Node: Comm, Next: CFI directives, Previous: Byte, Up: Pseudo Ops

### `.comm` symbol`,` length``

`.comm` 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 `ld` does not see a definition for the symbol-just one or more common symbols-then it will allocate length bytes of uninitialized memory. length must be an absolute expression. If `ld` sees multiple common symbols with the same name, and they do not all have the same size, it will allocate space using the largest size.

When using ELF, the `.comm` directive takes an optional third argument. This is the desired alignment of the symbol, specified as a byte boundary (for example, an alignment of 16 means that the least significant 4 bits of the address should be zero). The alignment must be an absolute expression, and it must be a power of two. If `ld` allocates uninitialized memory for the common symbol, it will use the alignment when placing the symbol. If no alignment is specified, `as` will set the alignment to the largest power of two less than or equal to the size of the symbol, up to a maximum of 16.

The syntax for `.comm` differs slightly on the HPPA. The syntax is ``symbol`.comm,` length``; symbol is optional.