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# # $Id$ # Directory Overview ================== This is the top level of the RTEMS directory structure. The following is a description of the files and directories in this directory: INSTALL Rudimentary installation instructions. For more detailed information please see the Release Notes. The Postscript version of this manual can be found in the file c_or_ada/doc/relnotes.tgz. LICENSE Required legalese. README This file. c This directory contains the source code for the C implementation of RTEMS as well as the test suites, sample applications, Board Support Packages, Device Drivers, and support libraries. doc This directory contains the PDL for the RTEMS executive. Ada versus C ============ There are two implementations of RTEMS in this source tree -- in Ada and in C. These two implementations are functionally and structurally equivalent. The C implementation follows the packaging conventions and hiearchical nature of the Ada implementation. In addition, a style has been followed which allows one to easily find the corresponding Ada and C implementations. File names in C and code placement was carefully designed to insure a close mapping to the Ada implementation. The following file name extensions are used: .adb - Ada body .ads - Ada specification .adp - Ada body requiring preprocessing .inc - include file for .adp files .c - C body (non-inlined routines) .inl - C body (inlined routines) .h - C specification In the executive source, XYZ.c and XYZ.inl correspond directly to a single XYZ.adb or XYZ.adp file. A .h file corresponds directly to the .ads file. There are only a handful of .inc files in the Ada source and these are used to insure that the desired simple inline textual expansion is performed. This avoids scoping and calling convention side-effects in carefully constructed tests which usually test context switch behavior. In addition, in Ada code and data name references are always fully qualified as PACKAGE.NAME. In C, this convention is followed by having the package name as part of the name itself and using a capital letter to indicate the presence of a "." level. So we have PACKAGE.NAME in Ada and _Package_Name in C. The leading "_" in C is used to avoid naming conflicts between RTEMS and user variables. By using these conventions, one can easily compare the C and Ada implementations. The most noticeable difference between the C and Ada83 code is the inability to easily obtain a "typed pointer" in Ada83. Using the "&" operator in C yields a pointer with a specific type. The 'Address attribute is the closest feature in Ada83. This returns a System.Address and this must be coerced via Unchecked_Conversion into an access type of the desired type. It is easy to view System.Address as similar to a "void *" in C, but this is not the case. A "void *" can be assigned to any other pointer type without an explicit conversion. The solution adopted to this problem was to provide two routines for each access type in the Ada implementation -- one to convert from System.Address to the access type and another to go the opposite direction. This results in code which accomplishes the same thing as the corresponding C but it is easier to get lost in the clutter of the apparent subprogram invocations than the "less bulky" C equivalent. A related difference is the types which are only in Ada which are used for pointers to arrays. These types do not exist and are not needed in the C implementation.