无法理解 libusb API

Question

I am working in libusb and have minimal knowledge on C++. I am trying to understand the API of libusb and write code accordingly. But I am unable to understand how to declare and use variables as variable or pointer or double or triple pointer.

1. The below code is from question . How to find what level of pointers to be used while declaring these variables from the API documentation. Is there any documentation or tutorial videos that explains these things.

    libusb_context *context = NULL; libusb_device_handle *dev_handle = NULL; libusb_device **devs; int rc = 0; ssize_t count; //holding number of devices in list

For example, consider libusb_device_handle . How to declare this and use it?

typedef struct libusb_device_handle libusb_device_handle

2. The syntax of libusb_strerror() is const char * libusb_strerror (int errcode) . The function returns constant string. Should I declare a char or char array or string to read the returned value. If the below way of usage right?

    char char *err_code;//declaration err_code = libusb_strerror(rc);

if the returned value is a string, then how can a characer pointer hold it?

An example would be really helpful.

3)Here is my entire code. I can open the device. But the bulk transfer command returns 5 and fails. I am not sure at which part I am making mistake.

#include <iostream>
#include "libusb.h"

#define IN_EP  0x81
#define OUT_EP 0x02

int main(){
    libusb_context *context = NULL;
    libusb_device_handle *dev_handle = NULL ;
    libusb_device **devs ;
    int rc = 100 ;
    //ssize_t count ; //holding number of devices in list
    unsigned int vid=0x1234;
    unsigned int pid=0x5678;
    unsigned char data[10];
    data[0]=128;
    int transferred = 0;
    unsigned int timeout = 5000;
    //std::string str[100];
    const char* err_code;
    

    rc = libusb_set_option(context, LIBUSB_OPTION_LOG_LEVEL,2);
    if (rc==0){    std::cout<<"libusb_setOption worked:"<<rc<<"\n"; }
    else{   std::cout<<"libusb_setOption_Failed:"<<rc<<"\n";    }


    rc = libusb_init(&context);
    if (rc==0){    std::cout<<"libusb_init worked:"<<rc<<"\n"; }
    else{   std::cout<<"libusb_init Failed"<<rc<<"\n";    }


    dev_handle = libusb_open_device_with_vid_pid(context,vid,pid);
    if (dev_handle == NULL){
        std::cout<<"libusb_open Failed"<<"\n";
        libusb_exit(context);std::cout<<"libusb_exit"<<"\n";exit(1);    }
    else{    std::cout<<"libusb_opened"<<"\n";    }

    
    rc = libusb_bulk_transfer(dev_handle,OUT_EP,data,1,&transferred, timeout);  //Send data to device
    if (rc==0){        std::cout<<"libusb_write worked:"<<rc<<"; Wrote "<<transferred<<" bytes\n";    }
    else{        std::cout<<"libusb__Write failed"<<rc<<"; Wrote "<<transferred<<" bytes\n";    }
    err_code = libusb_strerror(rc);
    
    rc = libusb_bulk_transfer(dev_handle,IN_EP,data,3,&transferred, timeout);   //Read data from device
    if (rc==0){        std::cout<<"libusb_read worked:"<<rc<<" ; Read"<<transferred<<" bytes; Data:"<<data<<"\n";
    std::cout<<data[0]<<" "<<data[1]<<" "<<data[2]<<"\n";  }
    else{        std::cout<<"libusb__read failed"<<rc<<"; Read "<<transferred<<" bytes\n";    }

    
    libusb_close(dev_handle);
    std::cout<<"libusb_close"<<"\n";
    
    
    libusb_exit(context);
    std::cout<<"libusb_close"<<"\n";
    
    
    return 0;
    }

Any help will be appreciated.

Answer 1

1. [...] For example, consider libusb_device_handle . How to declare this and use it?

You declare it exactly like in your code:

libusb_device_handle *dev_handle = NULL;

Because libusb_open_device_with_vid_pid() returns a pointer to libusb_device_handle , your dev_handle must also be a pointer to that.

You might be confused because of devs being a pointer-to-pointer. This is because it is actually returning a pointer to an array, but since you are not even using that in your code, I would forget about it for now.

2. The syntax of libusb_strerror() is const char * libusb_strerror (int errcode) . The function returns constant string. Should I declare a char or char array or string to read the returned value?

You should declare a variable exactly the same as the return type of libusb_strerror() , thus:

const char *err_string;
err_string = libusb_strerror(rc);

If the returned value is a string, then how can a characer pointer hold it?

The returned value is a pointer, you just make a copy of the pointer. The pointer points to some part of memory where the string is stored. You don't have to worry about how libusb allocated it.

3. I can open the device. But the bulk transfer command returns 5 and fails. I am not sure at which part I am making mistake.

The code looks mostly fine, except you want to call libusb_strerror() after the second call to libusb_bulk_transfer() , not right before it. Error code -5 is LIBUSB_ERROR_NOT_FOUND . This might mean it didn't find the endpoint. Are you sure IN_EP and OUT_EP are set correctly?