Versions of ld available

We need the syntax -l:filename if we find shared libraries using -L and want to use arbitrary file names for them. I list the Intel linkers here in case someone wants to try, for example, a quick test of new linker script techniques without setting up the cross-compilation environment.

Required command-line options

Driver control

C/C++ compilation

Assembly code

Static linker

Some of these options need to be prefaced with "-Wl,". However a series of options can be given following a single preface, e.g., "-Wl,-soname=foo,--hash-style=gnu,-zcombreloc".

Option	Needs -Wl,	Notes
-shared		To make a shared object.
-e or --entry		Specifies the entry point (use 0 if there is none).
-soname		Make sure SO names are used in needed lists.
-zcombreloc		Combine relocation entries into one or two tables and sort them by the index of the symbol referred to. This allows symbol lookups to be cached by the dynamic linker for efficiency.
--zmax-page-size=4096		This is the page size used in the Run Time Support Region, the part of memory into which shared objects are loaded. Controls the alignment of segments.
--hash-style=gnu		Provides more efficient symbol lookup.
-l:libname		Refer to a shared library libname that's to be found using the search path established using -L options.
-L dirname		Add dirname to the search path for ld, which uses the path for two purposes: to find libraries named using -l: and to find linker scripts named in include statements in other linker scripts (and named using -T).
-T scriptname		Use scriptname as the main linker script.
--no-undefined		Makes it an error if the resulting shared object has undefined references remaining after ld has finished making it. It allows needed objects to contain undefined references but presumably these too have been constructed using --no-undefined.

File names, sonames and the needed list

A shared library has both a file name and a so-called SO (shared object) name, or soname. The two need not be related as the soname can be set using the ld option -soname. The soname is contained in the dynamic string table at an offset given by the DT_SONAME entry in the dynamic section. Other entries of type DT_NEEDED, which also contain offsets into the dynamic string table, specify the needed list for the shared library. These are the shared libraries that must also be loaded for the shared library to work. Standard convention allows the needed list to contain both file names and sonames but we'll use only sonames.

File name conventions

File names need not begin with "lib"; we use -l: or just name the shared object file as input. We use the following extensions for the different kinds of shared objects:

• .svt for symbol-value tables.
• .exe for tasks.
• .drv for device drivers.
• .so for everything else.

Soname and needed list conventions

1. Every shared object must have a soname.
2. Sonames must be legal C identifiers.
3. Sonames must be no longer than 128 characters.
4. The needed list for an shared object must contain a reference to each other object on which it depends.
5. A shared object must have no undefined references left after linking with ld.
6. The following sonames are reserved:
   1. "RTEMS___" for the shared object containing RTEMS, newlib and other run-time support.
   2. "SYS" for the shared object containing the System Name Table.
   3. "APP" for the shared object containing the Application Name Table.

In general a shared object is found by using Svt_Translate() on the soname in order to obtain the full path name, where the soname is gotten from a needed list. The Symbol-Value tables used by Svt_Translate() are created by compiling C code which is why the sonames must be legal C identifiers. The length restriction on sonames comes from the RTEMS dynamic linker which for efficiency's sake uses a fixed-size buffer to create SVT keys from sonames. Sonames are also the keys used for the database of installed objects.

In order to have a shared object satisfy the requirements listed above we need to use -soname when building every shared object. Then whether you include the object as an input or search for it using -L and -l:, ld will put the soname on the needed list. Referencing an object that lacks an embedded soname will result in the file name of the object being put on the needed list, and that will cause the lookup with Svt_Translate() to fail.

All the direct dependencies of the shared object being built should be searched or included as inputs, and undefined references should cause the build to fail (--no-undefined). Almost always you should name only the objects to which the one being built makes symbolic references, as ld by default puts every object so named on the needed list. If you have to you can get ld to filter out those objects that don't satisfy symbolic references by using the option --as-needed early in the command line.  You can turn this mode off in the rare cases in which you really need an object that doesn't satisfy some symbolic reference: use --no-as-needed. If you need to build an object a.so that might make symbolic references to b.so, c.so or d.so, and which doesn't make such references to e.so but still needs it, you command line would look something like this:

arm-rtemsx.yy-g++ -shared -o a.so -soname A ... a.o  --as-needed b.so c.so d.so --no-as-needed e.so

Dynamic symbol table

1. Every shared object must have a GNU-style hash table (--hash-style=gnu). System V hash tables will be ignored and should not be generated.
2. Certain symbols are used for data and functions used by the dynamic linker. Currently these are:
   1. "lnk_preferences" which labels object preference data to be passed to lnk_prelude().
   2. "lnk_prelude" which labels a function to be called by the dynamic linker just after the functions pointed to by the .init_array have been run.

The special symbols must have global visibility in order for the dynamic linker to see them. However, it treats them as strictly local definitions and won't make cross-object references with them.

The ARM cross-compilers use .init_array to hold pointers to compiler-generated functions that run constructors for statically allocated C++ objects. Therefore .init_array is processed before calling lnk_prelude(). However, lnk_prelude() can't itself be called using the .init_array mechanism because it takes arguments, it returns a status value and there's no way to control the ordering of entries in .init_array.

RTEMS shared object

The shared object containing RTEMS, newlib and other run-time support is unique in that it has to be loaded at a fixed location, the start of the RTS Region. The loading is done by U-Boot which looks at the physical addresses of the loadable segments in order to determine where to put them. In order to set the physical addresses properly the linker script for the RTEMS object assigns all output to a region of memory, defined using the MEMORY directive, whose origin is at the required location. This has the side effect of giving the lowest loadable segment in the shared object a non-zero starting virtual address which has to be taken into account when using symbols defined by the object. All other shared objects are created with virtual addresses starting at zero.