ARM Compiler toolchain Using the Linker - ARM Information Center

4.22 About weak references and definitions
Image structure and generation
Weak references and definitions provide additional flexibility in the way the linker includes
various functions and variables in a build. These references are typically to library functions.
Weak references
If the linker cannot resolve normal, non-weak, references to symbols included in
the link, 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, the section it is resolved to 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.
Function or variable declarations in C source files can be marked with the __weak
qualifier. As with extern, this qualifier tells the compiler that a function or
variable is declared in another source file. Because the definition of this function
or variable might not be available to the compiler, it creates a weak reference to
be resolved by the linker.
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 is strongly referenced 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
A function definition, or an exported label in assembler, can also be marked as
weak, as can a variable definition. In this case, a weak symbol definition is
created in the object file.
A weak definition can be used 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 due to multiply-defined symbols.
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