@end ifset
@c man end
-@ifinfo
-@format
-START-INFO-DIR-ENTRY
+@ifnottex
+@dircategory Software development
+@direntry
* Ld: (ld). The GNU linker.
-END-INFO-DIR-ENTRY
-@end format
-@end ifinfo
+@end direntry
+@end ifnottex
@copying
This file documents the @sc{gnu} linker LD
architecture of an object file by using the @code{objdump} program with
the @samp{-f} option.
@end ifclear
+
+@item LD_FEATURE(@var{string})
+@kindex LD_FEATURE(@var{string})
+This command may be used to modify @command{ld} behavior. If
+@var{string} is @code{"SANE_EXPR"} then absolute symbols and numbers
+in a script are simply treated as numbers everywhere.
+@xref{Expression Section}.
@end table
@node Assignments
@cindex address, section
@cindex section address
The @var{address} is an expression for the VMA (the virtual memory
-address) of the output section. If you do not provide @var{address},
-the linker will set it based on @var{region} if present, or otherwise
-based on the current value of the location counter.
-
-If you provide @var{address}, the address of the output section will be
-set to precisely that. If you provide neither @var{address} nor
-@var{region}, then the address of the output section will be set to the
-current value of the location counter aligned to the alignment
-requirements of the output section. The alignment requirement of the
-output section is the strictest alignment of any input section contained
-within the output section.
-
-For example,
+address) of the output section. This address is optional, but if it
+is provided then the output address will be set exactly as specified.
+
+If the output address is not specified then one will be chosen for the
+section, based on the heuristic below. This address will be adjusted
+to fit the alignment requirement of the output section. The
+alignment requirement is the strictest alignment of any input section
+contained within the output section.
+
+The output section address heuristic is as follows:
+
+@itemize @bullet
+@item
+If an output memory @var{region} is set for the section then it
+is added to this region and its address will be the next free address
+in that region.
+
+@item
+If the MEMORY command has been used to create a list of memory
+regions then the first region which has attributes compatible with the
+section is selected to contain it. The section's output address will
+be the next free address in that region; @ref{MEMORY}.
+
+@item
+If no memory regions were specified, or none match the section then
+the output address will be based on the current value of the location
+counter.
+@end itemize
+
+@noindent
+For example:
+
@smallexample
.text . : @{ *(.text) @}
@end smallexample
+
@noindent
and
+
@smallexample
.text : @{ *(.text) @}
@end smallexample
+
@noindent
are subtly different. The first will set the address of the
@samp{.text} output section to the current value of the location
counter. The second will set it to the current value of the location
-counter aligned to the strictest alignment of a @samp{.text} input
-section.
+counter aligned to the strictest alignment of any of the @samp{.text}
+input sections.
The @var{address} may be an arbitrary expression; @ref{Expressions}.
For example, if you want to align the section on a 0x10 byte boundary,
difference is @code{SORT_BY_ALIGNMENT} will sort sections into
ascending order by alignment before placing them in the output file.
+@cindex SORT_BY_INIT_PRIORITY
+@code{SORT_BY_INIT_PRIORITY} is very similar to @code{SORT_BY_NAME}. The
+difference is @code{SORT_BY_INIT_PRIORITY} will sort sections into
+ascending order by numerical value of the GCC init_priority attribute
+encoded in the section name before placing them in the output file.
+
@cindex SORT
@code{SORT} is an alias for @code{SORT_BY_NAME}.
the input sections which map into it. You can override this by using
the section type. For example, in the script sample below, the
@samp{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
-@samp{ROM} section will appear in the linker output file as usual.
+need to be loaded when the program is run.
@smallexample
@group
SECTIONS @{
@cindex load address
@cindex section load address
Every section has a virtual address (VMA) and a load address (LMA); see
-@ref{Basic Script Concepts}. The address expression which may appear in
-an output section description sets the VMA (@pxref{Output Section
-Address}).
-
-The expression @var{lma} that follows the @code{AT} keyword specifies
-the load address of the section.
+@ref{Basic Script Concepts}. The virtual address is specified by the
+@pxref{Output Section Address} described earlier. The load address is
+specified by the @code{AT} or @code{AT>} keywords. Specifying a load
+address is optional.
-Alternatively, with @samp{AT>@var{lma_region}} expression, you may
-specify a memory region for the section's load address. @xref{MEMORY}.
-Note that if the section has not had a VMA assigned to it then the
-linker will use the @var{lma_region} as the VMA region as well.
+The @code{AT} keyword takes an expression as an argument. This
+specifies the exact load address of the section. The @code{AT>} keyword
+takes the name of a memory region as an argument. @xref{MEMORY}. The
+load address of the section is set to the next free address in the
+region, aligned to the section's alignment requirements.
If neither @code{AT} nor @code{AT>} is specified for an allocatable
-section, the linker will set the LMA such that the difference between
-VMA and LMA for the section is the same as the preceding output
-section in the same region. If there is no preceding output section
-or the section is not allocatable, the linker will set the LMA equal
-to the VMA.
-@xref{Output Section Region}.
+section, the linker will use the following heuristic to determine the
+load address:
+
+@itemize @bullet
+@item
+If the section has a specific VMA address, then this is used as
+the LMA address as well.
+
+@item
+If the section is not allocatable then its LMA is set to its VMA.
+
+@item
+Otherwise if a memory region can be found that is compatible
+with the current section, and this region contains at least one
+section, then the LMA is set so the difference between the
+VMA and LMA is the same as the difference between the VMA and LMA of
+the last section in the located region.
+
+@item
+If no memory regions have been declared then a default region
+that covers the entire address space is used in the previous step.
+
+@item
+If no suitable region could be found, or there was no previous
+section then the LMA is set equal to the VMA.
+@end itemize
@cindex ROM initialized data
@cindex initialized data in ROM
char *src = &_etext;
char *dst = &_data;
-/* ROM has data at end of text; copy it. */
-while (dst < &_edata) @{
+/* ROM has data at end of text; copy it. */
+while (dst < &_edata)
*dst++ = *src++;
-@}
-/* Zero bss */
+/* Zero bss. */
for (dst = &_bstart; dst< &_bend; dst++)
*dst = 0;
@end group
symbol will retain the same value throughout any further link
operations.
-Some terms in linker expressions are addresses. This is true of all
-symbols and for builtin functions that return an address, such as
-@code{ADDR}, @code{LOADADDR}, @code{ORIGIN} and @code{SEGMENT_START}.
-Other terms are simply numbers, or are builtin functions that return a
-non-address value, such as @code{LENGTH}.
-
-When the linker evaluates an expression, the result depends on where
-the expression is located in a linker script. Expressions appearing
-outside an output section definitions are evaluated with all terms
-first being converted to absolute addresses before applying operators,
-and evaluate to an absolute address result. Expressions appearing
-inside an output section definition are evaluated with more complex
-rules, but the aim is to treat terms as relative addresses and produce
-a relative address result. In particular, an assignment of a number
-to a symbol results in a symbol relative to the output section with an
-offset given by the number. So, in the following simple example,
+Some terms in linker expressions are addresses. This is true of
+section relative symbols and for builtin functions that return an
+address, such as @code{ADDR}, @code{LOADADDR}, @code{ORIGIN} and
+@code{SEGMENT_START}. Other terms are simply numbers, or are builtin
+functions that return a non-address value, such as @code{LENGTH}.
+One complication is that unless you set @code{LD_FEATURE ("SANE_EXPR")}
+(@pxref{Miscellaneous Commands}), numbers and absolute symbols are treated
+differently depending on their location, for compatibility with older
+versions of @code{ld}. Expressions appearing outside an output
+section definition treat all numbers as absolute addresses.
+Expressions appearing inside an output section definition treat
+absolute symbols as numbers. If @code{LD_FEATURE ("SANE_EXPR")} is
+given, then absolute symbols and numbers are simply treated as numbers
+everywhere.
+
+In the following simple example,
@smallexample
@group
@code{__data_start} are set to 0x10 relative to the @code{.data}
section in the second two assignments.
-For expressions appearing inside an output section definition
-involving numbers, relative addresses and absolute addresses, ld
-follows these rules to evaluate terms:
+For expressions involving numbers, relative addresses and absolute
+addresses, ld follows these rules to evaluate terms:
@itemize @bullet
@item
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