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### 7.30 `.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 or (as a GNU extension) PE, the `.comm` directive takes an optional third argument. This is the desired alignment of the symbol, specified for ELF as a byte boundary (for example, an alignment of 16 means that the least significant 4 bits of the address should be zero), and for PE as a power of two (for example, an alignment of 5 means aligned to a 32-byte boundary). 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 on ELF, or the default section alignment of 4 on PE1.

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

#### Footnotes

[1] This is not the same as the executable image file alignment controlled by `ld`'s ‘`--section-alignment`’ option; image file sections in PE are aligned to multiples of 4096, which is far too large an alignment for ordinary variables. It is rather the default alignment for (non-debug) sections within object (‘`*.o`’) files, which are less strictly aligned.