I was trying to understand the System.map file that gets created every time one compiles the Linux kernel, I was trying to understand the values presented in the System.map file.
Following is a sample information from it
000001d5 A kexec_control_code_size
00400000 A phys_startup_32
c0400000 T _text
c0400000 T startup_32
c04000b4 T start_cpu0
c04000c4 T startup_32_smp
c04000e0 t default_entry
c0400158 t enable_paging
c04001da t is486`
If you see the first line, the type of the symbol kexec_control_code_size is shown as A, I know that A means value of the symbol is absolute, but I wasn't able to completely decode what that exactly means. Does value mean the address of the symbol? Does absolute address mean that this symbol will be present at this address everytime the kernel gets loaded in to the memory?
Please forgive, if the questions are too basic.
You can examine symbol type via "man nm". nm tool shows all symbols in object file. Details about type of symbols you can find under man nm. Linux kernel modules .ko file and kernel object file can be examine with nm tool. Also you can investigate symbols from zImage or uImage or any kernel image and from kernel modules using objdump and readelf. Try use man pages for detail descriptions. Address of symbol can be calculated like offset from some main point for example section start. Other approach of symbols address calculation is absolute value of address (probably absolute value related to address space?). External symbols should be absolute. Symbols marked like absolute retain the same address through any link operation.
When the linker evaluates an expression, the result is either absolute or relative to some section. A relative expression is expressed as a fixed offset from the base of a section.
The position of the expression within the linker script determines whether it is absolute or relative. An expression which appears within an output section definition is relative to the base of the output section. An expression which appears elsewhere will be absolute.
A symbol set to a relative expression will be relocatable if you request relocatable output using the -r option. That means that a further link operation may change the value of the symbol. The symbol's section will be the section of the relative expression.
A symbol set to an absolute expression will retain the same value through any further link operation. The symbol will be absolute, and will not have any particular associated section. Taken from this manual
An example is here. Look for line "The following example shows how two absolute symbol definitions can be defined. "
