ARM Compiler toolchain Using the Linker - ARM Information Center

4.22.1 Example of a weak reference
Image structure and generation
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. Weak references can be used 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 --split_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().
• The linker feedback mechanism, --feedback, records init_foo() as being unused during
the link step. This causes the compiler to place init_foo() into its own section during
subsequent compilations, so that it can be removed.
• The compiler directive #pragma arm section also instructs the compiler to generate a
separate ELF section for some functions.
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:
__weak void init_foo(void);
int main(void)
{
init_foo();
// Rest of code that may make calls foo() directly or indirectly.
}
An example of a weak reference generated by the assembler is:
init.s:
IMPORT init_foo WEAK
AREA init, CODE, readonly
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