RTEMS 4.7.2 On-Line Library

RTEMS C User's Guide

22.2: Automatic Generation of System Configuration

RTEMS provides the confdefs.h C language header file that based on the setting of a variety of macros can automatically produce nearly all of the configuration tables required by an RTEMS application. Rather than building the individual tables by hand, the application simply specifies the values for the configuration parameters it wishes to set. In the following example, the configuration information for a simple system with a message queue and a time slice of 50 milliseconds is configured:

#define CONFIGURE_APPLICATION_NEEDS_CONSOLE_DRIVER
#define CONFIGURE_APPLICATION_NEEDS_CLOCK_DRIVER

#define CONFIGURE_MICROSECONDS_PER_TICK   1000 /* 1 millisecond */
#define CONFIGURE_TICKS_PER_TIMESLICE       50 /* 50 milliseconds */

#define CONFIGURE_MAXIMUM_TASKS 4
#define CONFIGURE_RTEMS_INIT_TASKS_TABLE

This system will begin execution with the single initialization task named Init. It will be configured to have both a console device driver (for standard I/O) and a clock tick device driver.

For each configuration parameter in the configuration tables, the macro corresponding to that field is discussed. Most systems can be easily configured using the confdefs.h mechanism.

The CONFIGURE_INIT constant must be defined in order to make confdefs.h instantiate the configuration data structures. This can only be defined in one source file per application that includes confdefs.h or the symbol table will be instantiated multiple times and linking errors produced.

The user should be aware that the defaults are intentionally set as low as possible. By default, no application resources are configured. The confdefs.h file ensures that at least one application tasks or thread is configured and that at least one of the initialization task/thread tables is configured.

The confdefs.h file estimates the amount of memory required for the RTEMS Executive Workspace. This estimate is only as accurate as the information given to confdefs.h and may be either too high or too low for a variety of reasons. Some of the reasons that confdefs.h may reserve too much memory for RTEMS are:

• All tasks/threads are assumed to be floating point.

Conversely, there are many more reasons, the resource estimate could be too low:

• Task/thread stacks greater than minimum size must be accounted for explicitly by developer.
• Memory for messages is not included.
• Device driver requirements are not included.
• Network stack requirements are not included.
• Requirements for add-on libraries are not included.

In general, confdefs.h is very accurate when given enough information. However, it is quite easy to use a library and not account for its resources.

The following subsection list all of the constants which can be set by the user.

RTEMS C User's Guide

Copyright © 1988-2004 OAR Corporation