BSP Developer's Guide

VxWorks 5.5
BSP Developer’s Guide
Analysis of Wind River’s existing code base reveals three main areas where
non-ANSI compiler features are used to generate run-time code:
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packed structure definitions
in-line assembly code
alignment of data elements
NOTE: This chapter is limited in scope to insuring that code is portable specifically
between the GNU and Diab compilers.
NOTE: The scope of this chapter is further limited to specify GCC version 2.96 and
Diab version 5.0 as the baseline compilers. Earlier versions are not truly compatible
with each other and may not have all the necessary support to implement the
conventions introduced here.
8.1 Portable C Code
8.1.1 Data Structures
Some structure definitions are bound by external restrictions. It is common
practice to use a structure definition to document the register layout of hardware
devices. Using a structure to define the layout of data packet contents received
from another system is also common. This can create problems because the ANSI
specification allows compilers to insert padding elements within structure
definitions in order to optimize data element accesses. In these situations, doing so
would make the structure declaration incompatible with the expectations and
restrictions of the outside world. The compiler offsets from the start of the structure
to the start of the data element would not match what is expected. A method is
required for identifying certain structures as requiring special compiler treatment,
while allowing other structures to be optimized for efficiency.
The common term for a structure definition without any padding is packed. Each of
the major compilers has a means to specify packing for a structure, but there is no
single recognized standard. GNU and Diab use an attribute statement as part of
the declaration. Microsoft compilers use #pragma statements.
To specify packing in a compiler-independent manner a macro has been created for
use when defining the structure. The _WRS_PACK_ALIGN(x) macro is used as an
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