On 1/10/20, Dan Cross <crossd(a)gmail.com> wrote: I think the situation you describe (common sections) is how this is done in ELF. a.out and COFF, as used on Unix, don't have common sections. Instead 'int x;' (without an initializer) becomes symbol 'x' in the object file's symbol table, with both the "external" and "undefined" attribute bits set, and with the symbol's value being the size of 'x' (typically 4 bites, in your example). It is the non-zero symbol value that distinguishes common symbols from ordinary external references, e.g., 'extern int x;' (without an initializer). At link time, common symbols are handled differently from ordinary external references: [1] When the linker is searching libraries, an ordinary external reference to 'x' will cause the linker to load an object that contains an external definition for 'x'. Common symbols do not trigger the loading of an object from a library. [2] After the linker has processed all of the files and libraries on the command line, if there is an external definition for 'x', all common symbol references to 'x' are treated as ordinary external references to 'x' and resolved against the definition. If no external definition is found, the linker allocates 'x' in BSS, using the maximum allocation size seen in any common symbol references to 'x'. All common symbol references and ordinary external references to 'x' are resolved to the newly-allocated space. Yes, the concept, nomenclature, and semantics come from FORTRAN, and they were included in a.out and COFF to support FORTRAN and other languages (such as PL/I) that have COMMON block-type semantics. I don't know why 'int x;' (without an initializer) in C was implemented as a common symbol. I suspect it was done to allow C and FORTRAN object modules linked together in the same executable to share external data. -Paul W.