20. I/O Manager — RTEMS Classic API Guide 5.4-rc2 (21st August 2026) documentation

# 20. I/O Manager

## 20.1. Introduction

The input/output interface manager provides a well-defined mechanism for accessing device drivers and a structured methodology for organizing device drivers. The directives provided by the I/O manager are:

- rtems_io_initialize - Initialize a device driver

- rtems_io_register_driver - Register a device driver

- rtems_io_unregister_driver - Unregister a device driver

- rtems_io_register_name - Register a device name

- rtems_io_lookup_name - Look up a device name

- rtems_io_open - Open a device

- rtems_io_close - Close a device

- rtems_io_read - Read from a device

- rtems_io_write - Write to a device

- rtems_io_control - Special device services

## 20.2. Background

### 20.2.1. Device Driver Table

Each application utilizing the RTEMS I/O manager must specify the address of a Device Driver Table in its Configuration Table. This table contains each device driver’s entry points that is to be initialised by RTEMS during initialization. Each device driver may contain the following entry points:

- Initialization

- Open

- Close

- Read

- Write

- Control

If the device driver does not support a particular entry point, then that entry in the Configuration Table should be NULL. RTEMS will return `RTEMS_SUCCESSFUL` as the executive’s and zero (0) as the device driver’s return code for these device driver entry points.

Applications can register and unregister drivers with the RTEMS I/O manager avoiding the need to have all drivers statically defined and linked into this table.

The `confdefs.h` entry `CONFIGURE_MAXIMUM_DRIVERS` configures the number of driver slots available to the application.

### 20.2.2. Major and Minor Device Numbers

Each call to the I/O manager must provide a device’s major and minor numbers as arguments. The major number is the index of the requested driver’s entry points in the Device Driver Table, and is used to select a specific device driver. The exact usage of the minor number is driver specific, but is commonly used to distinguish between a number of devices controlled by the same driver.

The data types `rtems_device_major_number` and `rtems_device_minor_number` are used to manipulate device major and minor numbers, respectively.

### 20.2.3. Device Names

The I/O Manager provides facilities to associate a name with a particular device. Directives are provided to register the name of a device and to look up the major/minor number pair associated with a device name.

### 20.2.4. Device Driver Environment

Application developers, as well as device driver developers, must be aware of the following regarding the RTEMS I/O Manager:

- A device driver routine executes in the context of the invoking task. Thus if the driver blocks, the invoking task blocks.

- The device driver is free to change the modes of the invoking task, although the driver should restore them to their original values.

- Device drivers may be invoked from ISRs.

- Only local device drivers are accessible through the I/O manager.

- A device driver routine may invoke all other RTEMS directives, including I/O directives, on both local and global objects.

Although the RTEMS I/O manager provides a framework for device drivers, it makes no assumptions regarding the construction or operation of a device driver.

### 20.2.5. Runtime Driver Registration

Board support package and application developers can select wether a device driver is statically entered into the default device table or registered at runtime.

Dynamic registration helps applications where:

- The BSP and kernel libraries are common to a range of applications for a specific target platform. An application may be built upon a common library with all drivers. The application selects and registers the drivers. Uniform driver name lookup protects the application.

- The type and range of drivers may vary as the application probes a bus during initialization.

- Support for hot swap bus system such as Compact PCI.

- Support for runtime loadable driver modules.

### 20.2.6. Device Driver Interface

When an application invokes an I/O manager directive, RTEMS determines which device driver entry point must be invoked. The information passed by the application to RTEMS is then passed to the correct device driver entry point. RTEMS will invoke each device driver entry point assuming it is compatible with the following prototype:

    rtems_device_driver io_entry(
        rtems_device_major_number  major,
        rtems_device_minor_number  minor,
        void                      *argument_block
    );

The format and contents of the parameter block are device driver and entry point dependent.

It is recommended that a device driver avoid generating error codes which conflict with those used by application components. A common technique used to generate driver specific error codes is to make the most significant part of the status indicate a driver specific code.

### 20.2.7. Device Driver Initialization

RTEMS automatically initializes all device drivers when multitasking is initiated via the `rtems_initialize_executive` directive. RTEMS initializes the device drivers by invoking each device driver initialization entry point with the following parameters:

`major`

  the major device number for this device driver.

`minor`

  zero.

`argument_block`

  will point to the Configuration Table.

The returned status will be ignored by RTEMS. If the driver cannot successfully initialize the device, then it should invoke the fatal_error_occurred directive.

## 20.3. Operations

### 20.3.1. Register and Lookup Name

The `rtems_io_register` directive associates a name with the specified device (i.e. major/minor number pair). Device names are typically registered as part of the device driver initialization sequence. The `rtems_io_lookup` directive is used to determine the major/minor number pair associated with the specified device name. The use of these directives frees the application from being dependent on the arbitrary assignment of major numbers in a particular application. No device naming conventions are dictated by RTEMS.

### 20.3.2. Accessing an Device Driver

The I/O manager provides directives which enable the application program to utilize device drivers in a standard manner. There is a direct correlation between the RTEMS I/O manager directives `rtems_io_initialize`, `rtems_io_open`, `rtems_io_close`, `rtems_io_read`, `rtems_io_write`, and `rtems_io_control` and the underlying device driver entry points.

## 20.4. Directives

This section details the I/O manager’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

### 20.4.1. IO_REGISTER_DRIVER - Register a device driver

CALLING SEQUENCE:

      rtems_status_code rtems_io_register_driver(
          rtems_device_major_number   major,
          rtems_driver_address_table *driver_table,
          rtems_device_major_number  *registered_major
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | successfully registered
  `RTEMS_INVALID_ADDRESS` | invalid registered major pointer
  `RTEMS_INVALID_ADDRESS` | invalid driver table
  `RTEMS_INVALID_NUMBER` | invalid major device number
  `RTEMS_TOO_MANY` | no available major device table slot
  `RTEMS_RESOURCE_IN_USE` | major device number entry in use

DESCRIPTION:

  This directive attempts to add a new device driver to the Device Driver Table. The user can specify a specific major device number via the directive’s `major` parameter, or let the registration routine find the next available major device number by specifing a major number of `0`. The selected major device number is returned via the `registered_major` directive parameter. The directive automatically allocation major device numbers from the highest value down.

  This directive automatically invokes the `IO_INITIALIZE` directive if the driver address table has an initialization and open entry.

  The directive returns `RTEMS_TOO_MANY` if Device Driver Table is full, and `RTEMS_RESOURCE_IN_USE` if a specific major device number is requested and it is already in use.

NOTES:

  The Device Driver Table size is specified in the Configuration Table condiguration. This needs to be set to maximum size the application requires.

### 20.4.2. IO_UNREGISTER_DRIVER - Unregister a device driver

CALLING SEQUENCE:

      rtems_status_code rtems_io_unregister_driver(
          rtems_device_major_number   major
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | successfully registered
  `RTEMS_INVALID_NUMBER` | invalid major device number

DESCRIPTION:

  This directive removes a device driver from the Device Driver Table.

NOTES:

  Currently no specific checks are made and the driver is not closed.

### 20.4.3. IO_INITIALIZE - Initialize a device driver

CALLING SEQUENCE:

      rtems_status_code rtems_io_initialize(
          rtems_device_major_number  major,
          rtems_device_minor_number  minor,
          void                      *argument
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | successfully initialized
  `RTEMS_INVALID_NUMBER` | invalid major device number

DESCRIPTION:

  This directive calls the device driver initialization routine specified in the Device Driver Table for this major number. This directive is automatically invoked for each device driver when multitasking is initiated via the initialize_executive directive.

  A device driver initialization module is responsible for initializing all hardware and data structures associated with a device. If necessary, it can allocate memory to be used during other operations.

NOTES:

  This directive may or may not cause the calling task to be preempted. This is dependent on the device driver being initialized.

### 20.4.4. IO_REGISTER_NAME - Register a device

CALLING SEQUENCE:

      rtems_status_code rtems_io_register_name(
          const char                *name,
          rtems_device_major_number  major,
          rtems_device_minor_number  minor
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | successfully initialized
  `RTEMS_TOO_MANY` | too many devices registered

DESCRIPTION:

  This directive associates name with the specified major/minor number pair.

NOTES:

  This directive will not cause the calling task to be preempted.

### 20.4.5. IO_LOOKUP_NAME - Lookup a device

CALLING SEQUENCE:

      rtems_status_code rtems_io_lookup_name(
          const char          *name,
          rtems_driver_name_t *device_info
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | successfully initialized
  `RTEMS_UNSATISFIED` | name not registered

DESCRIPTION:

  This directive returns the major/minor number pair associated with the given device name in `device_info`.

NOTES:

  This directive will not cause the calling task to be preempted.

### 20.4.6. IO_OPEN - Open a device

CALLING SEQUENCE:

      rtems_status_code rtems_io_open(
          rtems_device_major_number  major,
          rtems_device_minor_number  minor,
          void                      *argument
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | successfully initialized
  `RTEMS_INVALID_NUMBER` | invalid major device number

DESCRIPTION:

  This directive calls the device driver open routine specified in the Device Driver Table for this major number. The open entry point is commonly used by device drivers to provide exclusive access to a device.

NOTES:

  This directive may or may not cause the calling task to be preempted. This is dependent on the device driver being invoked.

### 20.4.7. IO_CLOSE - Close a device

CALLING SEQUENCE:

      rtems_status_code rtems_io_close(
          rtems_device_major_number  major,
          rtems_device_minor_number  minor,
          void                      *argument
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | successfully initialized
  `RTEMS_INVALID_NUMBER` | invalid major device number

DESCRIPTION:

  This directive calls the device driver close routine specified in the Device Driver Table for this major number. The close entry point is commonly used by device drivers to relinquish exclusive access to a device.

NOTES:

  This directive may or may not cause the calling task to be preempted. This is dependent on the device driver being invoked.

### 20.4.8. IO_READ - Read from a device

CALLING SEQUENCE:

      rtems_status_code rtems_io_read(
          rtems_device_major_number  major,
          rtems_device_minor_number  minor,
          void                      *argument
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | successfully initialized
  `RTEMS_INVALID_NUMBER` | invalid major device number

DESCRIPTION:

  This directive calls the device driver read routine specified in the Device Driver Table for this major number. Read operations typically require a buffer address as part of the argument parameter block. The contents of this buffer will be replaced with data from the device.

NOTES:

  This directive may or may not cause the calling task to be preempted. This is dependent on the device driver being invoked.

### 20.4.9. IO_WRITE - Write to a device

CALLING SEQUENCE:

      rtems_status_code rtems_io_write(
          rtems_device_major_number  major,
          rtems_device_minor_number  minor,
          void                      *argument
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | successfully initialized
  `RTEMS_INVALID_NUMBER` | invalid major device number

DESCRIPTION:

  This directive calls the device driver write routine specified in the Device Driver Table for this major number. Write operations typically require a buffer address as part of the argument parameter block. The contents of this buffer will be sent to the device.

NOTES:

  This directive may or may not cause the calling task to be preempted. This is dependent on the device driver being invoked.

### 20.4.10. IO_CONTROL - Special device services

CALLING SEQUENCE:

      rtems_status_code rtems_io_control(
          rtems_device_major_number  major,
          rtems_device_minor_number  minor,
          void                      *argument
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | successfully initialized
  `RTEMS_INVALID_NUMBER` | invalid major device number

DESCRIPTION:

  This directive calls the device driver I/O control routine specified in the Device Driver Table for this major number. The exact functionality of the driver entry called by this directive is driver dependent. It should not be assumed that the control entries of two device drivers are compatible. For example, an RS-232 driver I/O control operation may change the baud rate of a serial line, while an I/O control operation for a floppy disk driver may cause a seek operation.

NOTES:

  This directive may or may not cause the calling task to be preempted. This is dependent on the device driver being invoked.