libusb semi-working, but libusb_device_descriptor undeclared?

Question

I'm trying a C sample with libusb and things were working fine up to the point where I'm trying to do the following:

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

The compiler is telling me, that some elements defined in the same libusb.h header, as other libusb structs I am using, is not declared. WTH?

Here's the code in full:

#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;
    }
}

If I leave/comment out the printdev function, code compiles, and main() runs perfectly. But why on earth are structs used in main, like libusb_device and libusb_context defined, but libusb_device_descriptor is not?

Here's my compiler output:

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

I have studied the Libusb.h as well, and the libusb_config_descriptor is not type defined, so you would have to type struct before libusb_config_descriptor.