Communal Data Is Not Included in the LIB Listing

MS-DOS                                | OS/2

The information in this article applies to:

• Microsoft LIB for MS-DOS, versions 3.11, 3.17, 3.18, 3.2, 3.20.010, 3.31, and 3.4
• Microsoft LIB for OS/2, versions 3.11, 3.17, 3.18

SUMMARY

Communal data declarations generate no PUBLIC definitions. Therefore, although they can be placed in a library, they are not placed in the library dictionary and will not be a part of the library listing generated by the LIB utility.

MORE INFORMATION

Communal variables are supported only by QuickC, Microsoft C versions 5.0 and later, and MASM. They are, however, similar to common blocks in FORTRAN. In C, communal variables are the uninitialized global data items. In MASM, they are data items declared with the COMM qualifier.

The linker will assume the communal variables refer to an external defining declaration elsewhere. If there is no external definition, the linker allocates storage and initializes it to 0 (zero). If there is more than one declaration, storage is allocated for the largest data item declared. For example, if I is declared int I; in one module and char I; in another module, the linker will allocate 2 bytes for I because the int is 2 bytes but the char is only 1 byte. Because memory for communal variables may not be assigned until load time, their use may reduce the size of your executable file.

When a communal variable is declared in a library and defined in one of the object modules, references to that variable will be resolved with the defined variable. For this reason, communal variable declarations are not recommended for any file that might be placed in a library. If a variable defined in a .OBJ file accidentally had the same name as a communal variable in a library that it is being linked with, its value would unexplainably change every time you called a library function that used that communal variable! The linker does not issue a warning because the variable was defined only once, in the OBJ file.

In addition, a communal data item X in a library cannot be used to resolve an external reference to X in a module linked with that library. For example, linking the following:

   C File            Module in Library
   ------            -----------------

      extern int X;           int X;

      void main(void)
      {
         X = 5;
      }

   L2029 'X': unresolved external