Porting FreeRTOS+UDP to a New Embedded C Compiler
Summary Bullet Points
A FreeRTOS+UDP project has two compiler dependencies, as follows:
The syntax used by the compiler to pack structures.
This relates directly to the core FreeRTOS+UDP code, and
how FreeRTOS+UDP manages this dependency is
- The syntax used by the compiler to define an interrupt service routine (ISR). This only relates to ISRs used in Ethernet drivers, not the core core FreeRTOS+UDP code, and is therefore not covered on this page.
- The syntax used by the compiler to pack structures. This relates directly to the core FreeRTOS+UDP code, and how FreeRTOS+UDP manages this dependency is described below.
The syntax required to pack structures is placed in compiler
dependent header files to keep it out of the core FreeRTOS+UDP
Two header files are required for each compiler.
pack_struct_start.h contains any keywords that
are required to appear before the definition of a packed
pack_struct_end.h contains any keywords that are
required to appear at the end of the definition of packed
structure. pack_struct_end.h must also
contain the semicolon ( ; ) used to end the structure
- pack_struct_start.h contains any keywords that are required to appear before the definition of a packed structure.
Existing compiler specific header files are located in sub-directories
The first three members are 8 bits each, and will most likely appear in three consecutive bytes of the same 32-bit word. The forth member is 32-bits, and will probably be most efficiently accessed if it is placed on a 32-bit (four byte) aligned memory address. Therefore the compiler can add a ‘padding’ byte between Member3 and Member4 to move the start address of Member4 onto a 32-bit boundary. The structure will then map into memory as below:
However, the UDP/IP protocol headers do not have padding bytes, so the compiler must be told not to add them into structures that map onto the protocol headers. Structures that do not contain padding bytes are said to be ‘packed’. The syntax required to ensure structures are packed depends on the embedded C compiler. The FreeRTOS+UDP implementation avoids using any compiler specific syntax in the core files, and instead allows users to define their own packing directives in two very simple header files.
Structures that must be packed appear in the FreeRTOS+UDP code as follows:
A version of pack_struct_start.h and pack_struct_end.h that contains compiler specific syntax can then be provided for each compiler.
For example, the directory FreeRTOS-Plus/FreeRTOS-Plus-UDP/portable/Compiler/GCC contains definitions suitable for use with the GCC compiler. pack_struct_start.h is empty because GCC does not require any special syntax at the start of the structure. pack_struct_end.h contains the following single line of code:
__attribute__( (packed) );Therefore, after the pre-processor has included the header files, the C compiler sees the code as follows, which is valid GCC syntax for structure packing:
FreeRTOS-Plus/FreeRTOS-Plus-UDP/portable/Compiler/GCC already contains implementations for other compilers that can be used directly, or as a reference when adding new compiler ports.
pack_struct_end.h must, as a minimum, contain at least a semicolon ( ; ) to mark the end of the structure definition. It is sometimes valid for pack_struct_start.h to be empty, but it is never valid for pack_struct_end.h to be completely empty.