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(ld.info)Output Section Keywords

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Output section keywords
-----------------------

   There are a couple of keywords which can appear as output section
commands.

`CREATE_OBJECT_SYMBOLS'
     The command tells the linker to create a symbol for each input
     file.  The name of each symbol will be the name of the
     corresponding input file.  The section of each symbol will be the
     output section in which the `CREATE_OBJECT_SYMBOLS' command
     appears.

     This is conventional for the a.out object file format.  It is not
     normally used for any other object file format.

`CONSTRUCTORS'
     When linking using the a.out object file format, the linker uses an
     unusual set construct to support C++ global constructors and
     destructors.  When linking object file formats which do not support
     arbitrary sections, such as ECOFF and XCOFF, the linker will
     automatically recognize C++ global constructors and destructors by
     name.  For these object file formats, the `CONSTRUCTORS' command
     tells the linker to place constructor information in the output
     section where the `CONSTRUCTORS' command appears.  The
     `CONSTRUCTORS' command is ignored for other object file formats.

     The symbol `__CTOR_LIST__' marks the start of the global
     constructors, and the symbol `__DTOR_LIST' marks the end.  The
     first word in the list is the number of entries, followed by the
     address of each constructor or destructor, followed by a zero
     word.  The compiler must arrange to actually run the code.  For
     these object file formats GNU C++ normally calls constructors from
     a subroutine `__main'; a call to `__main' is automatically
     inserted into the startup code for `main'.  GNU C++ normally runs
     destructors either by using `atexit', or directly from the function
     `exit'.

     For object file formats such as `COFF' or `ELF' which support
     arbitrary section names, GNU C++ will normally arrange to put the
     addresses of global constructors and destructors into the `.ctors'
     and `.dtors' sections.  Placing the following sequence into your
     linker script will build the sort of table which the GNU C++
     runtime code expects to see.

                __CTOR_LIST__ = .;
                LONG((__CTOR_END__ - __CTOR_LIST__) / 4 - 2)
                *(.ctors)
                LONG(0)
                __CTOR_END__ = .;
                __DTOR_LIST__ = .;
                LONG((__DTOR_END__ - __DTOR_LIST__) / 4 - 2)
                *(.dtors)
                LONG(0)
                __DTOR_END__ = .;

     If you are using the GNU C++ support for initialization priority,
     which provides some control over the order in which global
     constructors are run, you must sort the constructors at link time
     to ensure that they are executed in the correct order.  When using
     the `CONSTRUCTORS' command, use `SORT(CONSTRUCTORS)' instead.
     When using the `.ctors' and `.dtors' sections, use
     `*(SORT(.ctors))' and `*(SORT(.dtors))' instead of just
     `*(.ctors)' and `*(.dtors)'.

     Normally the compiler and linker will handle these issues
     automatically, and you will not need to concern yourself with
     them.  However, you may need to consider this if you are using C++
     and writing your own linker scripts.