@ifinfo
This file documents the @sc{gnu} linker LD.
-Copyright (C) 1991, 92, 93, 94, 95, 1996 Free Software Foundation, Inc.
+Copyright (C) 1991, 92, 93, 94, 95, 96, 1997 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
@end tex
@vskip 0pt plus 1filll
-Copyright @copyright{} 1991, 92, 93, 94, 95, 1996 Free Software Foundation, Inc.
+Copyright @copyright{} 1991, 92, 93, 94, 95, 96, 1997 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
@item -E
@itemx --export-dynamic
When creating a dynamically linked executable, add all symbols to the
-dynamic symbol table. Normally, the dynamic symbol table contains only
-symbols which are used by a dynamic object. This option is needed for
-some uses of @code{dlopen}.
+dynamic symbol table. The dynamic symbol table is the set of symbols
+which are visible from dynamic objects at run time.
+
+If you do not use this option, the dynamic symbol table will normally
+contain only those symbols which are referenced by some dynamic object
+mentioned in the link.
+
+If you use @code{dlopen} to load a dynamic object which needs to refer
+back to the symbols defined by the program, rather than some other
+dynamic object, then you will probably need to use this option when
+linking the program itself.
@kindex -f
@kindex --auxiliary
implementation of certain functions, perhaps for debugging or for
machine specific performance.
+This option may be specified more than once. The DT_AUXILIARY entries
+will be created in the order in which they appear on the command line.
+
@kindex -F
@kindex --filter
@item -F @var{name}
necessary. This may be required if @code{ld} runs out of memory space
while linking a large executable.
+@kindex --no-warn-mismatch
+@item --no-warn-mismatch
+Normally @code{ld} will give an error if you try to link together input
+files that are mismatched for some reason, perhaps because they have
+been compiled for different processors or for different endiannesses.
+This option tells @code{ld} that it should silently permit such possible
+errors. This option should only be used with care, in cases when you
+have taken some special action that ensures that the linker errors are
+inappropriate.
+
@kindex --no-whole-archive
@item --no-whole-archive
Turn off the effect of the @code{--whole-archive} option for subsequent
supported. Display which input files can and cannot be opened. Display
the linker script if using a default builtin script.
+@kindex --version-script=@var{version-scriptfile}
+@cindex version script, symbol versions
+@itemx --version-script=@var{version-scriptfile}
+Specify the name of a version script to the linker. This is typically
+used when creating shared libraries to specify additional information
+about the version heirarchy for the library being created. This option
+is only meaningful on ELF platforms which support shared libraries.
+@xref{Version Script}.
+
@kindex --warn-comon
@cindex warnings, on combining symbols
@cindex combining symbols, warnings on
* SECTIONS:: SECTIONS Command
* PHDRS:: PHDRS Command
* Entry Point:: The Entry Point
+* Version Script:: Version Script
* Option Commands:: Option Commands
@end menu
@cindex memory region attributes
@item (@var{attr})
-is an optional list of attributes, permitted for compatibility with the
-AT&T linker but not used by @code{ld} beyond checking that the
-attribute list is valid. Valid attribute lists must be made up of the
-characters ``@code{LIRWX}''. If you omit the attribute list, you may
-omit the parentheses around it as well.
+is an optional list of attributes that specify whether to use a
+particular memory to place sections that are not listed in the linker
+script. Valid attribute lists must be made up of the characters
+``@code{ALIRWX}'' that match section attributes. If you omit the
+attribute list, you may omit the parentheses around it as well. The
+attributes currently supported are:
+
+@table @samp
+@item @code{Letter}
+@code{Section Attribute}
+
+@item @code{R}
+Read-only sections.
+
+@item @code{W}
+Read/write sections.
+
+@item @code{X}
+Sections containing executable code.
+
+@item @code{A}
+Allocated sections.
+
+@item @code{I}
+Initialized sections.
+
+@item @code{L}
+Same as @code{I}.
+
+@item @code{!}
+Invert the sense of any of the following attributes.
+@end table
@kindex ORIGIN =
@kindex o =
@end table
For example, to specify that memory has two regions available for
-allocation---one starting at 0 for 256 kilobytes, and the other
-starting at @code{0x40000000} for four megabytes:
+allocation---one starting at 0 for 256 kilobytes, and the other starting
+at @code{0x40000000} for four megabytes. The @code{rom} memory region
+will get all sections without an explicit memory register that are
+either read-only or contain code, while the @code{ram} memory region
+will get the sections.
@smallexample
@group
MEMORY
@{
- rom : ORIGIN = 0, LENGTH = 256K
- ram : org = 0x40000000, l = 4M
+ rom (rx) : ORIGIN = 0, LENGTH = 256K
+ ram (!rx) : org = 0x40000000, l = 4M
@}
@end group
@end smallexample
@noindent
@var{secname} is the name of the output section, and @var{contents} a
specification of what goes there---for example, a list of input files or
-sections of input files (@pxref{Section Placement}). As you might
-assume, the whitespace shown is optional. You do need the colon
-@samp{:} and the braces @samp{@{@}}, however.
+sections of input files (@pxref{Section Placement}). The whitespace
+around @var{secname} is required, so that the section name is
+unambiguous. The other whitespace shown is optional. You do need the
+colon @samp{:} and the braces @samp{@{@}}, however.
@var{secname} must meet the constraints of your output format. In
formats which only support a limited number of sections, such as
@item @var{filename}( @var{section} )
@itemx @var{filename}( @var{section} , @var{section}, @dots{} )
@itemx @var{filename}( @var{section} @var{section} @dots{} )
-You can name one or more sections from your input files, for
-insertion in the current output section. If you wish to specify a list
-of input-file sections inside the parentheses, you may separate the
-section names by either commas or whitespace.
+You can name one or more sections from your input files, for insertion
+in the current output section. If you wish to specify a list of
+input-file sections inside the parentheses, separate the section names
+with whitespace.
@cindex input sections to output section
@kindex *(@var{section})
@kindex SHORT(@var{expression})
@kindex LONG(@var{expression})
@kindex QUAD(@var{expression})
+@kindex SQUAD(@var{expression})
@cindex direct output
@item BYTE(@var{expression})
@itemx SHORT(@var{expression})
@itemx LONG(@var{expression})
@itemx QUAD(@var{expression})
+@itemx SQUAD(@var{expression})
By including one of these four statements in a section definition, you
-can explicitly place one, two, four, or eight bytes (respectively) at
-the current address of that section. @code{QUAD} is only supported when
-using a 64 bit host or target.
+can explicitly place one, two, four, eight unsigned, or eight signed
+bytes (respectively) at the current address of that section. When using
+a 64 bit host or target, @code{QUAD} and @code{SQUAD} are the same.
+When both host and target are 32 bits, @code{QUAD} uses an unsigned 32
+bit value, and @code{SQUAD} sign extends the value. Both will use the
+correct endianness when writing out the value.
@ifclear SingleFormat
Multiple-byte quantities are represented in whatever byte order is
@cindex prevent unnecessary loading
@cindex loading, preventing
@item (NOLOAD)
-Use @samp{(NOLOAD)} to prevent a section from being loaded into memory
-each time it is accessed. For example, in the script sample below, the
-@code{ROM} segment is addressed at memory location @samp{0} and does not
-need to be loaded into each object file:
+The @samp{(NOLOAD)} directive will mark a section to not be loaded at
+run time. The linker will process the section normally, but will mark
+it so that a program loader will not load it into memory. For example,
+in the script sample below, the @code{ROM} section is addressed at
+memory location @samp{0} and does not need to be loaded when the program
+is run. The contents of the @code{ROM} section will appear in the
+linker output file as usual.
@smallexample
@group
start = other_symbol ;
@end smallexample
+@node Version Script
+@section Version Script
+@kindex VERSION @{script text@}
+@cindex symbol versions
+@cindex version script
+@cindex versions of symbols
+The linker command script includes a command specifically for
+specifying a version script, and is only meaningful for ELF platforms
+that support shared libraries. A version script can be
+build directly into the linker script that you are using, or you
+can supply the version script as just another input file to the linker
+at the time that you link. The command script syntax is:
+@smallexample
+VERSION @{ version script contents @}
+@end smallexample
+The version script can also be specified to the linker by means of the
+@samp{--version-script} linker command line option.
+Version scripts are only meaningful when creating shared libraries.
+
+The format of the version script itself is identical to that used by
+Sun's linker in Solaris 2.5. Versioning is done by defining a tree of
+version nodes with the names and interdependencies specified in the
+version script. The version script can specify which symbols are bound
+to which version nodes, and it can reduce a specified set of symbols to
+local scope so that they are not globally visible outside of the shared
+library.
+
+The easiest way to demonstrate the version script language is with a few
+examples.
+
+@smallexample
+VERS_1.1 @{
+ global:
+ foo1;
+ local:
+ old*;
+ original*;
+ new*;
+@};
+
+VERS_1.2 @{
+ foo2;
+@} VERS_1.1;
+
+VERS_2.0 @{
+ bar1; bar2;
+@} VERS_1.2;
+@end smallexample
+
+In this example, three version nodes are defined. @samp{VERS_1.1} is the
+first version node defined, and has no other dependencies. The symbol
+@samp{foo1} is bound to this version node, and a number of symbols
+that have appeared within various object files are reduced in scope to
+local so that they are not visible outside of the shared library.
+
+Next, the node @samp{VERS_1.2} is defined. It depends upon
+@samp{VERS_1.1}. The symbol @samp{foo2} is bound to this version node.
+
+Finally, the node @samp{VERS_2.0} is defined. It depends upon
+@samp{VERS_1.2}. The symbols @samp{bar1} and @samp{bar2} are bound to
+this version node.
+
+Symbols defined in the library which aren't specifically bound to a
+version node are effectively bound to an unspecified base version of the
+library. It is possible to bind all otherwise unspecified symbols to a
+given version node using @samp{global: *} somewhere in the version
+script.
+
+Lexically the names of the version nodes have no specific meaning other
+than what they might suggest to the person reading them. The @samp{2.0}
+version could just as well have appeared in between @samp{1.1} and
+@samp{1.2}. However, this would be a confusing way to write a version
+script.
+
+When you link an application against a shared library that has versioned
+symbols, the application itself knows which version of each symbol it requires,
+and it also knows which version nodes it needs from each shared library it is
+linked against. Thus at runtime, the dynamic loader can make a quick check to
+make sure that the libraries you have linked against do in fact supply all
+of the version nodes that the application will need to resolve all of the
+dynamic symbols. In this way it is possible for the dynamic linker to know
+with certainty that all external symbols that it needs will be resolvable
+without having to search for each symbol reference.
+
+The symbol versioning is in effect a much more sophisticated way of
+doing minor version checking that SunOS does. The fundamental problem
+that is being addressed here is that typically references to external
+functions are bound on an as-needed basis, and are not all bound when
+the application starts up. If a shared library is out of date, a
+required interface may be missing; when the application tries to use
+that interface, it may suddenly and unexpectedly fail. With symbol
+versioning, the user will get a warning when they start their program if
+the libraries being used with the application are too old.
+
+There are several GNU extensions to Sun's versioning approach. The
+first of these is the ability to bind a symbol to a version node in the
+source file where the symbol is defined instead of in the versioning
+script. This was done mainly to reduce the burden on the library
+maintainer. This can be done by putting something like:
+
+@smallexample
+__asm__(".symver original_foo,foo@@VERS_1.1");
+@end smallexample
+
+in the C source file. This renamed the function @samp{original_foo} to
+be an alias for @samp{foo} bound to the version node @samp{VERS_1.1}.
+The @samp{local:} directive can be used to prevent the symbol
+@samp{original_foo} from being exported.
+
+The second GNU extension is to allow multiple versions of the same function
+to appear in a given shared library. In this way an incompatible change to
+an interface can take place without increasing the major version number of
+the shared library, while still allowing applications linked against the old
+interface to continue to function.
+
+This can only be accomplished by using multiple @samp{.symver}
+directives in the assembler. An example of this would be:
+
+@smallexample
+__asm__(".symver original_foo,foo@@");
+__asm__(".symver old_foo,foo@@VERS_1.1");
+__asm__(".symver old_foo1,foo@@VERS_1.2");
+__asm__(".symver new_foo,foo@@@@VERS_2.0");
+@end smallexample
+
+In this example, @samp{foo@@} represents the symbol @samp{foo} bound to the
+unspecified base version of the symbol. The source file that contains this
+example would define 4 C functions: @samp{original_foo}, @samp{old_foo},
+@samp{old_foo1}, and @samp{new_foo}.
+
+When you have multiple definitions of a given symbol, there needs to be
+some way to specify a default version to which external references to
+this symbol will be bound. This can be accomplished with the
+@samp{foo@@@@VERS_2.0} type of @samp{.symver} directive. Only one version of
+a symbol can be declared 'default' in this manner - otherwise you would
+effectively have multiple definitions of the same symbol.
+
+If you wish to bind a reference to a specific version of the symbol
+within the shared library, you can use the aliases of convenience
+(i.e. @samp{old_foo}), or you can use the @samp{.symver} directive to
+specifically bind to an external version of the function in question.
+
@node Option Commands
@section Option Commands
The command language includes a number of other commands that you can
to make @code{ld} assign space to common symbols even if a relocatable
output file is specified (@samp{-r}).
+@kindex INCLUDE @var{filename}
+@cindex including a linker script
+@item INCLUDE @var{filename}
+Include the linker script @var{filename} at this point. The file will
+be searched for in the current directory, and in any directory specified
+with the @code{-L} option. You can nest calls to @code{INCLUDE} up to
+10 levels deep.
+
@kindex INPUT ( @var{files} )
@cindex binary input files
@item INPUT ( @var{file}, @var{file}, @dots{} )
projects / binutils-gdb.git / commitdiff