by Ciro Santilli (@cirosantilli, 37)
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 _start
hello_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 _start
in 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.

Ancestors (12)

  1. .symtab
  2. Sections
  3. ELF Hello World Tutorial
  4. Executable and Linkable Format
  5. Executable file format
  6. Systems programming
  7. Software
  8. Computer
  9. Information technology
  10. Area of technology
  11. Technology
  12. Home

View article source

Discussion (0)

+ New discussion

There are no discussions about this article yet.

Articles by others on the same topic (0)

There are currently no matching articles.
See all articles in the same topic + Create my own version