ARM ELF

4.4.7.1 Mapping symbols

$a labels the first byte of a sequence of ARM instructions. Its type is STT_FUNC.

$d labels the first byte of a sequence of data items. Its type is STT_OBJECT.

$t labels the first byte of a sequence of Thumb instructions. Its type is STT_FUNC.

This list of mapping symbols may be extended in the future.

Section-relative mapping symbols

Mapping symbols defined in a section define a sequence of half-open address intervals that cover the address range of the section. Each interval starts at the address defined by a mapping symbol, and continues up to, but not including, the address defined by the next (in address order) mapping symbol or the end of the section. A corollary is that there must be a mapping symbol defined at the beginning of each section.

Consumers can ignore the size of a section-relative mapping symbol. Producers can set it to 0.

Absolute mapping symbols

Because of the need to crystallize a Thumb address with the Thumb-bit set, absolute symbol of type STT_FUNC (symbols of type STT_FUNC defined in section SHN_ABS) need to be mapped with $a or $t.

The extent of a mapping symbol defined in SHN_ABS is [st_value, st_value + st_size), or [st_value, st_value + 1) if st_size = 0, where [x, y) denotes the half-open address range from x, inclusive, to y, exclusive.

In the absence of a mapping symbol, a consumer can interpret a function symbol with an odd value as the Thumb code address obtained by clearing the least significant bit of the value. This interpretation is deprecated, and it may not work in the future.

4.4.7.2 Tagging symbols

Tagging symbols help consumers by tagging specific entities in a program. In each case, a consumer can ignore the size of a tagging symbol. A producer should set the size to the size of the tagged entity or 0.

$b labels a Thumb BL instruction. Its type is STT_FUNC, its size is 4 bytes.

$f labels a function pointer constant (static pointer to code). Its type is STT_OBJECT, its size is 4 bytes.

$p labels the final, PC-modifying instruction of an indirect function call. Its type is STT_FUNC.

(An indirect call is a call through a function pointer variable). $p does not label the PC-modifying instruction of a function return sequence. Its size is 2 bytes in Thumb-state, 4 bytes in ARM-state.

$m gives (in its st_value field) the total number of mapping and tagging symbols in the symbol table.

This list of tagging symbols is not exhaustive and others may be defined in the future.

4.4.8 Symbol table order

The order of symbols in the symbol table should be:

Local symbols

-$m

-Other mapping and tagging symbols sorted in virtual address order

-Other local symbols sorted in virtual address order (save that each symbol of type STT_FILE should precede all the symbols of type STT_SECTION that are associated with it).

Global symbols.