= RTEMS Trace Linker =

[[TOC(Developer/Tracing/Trace_Linker , depth=2)]]

The RTEMS Trace Linker is a tool that is part of the [wiki:Developer/Tools RTEMS Tool Project] and is central in the [wiki:Developer/Tracing RTEMS Tracing] framework.

The RTEMS Trace Linker is a post link tool that performs a link which produces a trace executable. A trace executable has been instrumented with additional code that provides software tracing capabilities. A key requirement of the trace process in RTEMS is to take existing code in a compiled format and to instrument it without rebuilding that code from source annotating it with trace code. The code being trace may be from a 3rd party or already certified or in the process of being certified and the trace testing is part of that process. 

The [wiki:Developer/Tools/RSB RTEMS Source Builder] builds and installs the [wiki:Developer/Tools RTEMS Tools] package so the RTEMS Trace Linker is available for you to use.

== Command Line ==

The current command line for the trace linker is:

{{{
$ rtems-tld -h
rtems-trace-ld [options] objects
Options and arguments:
 -h          : help (also --help)
 -V          : print linker version number and exit (also --version)
 -v          : verbose (trace import parts), can supply multiple times
               to increase verbosity (also --verbose)
 -w          : generate warnings (also --warn)
 -k          : keep temporary files (also --keep)
 -c compiler : target compiler is not standard (also --compiler)
 -l linker   : target linker is not standard (also --linker)
 -E prefix   : the RTEMS tool prefix (also --exec-prefix)
 -f cflags   : C compiler flags (also --cflags)
 -r path     : RTEMS path (also --rtems)
 -B bsp      : RTEMS arch/bsp (also --rtems-bsp)
 -W wrapper  : wrapper file name without ext (also --wrapper)
 -C ini      : user configuration INI file (also --config)
}}}

There are two parts to the command line passed to the trace linker. The first part controls the trace linker and provides the various options it needs and the second part is a standard linker command line you would use to link an RTEMS application. The first and second parts are separated by `--` the command line option escape sequence.

The trace linker generate code that needs to be compiled and linked your standard executable so it needs to know the target compiler and `CFLAGS`. There are a couple of ways to do this. The simplest is to provide the path to RTEMS using the `-r` option and the architecture and BSP name in the standard RTEMS format of `arch/bsp`. The trace linker will extract the compiler and flags used to build RTEMS and will use them. If you require specific options you can use the `-f`, `-c`, `-l` and `-E` options to provide them.

The trace linker requires you provide a configuration file using the `-C` or `--config`  option. This is an INI detailed in the Configuration section.

== Configuration ==

The trace linker uses the INI file format for configuration files. Users provide a top level configuration that defines the trace executable created.  The trace linker comes with a number of standard configurations that provide a range of functionality. A user can use those configurations or they can define a completely new set and produce a localised specific form or trace executable.

=== INI Files ===

[http://en.wikipedia.org/wiki/INI_file INI files format] consist of sections with a section name that groups keys A key has a name and value used as `name=value`. An example format is:

{{{
[section]
name1=value1
name2 = value2
}}}

The use of keys depends on the user of the INI file and in our case this is the trace linker. The trace linker can include other INI files using the `include` key name and a comma separated list of files to include:

{{{
include = rtems.ini, rtld-base.ini
}}}

The trace linker also uses values in key to specify other sections. In this example the `functions` name lists `test-trace-funcs` and that section a `headers` key that references a further section `test-headers`:

{{{
functions = test-trace-funcs, rtems-api

[test-trace-funcs]
; Parsed via the 'function-set', not parse as a 'trace'.
headers = test-headers

[test-headers] 
header = '#include "test-trace-1.h"'
}}}

The ability to include INI files and have key lists reference sections lets the trace linker provide base functionality a user can specialise.

=== Tracer Section ===

The top level section is `[tracer]`. The tracer section can contain the following keys:

 name:: A user defined name for the trace.
 bsp:: The BSP as an RTEMS standard `arch/bsp` pair. Currently not used.
 options:: Various trace linker defined options. Currently not used.
 traces:: List of trace sections. A trace section defines how functions are to be instrumented.
 functions:: List function sections. Function sections list the functions to be instrumented.

=== Trace Section ===

A trace section defines how trace wrapper functions are built.

== Wrapping ==

The trace linker's main role is to wrap functions in the existing executable with trace code. The trace linker executable does not know about the trace code added. That is provided by the `generator` configuration. The wrapping function uses a GNU linker option called `--wrap=symbol`. The GNU Ld manual states:

  Use a wrapper function for symbol. Any undefined reference to symbol will be resolved to !__wrap_symbol. Any undefined reference to !__real_symbol will be resolved to symbol.

The trace linker generates C code with a wrapper for each function to be instrumented. The trace code generated is driven by the configuration INI files.