jasonwhite · June 24, 2025 13:01 · Oct 19, 2019 · Oct 17, 2019 · Jun 10, 2019 · Dec 20, 2014
diff --git a/joystick.c b/joystick.c
@@ -128,6 +128,8 @@ int main(int argc, char *argv[])
                 /* Ignore init events. */
                 break;
         }
+
+        fflush(stdout);
     }
 
     close(js);

diff --git a/joystick.c b/joystick.c
@@ -15,6 +15,7 @@
  */
 #include <fcntl.h>
 #include <stdio.h>
+#include <unistd.h>
 #include <linux/joystick.h>
 
 

diff --git a/joystick.c b/joystick.c
@@ -121,7 +121,7 @@ int main(int argc, char *argv[])
             case JS_EVENT_AXIS:
                 axis = get_axis_state(&event, axes);
                 if (axis < 3)
-                    printf("Axis %u at (%6d, %6d)\n", axis, axes[axis].x, axes[axis].y);
+                    printf("Axis %zu at (%6d, %6d)\n", axis, axes[axis].x, axes[axis].y);
                 break;
             default:
                 /* Ignore init events. */

diff --git a/joystick.c b/joystick.c
@@ -36,6 +36,31 @@ int read_event(int fd, struct js_event *event)
     return -1;
 }
 
+/**
+ * Returns the number of axes on the controller or 0 if an error occurs.
+ */
+size_t get_axis_count(int fd)
+{
+    __u8 axes;
+
+    if (ioctl(fd, JSIOCGAXES, &axes) == -1)
+        return 0;
+
+    return axes;
+}
+
+/**
+ * Returns the number of buttons on the controller or 0 if an error occurs.
+ */
+size_t get_button_count(int fd)
+{
+    __u8 buttons;
+    if (ioctl(fd, JSIOCGBUTTONS, &buttons) == -1)
+        return 0;
+
+    return buttons;
+}
+
 /**
  * Current state of an axis.
  */
@@ -106,4 +131,4 @@ int main(int argc, char *argv[])
 
     close(js);
     return 0;
-}
+}
diff --git a/joystick.c b/joystick.c
@@ -0,0 +1,109 @@
+/**
+ * Author: Jason White
+ *
+ * Description:
+ * Reads joystick/gamepad events and displays them.
+ *
+ * Compile:
+ * gcc joystick.c -o joystick
+ *
+ * Run:
+ * ./joystick [/dev/input/jsX]
+ *
+ * See also:
+ * https://www.kernel.org/doc/Documentation/input/joystick-api.txt
+ */
+#include <fcntl.h>
+#include <stdio.h>
+#include <linux/joystick.h>
+
+
+/**
+ * Reads a joystick event from the joystick device.
+ *
+ * Returns 0 on success. Otherwise -1 is returned.
+ */
+int read_event(int fd, struct js_event *event)
+{
+    ssize_t bytes;
+
+    bytes = read(fd, event, sizeof(*event));
+
+    if (bytes == sizeof(*event))
+        return 0;
+
+    /* Error, could not read full event. */
+    return -1;
+}
+
+/**
+ * Current state of an axis.
+ */
+struct axis_state {
+    short x, y;
+};
+
+/**
+ * Keeps track of the current axis state.
+ *
+ * NOTE: This function assumes that axes are numbered starting from 0, and that
+ * the X axis is an even number, and the Y axis is an odd number. However, this
+ * is usually a safe assumption.
+ *
+ * Returns the axis that the event indicated.
+ */
+size_t get_axis_state(struct js_event *event, struct axis_state axes[3])
+{
+    size_t axis = event->number / 2;
+
+    if (axis < 3)
+    {
+        if (event->number % 2 == 0)
+            axes[axis].x = event->value;
+        else
+            axes[axis].y = event->value;
+    }
+
+    return axis;
+}
+
+int main(int argc, char *argv[])
+{
+    const char *device;
+    int js;
+    struct js_event event;
+    struct axis_state axes[3] = {0};
+    size_t axis;
+
+    if (argc > 1)
+        device = argv[1];
+    else
+        device = "/dev/input/js0";
+
+    js = open(device, O_RDONLY);
+
+    if (js == -1)
+        perror("Could not open joystick");
+
+    /* This loop will exit if the controller is unplugged. */
+    while (read_event(js, &event) == 0)
+    {
+        switch (event.type)
+        {
+            case JS_EVENT_BUTTON:
+                printf("Button %u %s\n", event.number, event.value ? "pressed" : "released");
+                break;
+            case JS_EVENT_AXIS:
+                axis = get_axis_state(&event, axes);
+                if (axis < 3)
+                    printf("Axis %u at (%6d, %6d)\n", axis, axes[axis].x, axes[axis].y);
+                break;
+            default:
+                /* Ignore init events. */
+                break;
+        }
+    }
+
+    close(js);
+    return 0;
+}