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(nasm.info)Section 6.5.2

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6.5.2. Position-Independent Code: `elf' Special Symbols and `WRT'
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   The `ELF' specification contains enough features to allow position-
independent code (PIC) to be written, which makes ELF shared libraries
very flexible. However, it also means NASM has to be able to generate a
variety of strange relocation types in ELF object files, if it is to be
an assembler which can write PIC.

   Since `ELF' does not support segment-base references, the `WRT'
operator is not used for its normal purpose; therefore NASM's `elf'
output format makes use of `WRT' for a different purpose, namely the
PIC-specific relocation types.

   `elf' defines five special symbols which you can use as the
right-hand side of the `WRT' operator to obtain PIC relocation types.
They are `..gotpc', `..gotoff', `..got', `..plt' and `..sym'. Their
functions are summarised here:

   * Referring to the symbol marking the global offset table base using
     `wrt ..gotpc' will end up giving the distance from the beginning of
     the current section to the global offset table.
     (`_GLOBAL_OFFSET_TABLE_' is the standard symbol name used to refer
     to the GOT.) So you would then need to add `$$' to the result to
     get the real address of the GOT.

   * Referring to a location in one of your own sections using `wrt
     ..gotoff' will give the distance from the beginning of the GOT to
     the specified location, so that adding on the address of the GOT
     would give the real address of the location you wanted.

   * Referring to an external or global symbol using `wrt ..got' causes
     the linker to build an entry _in_ the GOT containing the address
     of the symbol, and the reference gives the distance from the
     beginning of the GOT to the entry; so you can add on the address
     of the GOT, load from the resulting address, and end up with the
     address of the symbol.

   * Referring to a procedure name using `wrt ..plt' causes the linker
     to build a procedure linkage table entry for the symbol, and the
     reference gives the address of the PLT entry. You can only use
     this in contexts which would generate a PC-relative relocation
     normally (i.e. as the destination for `CALL' or `JMP'), since ELF
     contains no relocation type to refer to PLT entries absolutely.

   * Referring to a symbol name using `wrt ..sym' causes NASM to write
     an ordinary relocation, but instead of making the relocation
     relative to the start of the section and then adding on the offset
     to the symbol, it will write a relocation record aimed directly at
     the symbol in question. The distinction is a necessary one due to
     a peculiarity of the dynamic linker.

   A fuller explanation of how to use these relocation types to write
shared libraries entirely in NASM is given in *Note Section 8.2::.