ARM Compiler toolchain Using the Linker - ARM Information Center

3.5 Base Platform linking model
Linking models supported by armlink
Base Platform enables you to create dynamically linkable images that do not have the memory
map enforced by the System V (SysV) or Base Platform Application Binary Interface (BPABI)
linking models. It enables you to:
• Create images with a memory map described in a scatter file.
• Have dynamic relocations so the images can be dynamically linked. The dynamic
relocations can also target within the same image.
Note
The BPABI specification places constraints on the memory model that can be violated using
scatter-loading. However, because Base Platform is a superset of BPABI, it is possible to create
a BPABI conformant image with Base Platform.
To link with the Base Platform model, use the --base_platform command-line option.
If you specify this option, the linker acts as if you specified --bpabi, with the following
exceptions:
• Scatter-loading is available with --scatter, in addition to the following options:
— --dll
— --force_so_throw, --no_force_so_throw
— --pltgot=type is restricted to types none or direct
— --ro_base=address
— --rosplit
— --rw_base=address
— --rwpi.
• The default value of the --pltgot option is different to that for --bpabi:
— for --base_platform, the default is --pltgot=none
— for --bpabi the default is --pltgot=direct.
• If you do not use a scatter file, the linker can ensure that the Procedure Linkage Table
(PLT) section is placed correctly, and contains entries for calls only to imported symbols.
If you specify a scatter file, the linker might not be able to find a suitable location to place
the PLT.
Each load region containing code might require a PLT section to indirect calls from the
load region to functions where the address is not known at static link time. The PLT
section for a load region LR must be placed in LR and be accessible at all times to code
within LR.
To ensure calls between relocated load regions ar run-time:
— Use the --pltgot=direct option to turn on PLT generation.
— Use the --pltgot_opts=crosslr option to add entries in the PLT for calls between
RELOC load regions. The linker generates a PLT for each load region so that calls do
not have to be extended to reach a distant PLT.
Be aware of the following:
• The model assumes that shared objects cannot throw a C++ exception.
• Symbol versioning must be used to ensure that all the required symbols are available at
load time.
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