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* ld.texinfo: Clarify the CONSTRUCTORS command.

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Ian Lance Taylor 1996-05-29 17:14:24 +00:00
parent 3308a10725
commit a1d393cfb9
2 changed files with 58 additions and 12 deletions
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ld/ChangeLog
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@ -1,3 +1,7 @@
Wed May 29 13:13:35 1996 Ian Lance Taylor <ian@cygnus.com>
* ld.texinfo: Clarify the CONSTRUCTORS command.
Thu May 23 16:07:44 1996 Ian Lance Taylor <ian@cygnus.com>
* emultempl/aix.em (gld${EMULATION_NAME}_read_file): Initialize

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ld/ld.texinfo
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@ -340,6 +340,17 @@ the @code{GNUTARGET} environment variable) are more flexible, but
written to call the old linker.
@end ifclear
@kindex --force-exe-suffix
@item --force-exe-suffix
Make sure that an output file has a .exe suffix.
If a successfully built fully linked output file does not have a
@code{.exe} or @code{.dll} suffix, this option forces the linker to copy
the output file to one of the same name with a @code{.exe} suffix. This
option is useful when using unmodified Unix makefiles on a Microsoft
Windows host, since some versions of Windows won't run an image unless
it ends in a @code{.exe} suffix.
@kindex -g
@item -g
Ignored. Provided for compatibility with other tools.
@ -2481,18 +2492,49 @@ command-line options.
@cindex C++ constructors, arranging in link
@cindex constructors, arranging in link
@item CONSTRUCTORS
This command ties up C++ style constructor and destructor records. The
details of the constructor representation vary from one object format to
another, but usually lists of constructors and destructors appear as
special sections. The @code{CONSTRUCTORS} command specifies where the
linker is to place the data from these sections, relative to the rest of
the linked output. Constructor data is marked by the symbol
@w{@code{__CTOR_LIST__}} at the start, and @w{@code{__CTOR_LIST_END}} at
the end; destructor data is bracketed similarly, between
@w{@code{__DTOR_LIST__}} and @w{@code{__DTOR_LIST_END}}. (The compiler
must arrange to actually run this code; @sc{gnu} C++ calls constructors from
a subroutine @code{__main}, which it inserts automatically into the
startup code for @code{main}, and destructors from @code{_exit}.)
When linking using the @code{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 @code{ECOFF} and @code{XCOFF}, the linker
will automatically recognize C++ global constructors and destructors by
name. For these object file formats, the @code{CONSTRUCTORS} command
tells the linker where this information should be placed. The
@code{CONSTRUCTORS} command is ignored for other object file formats.
The symbol @w{@code{__CTOR_LIST__}} marks the start of the global
constructors, and the symbol @w{@code{__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 @sc{gnu} C++ calls constructors from a subroutine @code{__main};
a call to @code{__main} is automatically inserted into the startup code
for @code{main}. @sc{gnu} C++ runs destructors either by using
@code{atexit}, or directly from the function @code{exit}.
For object file formats such as @code{COFF} or @code{ELF} which support
multiple sections, @sc{gnu} C++ will normally arrange to put the
addresses of global constructors and destructors into the @code{.ctors}
and @code{.dtors} sections. Placing the following sequence into your
linker script will build the sort of table which the @sc{gnu} C++
runtime code expects to see.
@smallexample
__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__ = .;
@end smallexample
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.
@need 1000
@kindex FLOAT