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Preface Arm Compiler Tools Overview armclang Reference armlink Reference armlink Command-line Options --any_contingency --any_placement=algorithm --any_sort_order=order --api, --no_api --autoat, --no_autoat --bare_metal_pie --base_platform --be8 --be32 --bestdebug, --no_bestdebug --blx_arm_thumb, --no_blx_arm_thumb --blx_thumb_arm, --no_blx_thumb_arm --bpabi --branchnop, --no_branchnop --callgraph, --no_callgraph --callgraph_file=filename --callgraph_output=fmt --callgraph_subset=symbol[,symbol,...] --cgfile=type --cgsymbol=type --cgundefined=type --comment_section, --no_comment_section --cppinit, --no_cppinit --cpu=list (armlink) --cpu=name (armlink) --crosser_veneershare, --no_crosser_veneershare --dangling-debug-address=address --datacompressor=opt --debug, --no_debug --diag_error=tag[,tag,…] (armlink) --diag_remark=tag[,tag,…] (armlink) --diag_style={arm|ide|gnu} (armlink) --diag_suppress=tag[,tag,…] (armlink) --diag_warning=tag[,tag,…] (armlink) --dll --dynamic_linker=name --eager_load_debug, --no_eager_load_debug --eh_frame_hdr --edit=file_list --emit_debug_overlay_relocs --emit_debug_overlay_section --emit_non_debug_relocs --emit_relocs --entry=location --errors=filename --exceptions, --no_exceptions --export_all, --no_export_all --export_dynamic, --no_export_dynamic --filtercomment, --no_filtercomment --fini=symbol --first=section_id --force_explicit_attr --force_so_throw, --no_force_so_throw --fpic --fpu=list (armlink) --fpu=name (armlink) --got=type --gnu_linker_defined_syms --help (armlink) --import_cmse_lib_in=filename --import_cmse_lib_out=filename --import_unresolved, --no_import_unresolved --info=topic[,topic,…] (armlink) --info_lib_prefix=opt --init=symbol --inline, --no_inline --inline_type=type --inlineveneer, --no_inlineveneer input-file-list (armlink) --keep=section_id (armlink) --keep_intermediate --largeregions, --no_largeregions --last=section_id --legacyalign, --no_legacyalign --libpath=pathlist --library=name --library_security=protection --library_type=lib --list=filename --list_mapping_symbols, --no_list_mapping_symbols --load_addr_map_info, --no_load_addr_map_info --locals, --no_locals --lto, --no_lto --lto_keep_all_symbols, --no_lto_keep_all_symbols --lto_intermediate_filename --lto_level --lto_relocation_model --mangled, --unmangled --map, --no_map --max_er_extension=size --max_veneer_passes=value --max_visibility=type --merge, --no_merge --merge_litpools, --no_merge_litpools --muldefweak, --no_muldefweak -o filename, --output=filename (armlink) --output_float_abi=option --overlay_veneers --override_visibility -Omax (armlink) -Omin (armlink) --pad=num --paged --pagesize=pagesize --partial --pie --piveneer, --no_piveneer --pixolib --pltgot=type --pltgot_opts=mode --predefine="string" --preinit, --no_preinit --privacy (armlink) --ref_cpp_init, --no_ref_cpp_init --ref_pre_init, --no_ref_pre_init --reloc --remarks --remove, --no_remove --ro_base=address --ropi --rosplit --rw_base=address --rwpi --scanlib, --no_scanlib --scatter=filename --section_index_display=type --shared --show_cmdline (armlink) --show_full_path --show_parent_lib --show_sec_idx --soname=name --sort=algorithm --split --startup=symbol, --no_startup --stdlib --strict --strict_flags, --no_strict_flags --strict_ph, --no_strict_ph --strict_preserve8_require8 --strict_relocations, --no_strict_relocations --strict_symbols, --no_strict_symbols --strict_visibility, --no_strict_visibility --symbols, --no_symbols --symdefs=filename --symver_script=filename --symver_soname --sysv --tailreorder, --no_tailreorder --tiebreaker=option --unaligned_access, --no_unaligned_access --undefined=symbol --undefined_and_export=symbol --unresolved=symbol --use_definition_visibility --userlibpath=pathlist --veneerinject, --no_veneerinject --veneer_inject_type=type --veneer_pool_size=size --veneershare, --no_veneershare --verbose --version_number (armlink) --via=filename (armlink) --vsn (armlink) --xo_base=address --xref, --no_xref --xrefdbg, --no_xrefdbg --xref{from|to}=object(section) --zi_base=address Linking Models Supported by armlink Overview of linking models Bare-metal linking model overview Partial linking model overview Base Platform Application Binary Interface (BPABI) Base Platform linking model overview SysV linking model overview Concepts common to both BPABI and SysV linking mod Image Structure and Generation The structure of an Arm ELF image Views of the image at each link stage Input sections, output sections, regions, and prog Load view and execution view of an image Methods of specifying an image memory map with the Image entry points Restrictions on image structure Simple images Types of simple image Type 1 image structure, one load region and contig Type 2 image structure, one load region and non-co Type 3 image structure, multiple load regions and Section placement with the linker Default section placement Section placement with the FIRST and LAST attribut Section alignment with the linker Linker support for creating demand-paged files Linker reordering of execution regions containing Linker-generated veneers What is a veneer? Veneer sharing Veneer types Generation of position independent to absolute ven Reuse of veneers when scatter-loading Generation of secure gateway veneers Command-line options used to control the generatio Weak references and definitions How the linker performs library searching, selecti How the linker searches for the Arm standard libra Specifying user libraries when linking How the linker resolves references The strict family of linker options Linker Optimization Features Elimination of common section groups Elimination of unused sections Optimization with RW data compression How the linker chooses a compressor Options available to override the compression algo How compression is applied Considerations when working with RW data compressi Function inlining with the linker Factors that influence function inlining About branches that optimize to a NOP Linker reordering of tail calling sections Restrictions on reordering of tail calling section Linker merging of comment sections Merging identical constants Accessing and Managing Symbols with armlink About mapping symbols Linker-defined symbols Region-related symbols Types of region-related symbols Image$$ execution region symbols Load$$ execution region symbols Load$$LR$$ load region symbols Region name values when not scatter-loading Linker defined symbols and scatter files Methods of importing linker-defined symbols in C a Methods of importing linker-defined symbols in Arm Section-related symbols Types of section-related symbols Image symbols Input section symbols Access symbols in another image Creating a symdefs file Outputting a subset of the global symbols Reading a symdefs file Symdefs file format Edit the symbol tables with a steering file Specifying steering files on the linker command-li Steering file command summary Steering file format Hide and rename global symbols with a steering fil Use of $Super$$ and $Sub$$ to patch symbol definit Scatter-loading Features The scatter-loading mechanism Overview of scatter-loading When to use scatter-loading Linker-defined symbols that are not defined when s Placing the stack and heap with a scatter file Scatter-loading command-line options Scatter-loading images with a simple memory map Scatter-loading images with a complex memory map Root region and the initial entry point Effect of the ABSOLUTE attribute on a root region Effect of the FIXED attribute on a root region Methods of placing functions and data at specific Placing functions and data in a named section Placement of __at sections at a specific address Restrictions on placing __at sections Automatic placement of __at sections Manual placement of __at sections Place a key in flash memory with an __at section Example of how to explicitly place a named section Manual placement of unassigned sections Default rules for placing unassigned sections Command-line options for controlling the placement Prioritizing the placement of unassigned sections Specify the maximum region size permitted for plac Examples of using placement algorithms for .ANY se Example of next_fit algorithm showing behavior of Examples of using sorting algorithms for .ANY sect Behavior when .ANY sections overflow because of li Placing veneers with a scatter file Placement of CMSE veneer sections for a Secure ima Reserving an empty block of memory Characteristics of a reserved empty block of memor Example of reserving an empty block of memory Placement of Arm C and C++ library code Placement of code in a root region Placement of Arm C library code Placing Arm C++ library code Alignment of regions to page boundaries Alignment of execution regions and input sections Preprocessing a scatter file Default behavior for armclang -E in a scatter file Use of other preprocessors in a scatter file Example of using expression evaluation in a scatte Equivalent scatter-loading descriptions for simple Command-line options for creating simple images Type 1 image, one load region and contiguous execu Type 2 image, one load region and non-contiguous e Type 3 image, multiple load regions and non-contig How the linker resolves multiple matches when proc How the linker resolves path names when processing Scatter file to ELF mapping Scatter File Syntax BNF notation used in scatter-loading description s Syntax of a scatter file Load region descriptions Components of a load region description Syntax of a load region description Load region attributes Inheritance rules for load region address attribut Inheritance rules for the RELOC address attribute Considerations when using a relative address +offs Execution region descriptions Components of an execution region description Syntax of an execution region description Execution region attributes Inheritance rules for execution region address att Considerations when using a relative address +offs Input section descriptions Components of an input section description Syntax of an input section description Examples of module and input section specification Expression evaluation in scatter files Expression usage in scatter files Expression rules in scatter files Execution address built-in functions for use in sc ScatterAssert function and load address related fu Symbol related function in a scatter file AlignExpr(expr, align) function GetPageSize() function SizeOfHeaders() function Example of aligning a base address in execution sp Scatter files containing relative base address loa BPABI and SysV Shared Libraries and Executables About the Base Platform Application Binary Interfa Platforms supported by the BPABI Features common to all BPABI models About importing and exporting symbols for BPABI mo Symbol visibility for BPABI models Automatic import and export for BPABI models Manual import and export for BPABI models Symbol versioning for BPABI models RW compression for BPABI models SysV linking model SysV standard memory model Using the C and C++ libraries Using a dynamic Linker Automatic dynamic symbol table rules in the SysV l Symbol definitions defined for SysV compatibility Addressing modes in the SysV linking model Thread local storage in the SysV linking model Linker command-line options for the SysV linking m Bare metal and DLL-like memory models BPABI standard memory model Customization of the BPABI standard memory model Linker command-line options for bare metal and DLL Mandatory symbol versioning in the BPABI DLL-like Automatic dynamic symbol table rules in the BPABI Addressing modes in the BPABI DLL-like model C++ initialization in the BPABI DLL-like model Symbol versioning Overview of symbol versioning Embedded symbols The symbol versioning script file Example of creating versioned symbols Linker options for enabling implicit symbol versio Features of the Base Platform Linking Model Restrictions on the use of scatter files with the Scatter files for the Base Platform linking model Placement of PLT sequences with the Base Platform Linker Steering File Command Reference EXPORT steering file command HIDE steering file command IMPORT steering file command RENAME steering file command REQUIRE steering file command RESOLVE steering file command SHOW steering file command fromelf Reference armar Reference armasm Legacy Assembler Reference Appendixes

Weak references and definitions

C3.8 Weak references and definitions

Weak references and definitions provide additional flexibility in the way the linker includes various functions and variables in a build.

Weak references and definitions are typically used in connection with library functions.

Weak references

If the linker cannot resolve normal, non-weak, references to symbols from the content loaded so far, it attempts to do so by finding the symbol in a library:

  • If it is unable to find such a reference, the linker reports an error.

  • If such a reference is resolved, a section that is reachable from an entry point by at least one non-weak reference is marked as used. This ensures the section is not removed by the linker as an unused section. Each non-weak reference must be resolved by exactly one definition. If there are multiple definitions, the linker reports an error.

Symbols can be given weak binding by the compiler and assembler.

The linker does not load an object from a library to resolve a weak reference. It is able to resolve the weak reference only if the definition is included in the image for other reasons. The weak reference does not cause the linker to mark the section containing the definition as used, so it might be removed by the linker as unused. The definition might already exist in the image for several reasons:

  • The symbol has a non-weak reference from somewhere else in the code.

  • The symbol definition exists in the same ELF section as a symbol definition that is included for any of these reasons.

  • The symbol definition is in a section that has been specified using --keep, or contains an ENTRY point.

  • The symbol definition is in an object file included in the link and the --no_remove option is used. The object file is not referenced from a library unless that object file within the library is explicitly included on the linker command-line.

In summary, a weak reference is resolved if the definition is already included in the image, but it does not determine if that definition is included.

An unresolved weak function call is replaced with either:

  • A no-operation instruction, NOP.

  • A branch with link instruction, BL, to the following instruction. That is, the function call just does not happen.

Weak definitions

You can mark a function or variable definition as weak in a source file. A weak symbol definition is then present in the created object file.

You can use a weak definition to resolve any reference to that symbol in the same way as a normal definition. However, if another non-weak definition of that symbol exists in the build, the linker uses that definition instead of the weak definition, and does not produce an error because of multiply-defined symbols.

Example of a weak reference

A library contains a function foo(), that is called in some builds of an application but not in others. If it is used, init_foo() must be called first. You can use weak references to automate the call to init_foo().

The library can define init_foo() and foo() in the same ELF section. The application initialization code must call init_foo() weakly. If the application includes foo() for any reason, it also includes init_foo() and this is called from the initialization code. In any builds that do not include foo(), the call to init_foo() is removed by the linker.

Typically, the code for multiple functions defined within a single source file is placed into a single ELF section by the compiler. However, certain build options might alter this behavior, so you must use them with caution if your build is relying on the grouping of files into ELF sections. The compiler command-line option -ffunction-sections results in each function being placed in its own section. In this example, compiling the library with this option results in foo() and init_foo() being placed in separate sections. Therefore init_foo() is not automatically included in the build due to a call to foo().

In this example, there is no need to rebuild the initialization code between builds that include foo() and do not include foo(). There is also no possibility of accidentally building an application with a version of the initialization code that does not call init_foo(), and other parts of the application that call foo().

An example of foo.c source code that is typically built into a library is:

void init_foo()
    // Some initialization code
void foo()
     // A function that is included in some builds
     // and requires init_foo() to be called first.

An example of init.c is:

__attribute__((weak)) void init_foo(void);
int main(void)
    //  Rest of code that may make calls to foo() directly or indirectly.

An example of a weak reference generated by the assembler is:

    bl      init_foo
    // Rest of code

    .weak   init_foo

Example of a weak definition

You can provide a simple or dummy implementation of a function as a weak definition. This enables you to build software with defined behavior without having to provide a full implementation of the function. It also enables you to provide a full implementation for some builds if required.

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