usb- Communicate with USB devices

MaintainerBas van Dijk <>
Safe HaskellTrustworthy



This module provides functionality for initializing the usb library.



data Ctx Source

Abstract type representing a USB session.

The concept of individual sessions allows your program to use multiple threads that can independently use this library without interfering with eachother.

Sessions are created and initialized by newCtx and are automatically closed when they are garbage collected.

The only functions that receive a Ctx are setDebug and getDevices.


newCtx :: IO CtxSource

Create and initialize a new USB context.

This function may throw USBExceptions.

Note that the internal libusb event handling can return errors. These errors occur in the thread that is executing the event handling loop. newCtx will print these errors to stderr. If you need to handle the errors yourself (for example log them in an application specific way) please use newCtx'.

newCtx' :: (USBException -> IO ()) -> IO CtxSource

Like newCtx but enables you to specify the way errors should be handled that occur while handling libusb events.

setDebug :: Ctx -> Verbosity -> IO ()Source

Set message verbosity.

The default level is PrintNothing. This means no messages are ever printed. If you choose to increase the message verbosity level you must ensure that your application does not close the stdout/stderr file descriptors.

You are advised to set the debug level to PrintWarnings. Libusb is conservative with its message logging. Most of the time it will only log messages that explain error conditions and other oddities. This will help you debug your software.

The LIBUSB_DEBUG environment variable overrules the debug level set by this function. The message verbosity is fixed to the value in the environment variable if it is defined.

If libusb was compiled without any message logging, this function does nothing: you'll never get any messages.

If libusb was compiled with verbose debug message logging, this function does nothing: you'll always get messages from all levels.

data Verbosity Source

Message verbosity



No messages are ever printed by the library


Error messages are printed to stderr


Warning and error messages are printed to stderr


Informational messages are printed to stdout, warning and error messages are printed to stderr