5.3.2 .comm symbol, length [, algn]

Declares a common symbol named symbol. When linking, a common symbol in one object file may be merged with a defined or common symbol of the same name in another object file. If the linker does not see a definition for that symbol, then it will allocate length bytes of uninitialized memory. If the linker sees multiple common symbols with the same name, and they do not all have the same size, the linker will allocate space using the largest size.

If algn is specified, it is the address alignment required for symbol. The requested alignment must be a power of two. algn is supported when the object file format is ELF; otherwise, it is ignored.