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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 . Under such a system there [ELF]. There are two kinds of linking, or binding pieces of compiled code into a single functioning whole. The first kind of linking, " static " linking, creates the shared objects from object the ".o" files produced by the compiler. The second kind of linking, " dynamic " linking, loads shared objects into memory on demand while the program is already running. As the name implies a single copy of a shared object's code and data may be used by a number of other shared objects which reduces the use of memory. There is also a potential reduction gained from loading only those shared objects that contain what's actually needed.
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.
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The "run" command allows you to specify the Task name (up to four characters), scheduling priority and the size of the stack ( in bytes).
From C or C++ code
The C function lnk_load() is the main entry point of the dynamic linker. You give it the name of the shared object you want to load. The linker will attempt to find it and any other shared objects it needs according to the rules laid out in the next section.
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- The shared object whose name you give as an argument is never installed.
- If that shared object depends upon others then such of those others that are not already loaded are loaded.
- Shared objects that are loaded as dependencies are installed if they request it and if they are not already installed.
- A shared object isn't allowed to depend on list itself as a dependency.
Each shared object has a name, the so-called SO name or soname, used as follows:
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Notice that the soname "system:rtems.so" has two components separated by a colon. The first component, "system" give the namespace to which the shared object belongs. Objects wiin in the same namespace are found in the same place. Where that place is is determined by the system's namespace table; each namespace name is associated with a directory in the RTEMS filesystem. The second component of the soname, "rtems.so" is the filename of the shared object after all directories have been removed. If the system namespace table associates "/mnt/rtems" with the namespace "system" then "system:rtems.so" will be looked for at /mnt/rtems/rtems.so when the time comes to load it.
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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 is '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, taking the first definition that it finds. It doesn't check for duplication. The linker then puts the address of the definition it found into all the places in the first shared object that require it.
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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 take upneed. Dynamic linking has to allocate space for these variables and, in accordance with the C and C++ standards, initialize that space to all zeros.
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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 but with . With the "section" attribute you can place pointers of your own in the .init section.
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Shared objects to be run on a DAT system can 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.
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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. ThereIn 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.
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