Library initialization/deinitialization

Overview

This page details how to initialize and deinitialize libusb. More…

// typedefs

typedef struct libusb_context libusb_context;

// enums

enum libusb_log_level;

// structs

struct libusb_version;

// global functions

void
libusb_set_debug(
    libusb_context* ctx,
    int level
);

int
libusb_init(libusb_context** context);

void
libusb_exit(struct libusb_context* ctx);

Detailed Documentation

This page details how to initialize and deinitialize libusb.

Initialization must be performed before using any libusb functionality, and similarly you must not call any libusb functions after deinitialization.

Typedefs

typedef struct libusb_context libusb_context

Structure representing a libusb session.

The concept of individual libusb sessions allows for your program to use two libraries (or dynamically load two modules) which both independently use libusb. This will prevent interference between the individual libusb users - for example libusb_set_debug() will not affect the other user of the library, and libusb_exit() will not destroy resources that the other user is still using.

Sessions are created by libusb_init() and destroyed through libusb_exit(). If your application is guaranteed to only ever include a single libusb user (i.e. you), you do not have to worry about contexts: pass NULL in every function call where a context is required. The default context will be used.

For more information, see Contexts.

Global Functions

void
libusb_set_debug(
    libusb_context* ctx,
    int level
)

Set log message verbosity.

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

You are advised to use level LIBUSB_LOG_LEVEL_WARNING. libusb is conservative with its message logging and most of the time, will only log messages that explain error conditions and other oddities. This will help you debug your software.

If the LIBUSB_DEBUG environment variable was set when libusb was initialized, this function does nothing: the message verbosity is fixed to the value in the environment variable.

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.

Parameters:

ctx	the context to operate on, or NULL for the default context
level	debug level to set

int
libusb_init(libusb_context** context)

Initialize libusb.

This function must be called before calling any other libusb function.

If you do not provide an output location for a context pointer, a default context will be created. If there was already a default context, it will be reused (and nothing will be initialized/reinitialized).

Parameters:

context

Optional output location for context pointer. Only valid on return code 0.

Returns:

0 on success, or a LIBUSB_ERROR code on failure

See also:

libusb_contexts

void
libusb_exit(struct libusb_context* ctx)

Deinitialize libusb.

Should be called after closing all open devices and before your application terminates.

Parameters:

ctx	the context to deinitialize, or NULL for the default context