Include standard library functions in a specific memory section

Question

In C, we can force the linker to put a specific function in a specific section from the source code, using something like the following example.

Here, the function my_function is tagged with the preprocessor macro PUT_IN_USER_SECTION in order to tell the linker to put it in the section .user_section.

#define PUT_IN_USER_SECTION  __attribute__((__section__(".user_section"))) __attribute__ ((noinline))

double PUT_IN_USER_SECTION my_function(double a, double b)
{
    // Function content
}

Now, what I'd like to know is, when we use standard functions (for example log functions from the math.h library) from the GLIBC, MUSL, ... and we perform static linking: is it possible to put those functions in specific sections? and how to that?

@KamilCuk I thought about that, however, by default (using gcc + musl + static linking) the math functions are included in the `.text` section. How can I isolate only the math functions for example? — noureddine-as, Dec 10 '20 at 11:00
Well, you could list all math function (along `nm libm.a`) and compile with `--function-sections` then move all `.text.math_function_name` to other section with `objcopy`. But why not use a linker script? Something along `section { *libm.a:(.text) }` in linker script, I guess. — KamilCuk, Dec 10 '20 at 11:04

score 0 · Answer 1 · answered Dec 10 '20 at 12:56

There is a technique called overlaying that comes from a time when memory resources were limited. It is still used in Embedded Systems with restricted resources :

https://en.wikipedia.org/wiki/Overlay_(programming)