good explanation of __read_mostly, __init, __exit macros

GCC attributes are a general mechanism to give instructions to the compiler that are outside the specification of the language itself.

The common facility that the macros you list is the use of the __section__ attribute which is described as:

The section attribute specifies that a function lives in a particular section. For example, the declaration:

extern void foobar (void) __attribute__ ((section ("bar")));

puts the function foobar in the bar section.

So what does it mean to put something in a section? An object file is divided into sections: .text for executable machine code, .data for read-write data, .rodata for read-only data, .bss for data initialised to zero, etc. The names and purposes of these sections is a matter of platform convention, and some special sections can only be accessed from C using the __attribute__ ((section)) syntax.

In your example you can guess that .data..read_mostly is a subsection of .data for data that will be mostly read; .init.text is a text (machine code) section that will be run when the program is initialised, etc.

On Linux, deciding what to do with the various sections is the job of the kernel; when userspace requests to exec a program, it will read the program image section-by-section and process them appropriately: .data sections get mapped as read-write pages, .rodata as read-only, .text as execute-only, etc. Presumably .init.text will be executed before the program starts; that could either be done by the kernel or by userspace code placed at the program’s entry point (I’m guessing the latter).

If you want to see the effect of these attributes, a good test is to run gcc with the -S option to output assembler code, which will contain the section directives. You could then run the assembler with and without the section directives and use objdump or even hex dump the resulting object file to see how it differs.