\input texinfo
@setfilename ld.info
@c Copyright 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
-@c 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
+@c 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
@syncodeindex ky cp
+@c man begin INCLUDE
@include configdoc.texi
@c (configdoc.texi is generated by the Makefile)
@include ldver.texi
+@c man end
@c @smallbook
@c Configure for the generation of man pages
@set UsesEnvVars
@set GENERIC
-@set A29K
@set ARC
@set ARM
@set D10V
linker:
@table @gcctabopt
+@include @value{top_srcdir}/../libiberty/at-file.texi
+
@kindex -a@var{keyword}
@item -a@var{keyword}
This option is supported for HP/UX compatibility. The @var{keyword}
@itemize @bullet
@item
-Where object files and symbols are mapped into memory.
+Where object files are mapped into memory.
@item
How common symbols are allocated.
@item
All archive members included in the link, with a mention of the symbol
which caused the archive member to be brought in.
+@item
+The values assigned to symbols.
+
+Note - symbols whose values are computed by an expression which
+involves a reference to a previous value of the same symbol may not
+have correct result displayed in the link map. This is because the
+linker discards intermediate results and only retains the final value
+of an expression. Under such circumstances the linker will display
+the final value enclosed by square brackets. Thus for example a
+linker script containing:
+
+@smallexample
+ foo = 1
+ foo = foo * 4
+ foo = foo + 8
+@end smallexample
+
+will produce the following output in the link map if the @option{-M}
+option is used:
+
+@smallexample
+ 0x00000001 foo = 0x1
+ [0x0000000c] foo = (foo * 0x4)
+ [0x0000000c] foo = (foo + 0x8)
+@end smallexample
+
+See @ref{Expressions} for more information about expressions in linker
+scripts.
@end itemize
@kindex -n
Disallows undefined symbols in object files. Undefined symbols in
shared libraries are still allowed.
+@item execstack
+Marks the object as requiring executable stack.
+
@item initfirst
This option is only meaningful when building a shared object.
It marks the object so that its runtime initialization will occur
@item nodump
Marks the object can not be dumped by @code{dldump}.
+@item noexecstack
+Marks the object as not requiring executable stack.
+
+@item norelro
+Don't create an ELF @code{PT_GNU_RELRO} segment header in the object.
+
@item now
When generating an executable or shared library, mark it to tell the
dynamic linker to resolve all symbols when the program is started, or
@item origin
Marks the object may contain $ORIGIN.
+@item relro
+Create an ELF @code{PT_GNU_RELRO} segment header in the object.
+
@end table
Other keywords are ignored for Solaris compatibility.
on the command line after the @option{--as-needed} option. Normally,
the linker will add a DT_NEEDED tag for each dynamic library mentioned
on the command line, regardless of whether the library is actually
-needed. @option{--as-needed} causes DT_NEEDED tags to only be emitted
-for libraries that satisfy some reference from regular objects.
+needed. @option{--as-needed} causes DT_NEEDED tags to only be emitted
+for libraries that satisfy some symbol reference from regular objects
+which is undefined at the point that the library was linked.
@option{--no-as-needed} restores the default behaviour.
@kindex --add-needed
variants of this option are for compatibility with various systems. You
may use this option multiple times on the command line: it affects
library searching for @option{-l} options which follow it. This
-option also implies @option{--unresolved-symbols=report-all}.
+option also implies @option{--unresolved-symbols=report-all}. This
+option can be used with @option{-shared}. Doing so means that a
+shared library is being created but that all of the library's external
+references must be resolved by pulling in entries from static
+libraries.
@kindex -Bsymbolic
@item -Bsymbolic
@kindex --default-symver
@item --default-symver
Create and use a default symbol version (the soname) for unversioned
-symbols.
+exported symbols.
+
+@kindex --default-imported-symver
+@item --default-imported-symver
+Create and use a default symbol version (the soname) for unversioned
+imported symbols.
@kindex --no-warn-mismatch
@item --no-warn-mismatch
Compute and display statistics about the operation of the linker, such
as execution time and memory usage.
+@kindex --sysroot
+@item --sysroot=@var{directory}
+Use @var{directory} as the location of the sysroot, overriding the
+configure-time default. This option is only supported by linkers
+that were configured using @option{--with-sysroot}.
+
@kindex --traditional-format
@cindex traditional format
@item --traditional-format
file as @code{__wrap_malloc}; if you do, the assembler may resolve the
call before the linker has a chance to wrap it to @code{malloc}.
+@kindex --eh-frame-hdr
+@item --eh-frame-hdr
+Request creation of @code{.eh_frame_hdr} section and ELF
+@code{PT_GNU_EH_FRAME} segment header.
+
@kindex --enable-new-dtags
@kindex --disable-new-dtags
@item --enable-new-dtags
those options are only available for ELF systems.
@kindex --hash-size=@var{number}
+@item --hash-size=@var{number}
Set the default size of the linker's hash tables to a prime number
close to @var{number}. Increasing this value can reduce the length of
time it takes the linker to perform its tasks, at the expense of
@kindex --reduce-memory-overheads
@item --reduce-memory-overheads
This option reduces memory requirements at ld runtime, at the expense of
-linking speed. This was introduced to to select the old O(n^2) algorithm
+linking speed. This was introduced to select the old O(n^2) algorithm
for link map file generation, rather than the new O(n) algorithm which uses
about 40% more memory for symbol storage.
-Another affect of the switch is to set the default hash table size to
+Another effect of the switch is to set the default hash table size to
1021, which again saves memory at the cost of lengthening the linker's
run time. This is not done however if the @option{--hash-size} switch
has been used.
@itemx --subsystem @var{which}:@var{major}.@var{minor}
Specifies the subsystem under which your program will execute. The
legal values for @var{which} are @code{native}, @code{windows},
-@code{console}, and @code{posix}. You may optionally set the
-subsystem version also.
+@code{console}, @code{posix}, and @code{xbox}. You may optionally set
+the subsystem version also. Numeric values are also accepted for
+@var{which}.
[This option is specific to the i386 PE targeted port of the linker]
@end table
new undefined references are created. See the description of @samp{-(}
in @ref{Options,,Command Line Options}.
+@item AS_NEEDED(@var{file}, @var{file}, @dots{})
+@itemx AS_NEEDED(@var{file} @var{file} @dots{})
+@kindex AS_NEEDED(@var{files})
+This construct can appear only inside of the @code{INPUT} or @code{GROUP}
+commands, among other filenames. The files listed will be handled
+as if they appear directly in the @code{INPUT} or @code{GROUP} commands,
+with the exception of ELF shared libraries, that will be added only
+when they are actually needed. This construct essentially enables
+@option{--as-needed} option for all the files listed inside of it
+and restores previous @option{--as-needed} resp. @option{--no-as-needed}
+setting afterwards.
+
@item OUTPUT(@var{filename})
@kindex OUTPUT(@var{filename})
@cindex output file name in linker scripot
@cindex symbol definition, scripts
@cindex variables, defining
You may assign a value to a symbol in a linker script. This will define
-the symbol as a global symbol.
+the symbol and place it into the symbol table with a global scope.
@menu
* Simple Assignments:: Simple Assignments
* PROVIDE:: PROVIDE
+* PROVIDE_HIDDEN:: PROVIDE_HIDDEN
+* Source Code Reference:: How to use a linker script defined symbol in source code
@end menu
@node Simple Assignments
defined, and the value will be adjusted accordingly.
The special symbol name @samp{.} indicates the location counter. You
-may only use this within a @code{SECTIONS} command.
+may only use this within a @code{SECTIONS} command. @xref{Location Counter}.
The semicolon after @var{expression} is required.
If the program references @samp{etext} but does not define it, the
linker will use the definition in the linker script.
+@node PROVIDE_HIDDEN
+@subsection PROVIDE_HIDDEN
+@cindex PROVIDE_HIDDEN
+Similar to @code{PROVIDE}. For ELF targeted ports, the symbol will be
+hidden and won't be exported.
+
+@node Source Code Reference
+@subsection Source Code Reference
+
+Accessing a linker script defined variable from source code is not
+intuitive. In particular a linker script symbol is not equivalent to
+a variable declaration in a high level language, it is instead a
+symbol that does not have a value.
+
+Before going further, it is important to note that compilers often
+transform names in the source code into different names when they are
+stored in the symbol table. For example, Fortran compilers commonly
+prepend or append an underscore, and C++ performs extensive @samp{name
+mangling}. Therefore there might be a discrepancy between the name
+of a variable as it is used in source code and the name of the same
+variable as it is defined in a linker script. For example in C a
+linker script variable might be referred to as:
+
+@smallexample
+ extern int foo;
+@end smallexample
+
+But in the linker script it might be defined as:
+
+@smallexample
+ _foo = 1000;
+@end smallexample
+
+In the remaining examples however it is assumed that no name
+transformation has taken place.
+
+When a symbol is declared in a high level language such as C, two
+things happen. The first is that the compiler reserves enough space
+in the program's memory to hold the @emph{value} of the symbol. The
+second is that the compiler creates an entry in the program's symbol
+table which holds the symbol's @emph{address}. ie the symbol table
+contains the address of the block of memory holding the symbol's
+value. So for example the following C declaration, at file scope:
+
+@smallexample
+ int foo = 1000;
+@end smallexample
+
+creates a entry called @samp{foo} in the symbol table. This entry
+holds the address of an @samp{int} sized block of memory where the
+number 1000 is initially stored.
+
+When a program references a symbol the compiler generates code that
+first accesses the symbol table to find the address of the symbol's
+memory block and then code to read the value from that memory block.
+So:
+
+@smallexample
+ foo = 1;
+@end smallexample
+
+looks up the symbol @samp{foo} in the symbol table, gets the address
+associated with this symbol and then writes the value 1 into that
+address. Whereas:
+
+@smallexample
+ int * a = & foo;
+@end smallexample
+
+looks up the symbol @samp{foo} in the symbol table, gets it address
+and then copies this address into the block of memory associated with
+the variable @samp{a}.
+
+Linker scripts symbol declarations, by contrast, create an entry in
+the symbol table but do not assign any memory to them. Thus they are
+an address without a value. So for example the linker script definition:
+
+@smallexample
+ foo = 1000;
+@end smallexample
+
+creates an entry in the symbol table called @samp{foo} which holds
+the address of memory location 1000, but nothing special is stored at
+address 1000. This means that you cannot access the @emph{value} of a
+linker script defined symbol - it has no value - all you can do is
+access the @emph{address} of a linker script defined symbol.
+
+Hence when you are using a linker script defined symbol in source code
+you should always take the address of the symbol, and never attempt to
+use its value. For example suppose you want to copy the contents of a
+section of memory called .ROM into a section called .FLASH and the
+linker script contains these declarations:
+
+@smallexample
+@group
+ start_of_ROM = .ROM;
+ end_of_ROM = .ROM + sizeof (.ROM) - 1;
+ start_of_FLASH = .FLASH;
+@end group
+@end smallexample
+
+Then the C source code to perform the copy would be:
+
+@smallexample
+@group
+ extern char start_of_ROM, end_of_ROM, start_of_FLASH;
+
+ memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
+@end group
+@end smallexample
+
+Note the use of the @samp{&} operators. These are correct.
+
@node SECTIONS
@section SECTIONS Command
@kindex SECTIONS
@smallexample
@group
@var{section} [@var{address}] [(@var{type})] :
- [AT(@var{lma})] [SUBALIGN(@var{subsection_align})]
+ [AT(@var{lma})] [ALIGN(@var{section_align})] [SUBALIGN(@var{subsection_align})]
@{
@var{output-section-command}
@var{output-section-command}
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
+constructors, and the symbol @w{@code{__CTOR_END__}} marks the end.
+Similarly, @w{@code{__DTOR_LIST__}} and @w{@code{__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
@smallexample
@group
@var{section} [@var{address}] [(@var{type})] :
- [AT(@var{lma})] [SUBALIGN(@var{subsection_align})]
+ [AT(@var{lma})] [ALIGN(@var{section_align})] [SUBALIGN(@var{subsection_align})]
@{
@var{output-section-command}
@var{output-section-command}
@menu
* Output Section Type:: Output section type
* Output Section LMA:: Output section LMA
+* Forced Output Alignment:: Forced Output Alignment
* Forced Input Alignment:: Forced Input Alignment
* Output Section Region:: Output section region
* Output Section Phdr:: Output section phdr
@end group
@end smallexample
+@node Forced Output Alignment
+@subsubsection Forced Output Alignment
+@kindex ALIGN(@var{section_align})
+@cindex forcing output section alignment
+@cindex output section alignment
+You can increase an output section's alignment by using ALIGN.
+
@node Forced Input Alignment
@subsubsection Forced Input Alignment
@kindex SUBALIGN(@var{subsection_align})
@kindex ORIGIN =
@kindex o =
@kindex org =
-The @var{origin} is an expression for the start address of the memory
-region. The expression must evaluate to a constant before memory
-allocation is performed, which means that you may not use any section
-relative symbols. The keyword @code{ORIGIN} may be abbreviated to
-@code{org} or @code{o} (but not, for example, @code{ORG}).
+The @var{origin} is an numerical expression for the start address of
+the memory region. The expression must evaluate to a constant and it
+cannot involve any symbols. The keyword @code{ORIGIN} may be
+abbreviated to @code{org} or @code{o} (but not, for example,
+@code{ORG}).
@kindex LENGTH =
@kindex len =
@kindex l =
The @var{len} is an expression for the size in bytes of the memory
region. As with the @var{origin} expression, the expression must
-evaluate to a constant before memory allocation is performed. The
-keyword @code{LENGTH} may be abbreviated to @code{len} or @code{l}.
+be numerical only and must evaluate to a constant. The keyword
+@code{LENGTH} may be abbreviated to @code{len} or @code{l}.
In the following example, we specify that there are two memory regions
available for allocation: one starting at @samp{0} for 256 kilobytes,
output sections directed to a memory region are too large for the
region, the linker will issue an error message.
+It is possible to access the origin and length of a memory in an
+expression via the @code{ORIGIN(@var{memory})} and
+@code{LENGTH(@var{memory})} functions:
+
+@smallexample
+@group
+ _fstack = ORIGIN(ram) + LENGTH(ram) - 4;
+@end group
+@end smallexample
+
@node PHDRS
@section PHDRS Command
@kindex PHDRS
VERS_2.0 @{
bar1; bar2;
+ extern "C++" @{
+ ns::*;
+ "int f(int, double)";
+ @}
@} VERS_1.2;
@end smallexample
symbol whose name begins with @samp{old}, @samp{original}, or @samp{new}
is matched. The wildcard patterns available are the same as those used
in the shell when matching filenames (also known as ``globbing'').
+However, if you specify the symbol name inside double quotes, then the
+name is treated as literal, rather than as a glob pattern.
Next, the version script defines node @samp{VERS_1.2}. This node
depends upon @samp{VERS_1.1}. The script binds the symbol @samp{foo2}
demangle them according to @samp{lang} before matching them to the
patterns specified in @samp{version-script-commands}.
+Demangled names may contains spaces and other special characters. As
+described above, you can use a glob pattern to match demangled names,
+or you can use a double-quoted string to match the string exactly. In
+the latter case, be aware that minor differences (such as differing
+whitespace) between the version script and the demangler output will
+cause a mismatch. As the exact string generated by the demangler
+might change in the future, even if the mangled name does not, you
+should check that all of your version directives are behaving as you
+expect when you upgrade.
+
@node Expressions
@section Expressions in Linker Scripts
@cindex expressions
@menu
* Constants:: Constants
* Symbols:: Symbol Names
+* Orphan Sections:: Orphan Sections
* Location Counter:: The Location Counter
* Operators:: Operators
* Evaluation:: Evaluation
to delimit symbols with spaces. For example, @samp{A-B} is one symbol,
whereas @samp{A - B} is an expression involving subtraction.
+@node Orphan Sections
+@subsection Orphan Sections
+@cindex orphan
+Orphan sections are sections present in the input files which
+are not explicitly placed into the output file by the linker
+script. The linker will still copy these sections into the
+output file, but it has to guess as to where they should be
+placed. The linker uses a simple heuristic to do this. It
+attempts to place orphan sections after non-orphan sections of the
+same attribute, such as code vs data, loadable vs non-loadable, etc.
+If there is not enough room to do this then it places
+at the end of the file.
+
+For ELF targets, the attribute of the section includes section type as
+well as section flag.
+
@node Location Counter
@subsection The Location Counter
@kindex .
the values from the @samp{.data} input sections and before the end of
the @samp{.data} output section itself.
+@cindex dot outside sections
+Setting symbols to the value of the location counter outside of an
+output section statement can result in unexpected values if the linker
+needs to place orphan sections. For example, given the following:
+
+@smallexample
+SECTIONS
+@{
+ start_of_text = . ;
+ .text: @{ *(.text) @}
+ end_of_text = . ;
+
+ start_of_data = . ;
+ .data: @{ *(.data) @}
+ end_of_data = . ;
+@}
+@end smallexample
+
+If the linker needs to place some input section, e.g. @code{.rodata},
+not mentioned in the script, it might choose to place that section
+between @code{.text} and @code{.data}. You might think the linker
+should place @code{.rodata} on the blank line in the above script, but
+blank lines are of no particular significance to the linker. As well,
+the linker doesn't associate the above symbol names with their
+sections. Instead, it assumes that all assignments or other
+statements belong to the previous output section, except for the
+special case of an assignment to @code{.}. I.e., the linker will
+place the orphan @code{.rodata} section as if the script was written
+as follows:
+
+@smallexample
+SECTIONS
+@{
+ start_of_text = . ;
+ .text: @{ *(.text) @}
+ end_of_text = . ;
+
+ start_of_data = . ;
+ .rodata: @{ *(.rodata) @}
+ .data: @{ *(.data) @}
+ end_of_data = . ;
+@}
+@end smallexample
+
+This may or may not be the script author's intention for the value of
+@code{start_of_data}. One way to influence the orphan section
+placement is to assign the location counter to itself, as the linker
+assumes that an assignment to @code{.} is setting the start address of
+a following output section and thus should be grouped with that
+section. So you could write:
+
+@smallexample
+SECTIONS
+@{
+ start_of_text = . ;
+ .text: @{ *(.text) @}
+ end_of_text = . ;
+
+ . = . ;
+ start_of_data = . ;
+ .data: @{ *(.data) @}
+ end_of_data = . ;
+@}
+@end smallexample
+
+Now, the orphan @code{.rodata} section will be placed between
+@code{end_of_text} and @code{start_of_data}.
+
@need 2000
@node Operators
@subsection Operators
@end group
@end smallexample
+@item LENGTH(@var{memory})
+@kindex LENGTH(@var{memory})
+Return the length of the memory region named @var{memory}.
+
@item LOADADDR(@var{section})
@kindex LOADADDR(@var{section})
@cindex section load address in expression
use the @code{MEMORY} command to define discontinuous memory for the
output file, the two functions are equivalent.
+@item ORIGIN(@var{memory})
+@kindex ORIGIN(@var{memory})
+Return the origin of the memory region named @var{memory}.
+
+@item SEGMENT_START(@var{segment}, @var{default})
+@kindex SEGMENT_START(@var{segment}, @var{default})
+Return the base address of the named @var{segment}. If an explicit
+value has been given for this segment (with a command-line @samp{-T}
+option) that value will be returned; otherwise the value will be
+@var{default}. At present, the @samp{-T} command-line option can only
+be used to set the base address for the ``text'', ``data'', and
+``bss'' sections, but you use @code{SEGMENT_START} with any segment
+name.
+
@item SIZEOF(@var{section})
@kindex SIZEOF(@var{section})
@cindex section size
@samp{R_ARM_GOT_PREL} (arm*-*-linux, arm*-*-*bsd)
@end table
+@cindex FIX_V4BX
+@kindex --fix-v4bx
+The @samp{R_ARM_V4BX} relocation (defined by the ARM AAELF
+specification) enables objects compiled for the ARMv4 architecture to be
+interworking-safe when linked with other objects compiled for ARMv4t, but
+also allows pure ARMv4 binaries to be built from the same ARMv4 objects.
+
+In the latter case, the switch @option{--fix-v4bx} must be passed to the
+linker, which causes v4t @code{BX rM} instructions to be rewritten as
+@code{MOV PC,rM}, since v4 processors do not have a @code{BX} instruction.
+
+In the former case, the switch should not be used, and @samp{R_ARM_V4BX}
+relocations are ignored.
+
+@cindex USE_BLX
+@kindex --use-blx
+The @samp{--use-blx} switch enables the linker to use ARM/Thumb
+BLX instructions (available on ARMv5t and above) in various
+situations. Currently it is used to perform calls via the PLT from Thumb
+code using BLX rather than using BX and a mode-switching stub before
+each PLT entry. This should lead to such calls executing slightly faster.
+
+This option is enabled implicitly for SymbianOS, so there is no need to
+specify it if you are using that target.
+
@ifclear GENERIC
@lowersections
@end ifclear
projects / binutils-gdb.git / blobdiff