c - 未声明变量的操纵杆代码错误

Question

我正在尝试编写代码来读取操纵杆轴值，我最终希望能够使用这些值来控制电机。经过大量尝试弄清楚 C 语言以及到底如何使用操纵杆 api 之后，我编写了这段代码。首先它只出现了一个变量未声明的错误，然后当我改进代码时，使其更具可读性和更容易理解，当我去编译它时，我得到了另一个相同的错误，希望第一个会有走了! 这是我的代码(请原谅评论):

#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 */

我从 gcc 返回的错误是:

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)

有人可以解释一下为什么我会收到这些错误并提出修复建议吗？ 预先感谢您。

Answer 1

open_joystick 设置 fd，但 fd 是 open_joystick 本地的，因此 无法读取read_joystick_thrust_axis。转换 read_joystick_thrust_axis 以允许将 fd 作为参数传递，并传递 open_joystick 的返回值，如下所示:

更改:

int read_joystick_thrust_axis(struct js_event js)

至

int read_joystick_thrust_axis(int fd, struct js_event js)

然后当你调用它时(从 main 或其他地方)，执行

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

关于js_event错误，变量名为js，类型为struct js_event。因此，您需要引用 js.type 而不是 js_event.type。