Clemson University -- CPSC 231 -- Fall 2009


Scope (visibility)

  scope in C is simple:
     normal declaration inside a procedure is visible only within that procedure
     normal declaration outside a procedure is visible to all code below that
        point in that source file
     external declaration (extern) informs the linker of a reference to a non-
        static/non-local variable or function in another source file

  scope in assembly is similar:
     unless otherwise specified, symbols are visible only within the source file
        (thus, you can't use "loop" as a label twice within the same source
        file, but each separate source file can use "loop" as a local label)

     for external references (i.e., between source files) a pseudo-op is used in
        one source file to indicate the global availability of a symbol defined
        in that file (i.e., exporting the symbol) and another pseudo-op is used
        in another source file to indicate use of that symbol (i.e., importing
        that symbol); up to the linker to match the uses with the definitions

     in MASM, the two pseudo-ops PUBLIC and EXTERN are used for these purposes

     in SPARC assembly, .global is used both in the exporting file and in the
        importing file


     using MASM-like pseudo ops:   in C:
       define(EXTERN,global)
       define(PUBLIC,global)

     file a: .EXTERN sqrt          file d: extern double sqrt(double); 
             .EXTERN x                       /* can omit "extern" here;    */
             ...                             /* "static" means not public  */
             call sqrt                     extern int x;  /* "referencing" */
             ...                           ...            /*  declaration  */
             set  x,%o0                    int w;
             ld   [%o0],%x_r               ...
             set  w,%o1                    z = sqrt (y);
             st   %x_r,[%o1]               ...
             ...                           w = x;
             w: .word 0                    ...

     file b: .PUBLIC sqrt           file e: double sqrt(double argument){   
             sqrt:                         ...
             ...

     file c: .PUBLIC x              file f: int x;  /* "defining" decl. */
             ...                           ...
             x: .word 0
             ...


note: C has static scoping; there are languages with dynamic scoping, such
  as APL and Perl, in which the variable names have to be resolved at run
  time based on the current call chain