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 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.hmust also
contain the semicolon ( ; ) used to end the structure
Existing compiler specific header files are located in sub-directories
The FreeRTOS+UDP implementation uses C structures to map onto the UDP, IP,
ARP, DNS and DHCP protocol headers. C compilers normally lay out
the memory used to hold a C structure in a way that allows optimal access
to the structure's members, and that often results in the internal
representation of the structure being padded to achieve optimal byte alignment.
For example, consider the following
structure defined on a 32-bit microcontroller architecture:
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:
uint8_t Member1; At address 0 (first byte of first 32-bit word)
uint8_t Member2; At address 1 (second byte of first 32-bit word)
uint8_t Member3; At address 2 (third byte of first 32-bit word)Padding byte At address 4 (forth byte of the first 32-bit word)
uint32_t Member4; Now at address 4, the first byte of the second 32-bit word
Example of how the C structure will appear in the microcontroller memory
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 structure wrapped in pack_struct_start and pack_struct_end header file inclusions
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:
How GCC sees a packed structure after pack_struct_start.h and pack_struct_end.h have been included
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.