12.2. RTEMS Symbols

The RTEMS Symbols (rtems-syms) command is an RTEMS tool to generate symbol tables used by the RTEMS Runtime Loader (RTL). The symbol table contains the exported base kernel symbols user code dynamically loaded can reference.

The RTEMS Runtime Loader supports two methods of loading a symbol table, embedded and runtime loading. Embedding the table requires linking the symbol table with the base image and runtime loading loads the table using the dynamic loader when RTEMS is running.

Filtering Symbols

Currently there is no filtering of symbols in the symbol table. This means all base kernel image symbols are present in the symbol table when only a sub-set of the symbols are referenced.

Embedding the symbol table creates self contained images. A target may not have any external media, for example RTEMS tests, or there is a requirement to avoid the management need to match the symbol table with the kernel base image. Embedding the symbol table requires a 2-pass link process making the application’s build system more complicated.

A dynamically loadable symbol table is simpler to create however the symbol table and the kernel base image must match or the behaviour is undefined. There is currently no mechnanisum to ensure the symbol table and the kernel image match The rtems-syms command is run against the base kernel image and the generated symbol table is installed on to the target hardware and loaded before any other modules.

12.2.1. Symbol Table

The symbol table is an ELF object file in the target’s ELF format and is built using the target’s RTEMS C compiler. The rtems-syms command searches for the C compller under the prefix this command is installed under or the system path. If the target’s C compiler is not located in either of these paths use the option -c or --cc to specify the path to the compiler.

The rtems-syms command loads the base kernel image’s ELF file and reads the global or public symbols, creates a temporary C file and then compiles it using the target’s RTEMS C compiler. The command automatically detects the architecture from the base kernel image’s ELF file and uses it to create the C compiler’s name. The option -E or --exec-prefix can be used to override the executable prefix used.

It is important to supply suitable C compiler flags (cflags) that match the kernel image’s so the symbol table can be linked or loaded.

12.2.2. 2-Pass Linking

2-Pass linking is used to embed a symbol table in a base kernel image. The first link pass is a normal RTEMS kernel link process. The link output is passed to the rtems-syms command and the -e or --embed option is used. The symbol table object file created by rtems-syms is added to the linker command used in the first pass to create the second pass. The address map will change between the first pass and second pass without causing a problem, the symbol table embedded in the second link pass will adjust the symbol addresses to match.

12.2.3. Command

rtems-syms [options] kernel

-V, --version

Display the version information and then exit.

-v, --verbose

Increase the verbose level by 1. The option can be used more than once to get more detailed trace and debug information.

-w, --warn

Enable build warnings. This is useful when debugging symbol table generation.

-k, --keep

Do not delete temporary files on exit, keep them.

-e, --embed

Create a symbol table that can be embedded in the base kernel image using a 2-pass link process.

-S, --symc

Specify the symbol’s C source file. The defautl is to use a temporary file name.

-o, --output

Specify the ELF output file name.

-m, --map

Create a map file using the provided file name.

-C, --cc

Specify the C compile executable file name. The file can be absolute and no path is search or relative and the environment’s path is searched.

-E, --exec-prefix

Specify the RTEMS tool prefix. For example for RTEMS 5 and the SPARC architecture the prefix is sparc-rtems5.

-c, --cflags

Specify the C compiler flags used to build the symbol table with. These should be the same or compatible with the flags used to build the RTEMS kernel.

-?, -h

Reort the usage help.

12.2.4. Examples

Create a dynamlically loaded symbol table for the minimum.exe sample program for the i386/pc686 BSP:

$ rtems-syms -o ms.o i386-rtems5/c/pc686/testsuites/samples/minimum/minimum.exe
$ file ms.o
ms.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped

Run the same command, this time create a map file:

$ rtems-syms -o ms.o -m ms.map i386-rtems5/c/pc686/testsuites/samples/minimum/minimum.exe
$ head -10 ms.map
RTEMS Kernel Symbols Map
 kernel: i386-rtems5/c/pc686/testsuites/samples/minimum/minimum.exe

Globals:
 No.  Index Scope      Type        SHNDX  Address    Size    Name
    0   931 STB_GLOBAL STT_OBJECT      11 0x0012df08       4 BSPBaseBaud   (minimum.exe)
    1  1124 STB_GLOBAL STT_OBJECT      11 0x0012d894       4 BSPPrintkPort   (minimum.exe)
    2   836 STB_GLOBAL STT_FUNC         1 0x00104b00     302 BSP_dispatch_isr   (minimum.exe)
    3  1156 STB_GLOBAL STT_FUNC         1 0x001082d0      92 BSP_install_rtems_shared_irq_handler   (minimum.exe)
    4   876 STB_GLOBAL STT_FUNC         1 0x00106500     138 BSP_outch   (minimum.exe)

Run the same command with a raise verbose level to observe the stages the command performs:

$ rtems-syms -vvv -o ms.o i386-rtems5/c/pc686/testsuites/samples/minimum/minimum.exe
RTEMS Kernel Symbols 5.a72a462adc18
kernel: i386-rtems5/c/pc686/testsuites/samples/minimum/minimum.exe
cache:load-sym: object files: 1
cache:load-sym: symbols: 1043
symbol C file: /tmp/rld--X7paaa.c
symbol O file: ms.o
execute: i386-rtems5-gcc -O2 -c -o ms.o /tmp/rld--X7paaa.c
execute: status: 0