Using a variable defined in another function

Question

I'm trying to control a joystick, written some code using the API to do so. I've read the number of axes on the joystick in function a using ioctl(fd, JSIOCGAXES, &axes); and want to then print the axis that was moved in the event-handling function (function b):

char whichaxis[axes] = {'X','Y','Y','X'};
printf("%c%c |%8hd\n",whichjs,whichaxis[jse.number],jse.value);

This code should print something like LX| -32768, saying that the left joystick has moved in the x direction.

However, this returns an error, as I am calling axes in function b but it is not defined in function b. So my question is, how can I use axes despite it not being defined in function b?

Here is the code

// Returns info about the joystick device
void print_device_info(int fd) {
    int axes=0, buttons=0;
    char name[128];
    ioctl(fd, JSIOCGAXES, &axes);
    ioctl(fd, JSIOCGBUTTONS, &buttons);
    ioctl(fd, JSIOCGNAME(sizeof(name)), &name);
    printf("%s\n  %d Axes %d Buttons\n", name, axes, buttons);
}

// Event handler
void process_event(struct js_event jse) {
     // Define which axis is which
     //        Axis number {0,1,2,3} */
     char whichaxis[axes] = {'X','Y','Y','X'};
     //Define which joystick is moved
     char whichjs = '*';
     switch(jse.number) {
         case 0: case 1:
             whichjs = 'L';
             break;
         case 2: case 3:
             whichjs = 'R';
             break;
         default:
             whichjs = '*';
             break;
     }
     // Print which joystick, axis and value of the joystick position
     printf("%c%c |%8hd\n",whichjs,whichaxis[jse.number],jse.value);
}

Please post some details (function where it is defined and function b). — doptimusprime
– doptimusprime, Commented Nov 5, 2013 at 7:48
In printf("%c%c |%8hd\n",whichjs,whichaxis,jse.value); whichaxis should be whichaxis[jse.number]. whichaxis is a pointer. — doptimusprime
– doptimusprime, Commented Nov 5, 2013 at 8:32

doptimusprime · Accepted Answer · 2013-11-05 07:47:52Z

2

Either, you can use a global variable axes or pass axes as a parameter in both or one of the function.

If it is not possible to pass as a parameter (because it is a callback), then you can make function such as GetAxes() to give current axes or use a global variable.

answered Nov 5, 2013 at 7:47

doptimusprime

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3 Comments

Mihai Stancu Over a year ago

I'd say use a global or a non-const pointer to axes, so you can modify it.

theo-brown Over a year ago

Also, how do I pass axes as a parameter?

doptimusprime Over a year ago

@theoB610: In your case, it seems a problem to pass as parameter to handler function. You can use a global variable.

Sunil Bojanapally · Accepted Answer · 2013-11-05 08:13:03Z

1

axes is a local variable that is declared inside a function. A local variable can only be used in the function where it is declared. A global variable is a variable that is declared outside all functions. So make axes a global variable which can be used in all functions.

int axes; // Global declaration makes axes which as scope in all below functions

void print_device_info(int fd) {
    ...
    ioctl(fd, JSIOCGAXES, &axes);
    ...

void process_event(struct js_event jse) {
    char whichaxis[axes] = {'X','Y','Y','X'};
    ...

edited Nov 5, 2013 at 8:13

answered Nov 5, 2013 at 7:51

Sunil Bojanapally

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2 Comments

theo-brown Over a year ago

how? logically it makes sense to define it in the device info function

Sunil Bojanapally Over a year ago

@theoB610 int axes in function print_device_info() has local scope, making the variable is valid and can use within that function. If this is declared as global it is accessible in multiple scopes.

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