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(nasm.info)Section 8.2

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8.2. Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF Shared Libraries
==================================================================

   `ELF' replaced the older `a.out' object file format under Linux
because it contains support for position-independent code (PIC), which
makes writing shared libraries much easier. NASM supports the `ELF'
position-independent code features, so you can write Linux `ELF' shared
libraries in NASM.

   NetBSD, and its close cousins FreeBSD and OpenBSD, take a different
approach by hacking PIC support into the `a.out' format. NASM supports
this as the `aoutb' output format, so you can write BSD shared
libraries in NASM too.

   The operating system loads a PIC shared library by memory-mapping the
library file at an arbitrarily chosen point in the address space of the
running process. The contents of the library's code section must
therefore not depend on where it is loaded in memory.

   Therefore, you cannot get at your variables by writing code like
this:

             mov     eax,[myvar]             ; WRONG

   Instead, the linker provides an area of memory called the _global
offset table_, or GOT; the GOT is situated at a constant distance from
your library's code, so if you can find out where your library is
loaded (which is typically done using a `CALL' and `POP' combination),
you can obtain the address of the GOT, and you can then load the
addresses of your variables out of linker-generated entries in the GOT.

   The _data_ section of a PIC shared library does not have these
restrictions: since the data section is writable, it has to be copied
into memory anyway rather than just paged in from the library file, so
as long as it's being copied it can be relocated too. So you can put
ordinary types of relocation in the data section without too much worry
(but see *Note Section 8.2.4:: for a caveat).

Section 8.2.1

Obtaining the Address of the GOT

Section 8.2.2

Finding Your Local Data Items

Section 8.2.3

Finding External and Common Data Items

Section 8.2.4

Exporting Symbols to the Library User

Section 8.2.5

Calling Procedures Outside the Library

Section 8.2.6

Generating the Library File