Then come the most important symbols:
Num: Value Size Type Bind Vis Ndx Name
4: 0000000000000000 0 NOTYPE LOCAL DEFAULT 1 hello_world
5: 000000000000000d 0 NOTYPE LOCAL DEFAULT ABS hello_world_len
6: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 2 _starthello_world string is in the .data section (index 1). It's value is 0: it points to the first byte of that section._start is marked with GLOBAL visibility since we wrote:global _startin NASM. This is necessary since it must be seen as the entry point. Unlike in C, by default NASM labels are local.
hello_world_len points to the special st_shndx == SHN_ABS == 0xF1FF.0xF1FF is chosen so as to not conflict with other sections.st_value == 0xD == 13 which is the value we have stored there on the assembly: the length of the string Hello World!.This means that relocation will not affect this value: it is a constant.
This is small optimization that our assembler does for us and which has ELF support.
If we had used the address of
hello_world_len anywhere, the assembler would not have been able to mark it as SHN_ABS, and the linker would have extra relocation work on it later. Articles by others on the same topic
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