libusb半工作，但libusb_device_descriptor未申报？

Question

我正在使用libusb尝试一个C样例，直到我尝试执行以下操作时，一切都进行得很好：

libusb_device_descriptor descriptor;
int result = libusb_get_device_descriptor(usb_device, &descriptor);

编译器告诉我，在同一个libusb.h头中定义的某些元素(与我正在使用的其他libusb结构一样)是没有声明的。这是什么？

下面是完整的代码：

#include <stdio.h>
#include <stdlib.h>
#include <libusb.h>

void printdev(libusb_device *usb_device);

/**
 * main
 */
int main(void) {
    puts("USB Test v0.0.1");

    libusb_device **usb_devices;
    libusb_context *usb_context = NULL;
    int result;
    result = libusb_init(&usb_context);
    ssize_t device_count;

    if (result < 0) {
        puts("USB initialization error!");
        return EXIT_FAILURE;
    }

    libusb_set_debug(usb_context, 1);
    device_count = libusb_get_device_list(usb_context, &usb_devices);

    if (device_count < 0) {
        puts("Unable to get USB device list!");
        return EXIT_FAILURE;
    }

    char message[24];
    sprintf(message, "%d USB Devices found", (int)device_count);
    puts(message);

    ssize_t i;
    for (i = 0; i < device_count; i++) {

    }

    libusb_free_device_list(usb_devices, 1);
    libusb_exit(usb_context);

    return EXIT_SUCCESS;
}

/**
 *
 */
void printdev(libusb_device *usb_device)
{
    libusb_device_descriptor descriptor;
    int result = libusb_get_device_descriptor(usb_device, &descriptor);

    if (result < 0) {
        puts("Failed to get device descriptor");
        return;
    }
}

如果我将printdev函数去掉/注释掉，代码编译和main()将完美地运行。但是为什么在main中使用结构，比如libusb_device和libusb_context定义了，而libusb_device_descriptor却没有呢？

下面是我的编译器输出：

23:18:07 **** Incremental Build of configuration debug for project usb_test ****
make all 
Building file: ../src/usb_test.c
Invoking: GCC C Compiler
gcc -I/usr/local/Cellar/libusb/1.0.9/include/libusb-1.0 -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/usb_test.d" -MT"src/usb_test.d" -o "src/usb_test.o" "../src/usb_test.c"
../src/usb_test.c: In function 'printdev':
../src/usb_test.c:52: error: 'libusb_device_descriptor' undeclared (first use in this function)
../src/usb_test.c:52: error: (Each undeclared identifier is reported only once
../src/usb_test.c:52: error: for each function it appears in.)
../src/usb_test.c:52: error: expected ';' before 'descriptor'
../src/usb_test.c:53: error: 'descriptor' undeclared (first use in this function)
make: *** [src/usb_test.o] Error 1

23:18:07 Build Finished (took 73ms)

Answer 1

我也研究过Libusb.h，而且libusb_config_descriptor没有定义类型，所以您必须在libusb_config_descriptor之前输入struct。

Answer 2

在libusb.h中没有libusb_device_descriptor类型。

然而，有一个struct libusb_device_descriptor