# 26.1. Introduction¶

The application configuration information group provides an API to get the configuration of an application.

RTEMS must be configured for an application. This configuration encompasses a variety of information including the length of each clock tick, the maximum number of each information RTEMS object that can be created, the application initialization tasks, the task scheduling algorithm to be used, and the device drivers in the application.

Although this information is contained in data structures that are used by RTEMS at system initialization time, the data structures themselves must not be generated by hand. RTEMS provides a set of macros system which provides a simple standard mechanism to automate the generation of these structures.

The RTEMS header file <rtems/confdefs.h> is at the core of the automatic generation of system configuration. It is based on the idea of setting macros which define configuration parameters of interest to the application and defaulting or calculating all others. This variety of macros can automatically produce all of the configuration data required for an RTEMS application. The term confdefs is shorthand for a Configuration Defaults.

As a general rule, application developers only specify values for the configuration parameters of interest to them. They define what resources or features they require. In most cases, when a parameter is not specified, it defaults to zero (0) instances, a standards compliant value, or disabled as appropriate. For example, by default there will be 256 task priority levels but this can be lowered by the application. This number of priority levels is required to be compliant with the RTEID/ORKID standards upon which the Classic API is based. There are similar cases where the default is selected to be compliant with the POSIX standard.

For each configuration parameter in the configuration tables, the macro corresponding to that field is discussed. The RTEMS Maintainers expect that all systems can be easily configured using the <rtems/confdefs.h> mechanism and that using this mechanism will avoid internal RTEMS configuration changes impacting applications.

Some application configuration settings and other system parameters can be queried by the application. The directives provided by the Application Configuration Information are: