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(as.info)Comm

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`.comm SYMBOL , LENGTH '
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   `.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.