Syntax

              .common symbol,size in bytes[, alignment]

              .common symbol,structure tag[, alignment]

Description

The .common directive creates a common symbol in a common block, rather than placing the variable in a memory section.

The benefit of common symbols is that generated code can remove unused variables that would otherwise increase the size of the .bss section. (Uninitialized variables of a size larger than 32 bytes are separately optimized through placement in separate subsections that can be omitted from a link.)

• The symbol is a required parameter. It defines a name for the symbol created by this directive. The symbol name must correspond to the variable that you are reserving space for.
• The size in bytes is a required parameter; it must be an absolute expression. The assembler allocates size bytes in the section used for common symbols. There is no default size.
• A structure tag can be used in place of a size to specify a structure created with the .struct directive. Either a size or a structure tag is required for this argument.
• The alignment is an optional parameter that ensures that the space allocated to the symbol occurs on the specified boundary. The boundary must be set to a power of 2 between 2⁰ and 2¹⁵, inclusive. If the SPC is already aligned at the specified boundary, it is not incremented.

Common symbols are symbols that are placed in the symbol table of an ELF object file. They represent an uninitialized variable. Common symbols do not reference a section. (In contrast, initialized variables need to reference a section that contains the initialized data.) The value of a common symbol is its required alignment; it has no address and stores no address. While symbols for an uninitialized common block can appear in executable object files, common symbols may only appear in relocatable object files. Common symbols are preferred over weak symbols. See the section on the "Symbol Table" in the System V ABI specification for more about common symbols.

When object files containing common symbols are linked, space is reserved in an uninitialized section (.common) for each common symbol. A symbol is created in place of the common symbol to refer to its reserved location.