libusb-1.0 libusb_get_device_list() failure

BombaD · 02-04-2015, 01:03 PM

Also tested with -Wall and -std=c99 and c11 (not working of course)

Code:

$ gcc -o usb -L/usr/local/lib -lusb-1.0 -I/usr/local/include usbtest.c

Output:

Code:

usbtest.c: In function ‘main’:
usbtest.c:14:1: error: label ‘brd_ftdi’ used but not defined
 libusb_get_device_list(context, &&brd_ftdi);
 ^

usbtest.c:

Code:

#include <stdio.h>
#include <libusb-1.0/libusb.h>
int main() {
libusb_device* brd_ftdi;
libusb_device_handle** brd_ftdi_handle;
libusb_context* context;

//libusb_device *** list = &&brd_ftdi;


//libusb_init (libusb_context **);
libusb_init (&context);

libusb_get_device_list(context, &&brd_ftdi);
//libusb_open(brd_ftdi, brd_ftdi_handle);
    
//libusb_exit(struct libusb_context *); 
libusb_exit(context);

return 0;
}

Not defined? I know, but libusb_get_device_list() is supposed to do it (put the list in there), so WTF? I also tested declaring a pointer to pointer to brd_ftdi and passing that on instead but same result.

smallpond · 02-04-2015, 01:51 PM

You are trying to create a pointer to a pointer out of thin air. Where does it put that intermediate pointer?
Try:

Code:

libusb_device **brd_ftdi;
n = libusb_get_device_list(context, &brd_ftdi);

BombaD · 02-16-2015, 08:30 AM

Somewhere in the memory I guess. Isn't your method the same thing? you're using a double pointer instead of a single like I did and hence pass only one & to the method.

pan64 · 02-16-2015, 09:07 AM

no you have misunderstood:
& means address of, so &brd_ftdi means the location where the variable brd_ftdi is stored.
* means pointer, ** means a pointer to a pointer (* means a variable which contains the address of another thing, ** means a variable which contains the address of another variable, which contains the address of another one...)
But && as (address of the address of something) has no any meaning, this syntax does not exist - because the address of variable is not yet stored anywhere. && is the logical and operator.
The line cannot be properly parsed and an error message was dropped....

BombaD · 03-27-2015, 08:06 AM

Using * gives

Code:

schtern@frmscrpyrd:~$ gcc -o usb -Wall -L/usr/local/lib -lusb-1.0 -I/usr/local/include usbtest.c
usbtest.c: In function main:
usbtest.c:12:14: error: dereferencing pointer to incomplete type
 libusb_init (*context);
              ^
usbtest.c:14:33: error: dereferencing pointer to incomplete type
 libusb_get_device_list(context,**brd_ftdi);
                                 ^
usbtest.c:16:25: error: dereferencing pointer to incomplete type
 libusb_free_device_list(*brd_ftdi,0);
                         ^
usbtest.c:5:24: warning: unused variable brd_ftdi_handle [-Wunused-variable]
 libusb_device_handle** brd_ftdi_handle;
                        ^

Using & gives

Code:

$ gcc -o usb -Wall -L/usr/local/lib -lusb-1.0 -I/usr/local/include usbtest.c


usbtest.c: In function main:
usbtest.c:14:1: error: label brd_ftdi used but not defined
 libusb_get_device_list(context,&&brd_ftdi);
 ^
usbtest.c:5:24: warning: unused variable brd_ftdi_handle [-Wunused-variable]
 libusb_device_handle** brd_ftdi_handle;
                        ^