LD: Output Section Keywords

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3.6.6 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 __CTOR_END__ marks the end. Similarly, __DTOR_LIST__ and __DTOR_END__ mark the start and end of the global destructors. 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_BY_NAME(CONSTRUCTORS)’ instead. When using the .ctors and .dtors sections, use ‘*(SORT_BY_NAME(.ctors))’ and ‘*(SORT_BY_NAME(.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.

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