i eventually hope to be able to use the values for controlling motors. After copious amounts of trying to figure out C and how on earth to use the joystick api I wrote this code. to start with it came only with one variable undeclared error then as I improved code, making it more readable and easier to understand I got another one of the same when I went to compile it hoping the first would have gone away! This is my code (excuse the commenting):

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <linux/joystick.h> /* lots of included headers because */ 
                            /*  I wasn't sure which I needed!   */

#define JS_EVENT_AXIS           0x02    /* joystick moved */

int open_joystick() {

    int fd;                             /* fd declared I thought */
        fd = open ("/dev/js0", O_RDONLY | O_NONBLOCK);

    return fd;                          /* code likes something to return */
}

int read_joystick_thrust_axis(struct js_event js) /* declare r_j_t_a variable and the js instance */


    while (1) {                                          /* loop forever */
        while (read (fd, &js, sizeof(js)) > 0) {         /* while there is an event */
          if (js_event.type == JS_EVENT_AXIS) {          /* and if that is an axis event */
            if (js_event.number == 1) {                  /* and if that event is on the right axis */  
               printf ("Joystick at %8hb\n", js.value);  /* print that instance of the joysticks value */
                                        }
                                    }
                                }
                            }
    return 0; }            /* keeping C happy by returning something */

And the errors i get back from gcc are:

pi@raspberrypi ~/rc $ gcc joystick.c
joystick.c: In function ‘read_joystick_thrust_axis’:
joystick.c:24:16: error: ‘fd’ undeclared (first use in this function)
joystick.c:24:16: note: each undeclared identifier is reported only once for each function it appears in
joystick.c:25:8: error: ‘js_event’ undeclared (first use in this function)

Could someone please explain why I am getting these errors and suggest a fix? Thank you in advance.

Answer 1

open_joystick sets fd, but fd is local to open_joystick, and therefore not readable by read_joystick_thrust_axis. Convert read_joystick_thrust_axis to allow fd to be passed as an argument, and pass the return value of open_joystick, like this:

Change:

int read_joystick_thrust_axis(struct js_event js)

to

int read_joystick_thrust_axis(int fd, struct js_event js)

Then when you call it (from main or whatever), do

int fd;
fd = open_joystick();
...
int read_joystick_thrust_access (fd, whatever);

Re the js_event error, the variable is named js, and is of type struct js_event. Therefore you want to refer to js.type not js_event.type.