# Linkers part 17

## Warning Symbols

The GNU linker supports a weird extension to ELF used to issue warnings when
symbols are referenced at link time. This was originally implemented for a.out
using a special symbol type. For ELF, I implemented it using a special section
name.

If you create a section named `.gnu.warning.SYMBOL`, then if and when the
linker sees an undefined reference to `SYMBOL`, it will issue a warning. The
warning is triggered by seeing an undefined symbol with the right name in an
object file.  Unlike the warning about an undefined symbol, it is not triggered
by seeing a relocation entry. The text of the warning is simply the contents of
the `.gnu.warning.SYMBOL` section.

The GNU C library uses this feature to warn about references to symbols like
`gets` which are required by standards but are generally considered to be
unsafe.  This is done by creating a section named `.gnu.warning.gets` in the
same object file which defines `gets`.

The GNU linker also supports another type of warning, triggered by sections
named `.gnu.warning` (without the symbol name). If an object file with a
section of that name is included in the link, the linker will issue a warning.
Again, the text of the warning is simply the contents of the `.gnu.warning`
section. I don’t know if anybody actually uses this feature.

Short entry today, more tomorrow.