Overview
The DAT group has divided RTEMS and DAT support code into a number of "dynamically shared objects" which are structurally the same as the shared libraries one uses under Linux [ELF]
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The system boot loader loads and starts the principle shared object containing RTEMS and the DAT dynamic linker. Then the application initialization code written by the DAT group uses the dynamic linker to load the remaining shared objects specified by the system configuration, itself contained in several shared objects.
Using the dynamic linker
The dynamic linker is available for use in applications as well as in system start-up.
From the RTEMS shell
The shell provides the "run" command in order to let you run a special kind of shared object called a Task. What makes a Task special is the way its code is organized and the name of the entry point. Most of the organization is taken care of by including a special Task-stub library when you create the shared object.
The "run" command allows you to specify the Task name (up to four characters), scheduling priority and the size of the stack in bytes [run]
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From C or C++ code
The C function lnk_load() is the main entry point of the dynamic linker [linkerCall]
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SO names, dependencies, namespaces and installation
Whether you call the linker from the shell or from your own code it obeys the same rules:
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Other shared objects besides system:rtems.so may be installed. In general, a shared object that the dynamic linker loads in order to satisfy a dependency it will also install if the shared object requests it. In such a case that single loaded copy is used whenever named as a dependency not only in that run of the linker but in all subsequent runs. In any given run of the linker it will load any object only once, but those objects that are not installed will be re-loaded if need be in subsequent runs, ignoring any copies loaded in previous runs. In this case shared objects are really unshared.
Resolving symbolic references
Though each shared object contains a list of the sonames of all the other shared objects it depends on, the so-called needed-list, this doesn't tell the dynamic linker just what the object requires from the other objects. It could be user data, functions, classes, etc. Each of these items has a symbolic name which appears in a symbol table built into the shared object that defines them. The object that needs to use them also has the symbolic names in its table, though marked as "undefined", that is to say "not defined in this shared object". There's no indication of which shared object is the definer. Instead, for each undefined symbol in a newly-loaded object the dynamic linker searches for a definition in each of the objects on the first object's needed-list [scope]
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Initialization of newly loaded shared objects
The following initializations are performed in the order listed here. They're only performed once when the object is loaded.
Uninitialized variables
A shared object may define statically allocated variables that are given no explicit initial values in source code. Such variables take up no space in the shared object file; there's only a count of how much space they need. Dynamic linking has to allocate space for these variables and, in accordance with the C and C++ standards, initialize that space to all zeros.
The .init section and C++ static constructors
A shared object is divided into many named "sections" some of which have special meaning for the dynamic linker. One of these is named ".init" and contains pointers to functions that must be called before the shared object can be considered usable. Normally the .init section is filled by the compiler and contains pointers to functions that run static C++ constructors. With the "section" attribute [attr]
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Installation
If the shared object was loaded as a dependency, contains a global variable named lnk_options and that variable's (integer) value has the bit LNK_INSTALL set then the shared object's soname and location are recorded in the table of installed objects.
The prelude and preferences functions
Shared objects to be run on a DAT system may have two optional initialization functions which if present are called by the dynamic linker. The first, lnk_preferences() , returns a 32-bit preference datum which may be an int or a pointer and is passed as an argument to lnk_prelude(). The latter function is a general initialization function designed to be more easily used than the .init section. Since most other initialization for the shared object has been done, lnk_prelude() can do most anything: start tasks, load other shared objects, print messages, etc.
Memory access permissions
A shared object file's loadable content is divided into a small number of "segments". Each segment has a set of permission flags: (R)eadable, (W)riteable ans e(X)ecutable. In shared objects built for DAT systems there's normally one RX segment containing instructions and read-only data and one RW section containing non-constant data. The dynamic linker uses the CPU's memory management unit (MMU) to set the access type of the memory allocated to each segment to match the segment's permission flags.
Linker actions step-by-step example
Suppose you use the dynamic linker to load a shared object A, which makes use of objects B and D. B uses object C which in turn uses E. D and E use no other objects. Then the needed-lists in the files containing these objects may look like this (the ordering depends on the order in which the shared objects are mentioned on the command lines used with the static linker):
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Note that object A, because it's the object whose name is passed to the linker as an argument, is not installed.
Notes
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