struct  TAG_1
{
   int   a;           /* 0-byte  enclosing_struct offset */
   char  *b;          /* 4-byte  enclosing_struct offset */
   variant_union
   {
      int  c;         /* 8-byte  enclosing_struct offset */
      struct  TAG_2   /* 8-byte  enclosing-struct offset */
      {
         int  d;      /* 0-byte   nested_struct offset */
         int  e;      /* 4-byte   nested_struct offset */
      }  nested_struct;
   } nested_union;
} enclosing_struct;

In this case, to access member d , use the following notation:

enclosing_struct.nested_union.nested_struct.d

4.8 Program Sections

The following sections describe program-section attributes and program sections created by Compaq C for OpenVMS Systems.

4.8.1 Attributes of Program Sections

As the Compaq C compiler creates an object module, it groups data into contiguous program sections, or psects. The grouping depends on the attributes of the data and on whether the psects contain executable code or read/write variables.

The compiler also writes into each object module information about the program sections contained in it. The linker uses this information when it binds object modules into an executable image. As the linker allocates virtual memory for the image, it groups together program sections that have similar attributes.

Table 4-3 lists the attributes that can be applied to program sections.

4.8.2 Program Sections Created by Compaq C

If necessary, Compaq C creates the following program sections:

• $CODE (VAX ONLY)---Contains all executable code and constant data (including variables defined with the readonly modifier or const type qualifier).
• $CODE$ (ALPHA ONLY)---Contains all executable code.
• $READONLY$ (ALPHA ONLY)---Contains all constant data defined with the readonly modifier or const type qualifier.
• $DATA (VAX ONLY) or $DATA$ (ALPHA ONLY)---Contains all static variables, as well as global variables defined without the readonly modifier or const type qualifier. $DATA also contains character-string constants when /ASSUME=WRITABLE_STRING_LITERALS is specified.
• $LITERAL$ (ALPHA ONLY)---Contains character-string constants when /ASSUME=NOWRITABLE_STRING_LITERALS is specified.
• Compaq C also creates additional program sections for variables declared with the globaldef keyword if the optional psect name in braces is specified, or for variables declared with the extern storage class, depending on the external model.

All program sections created by Compaq C have the PIC, REL, RD, USR, and NOVEC attributes. On OpenVMS VAX systems, the $CODE psect is aligned on a byte boundary; all other psects generated by Compaq C are aligned on longword boundaries. On OpenVMS Alpha systems, all psects generated by Compaq C are aligned on octaword boundaries. Note that use of the _align storage-class modifier can cause a psect to be aligned on greater than a longword boundary on OpenVMS VAX systems. The $CHAR_STRING_CONSTANTS psect has the same attributes as the $DATA (VAX ONLY) and $DATA$ (ALPHA ONLY) psects.

Tables 4-4, 4-5, 4-6, and 4-7 summarize the differences in psects created by different declarations:

• Table 4-4, Table 4-5 (ALPHA ONLY), and Table 4-6 (VAX ONLY) show different cases of variable definitions and assign to them a storage-class code number:
  • Table 4-4 shows the effect of each #pragma extern_model preprocessor directive on the storage-class code number for external variable definitions that have an extern storage class.
  • Table 4-5 shows the storage-class code number for variable definitions that do not have the extern storage class on OpenVMS Alpha systems.
  • Table 4-6 shows the storage-class code number for variable definitions that do not have the extern storage class on OpenVMS VAX systems.
• Table 4-7 shows the psect name and attributes associated with each storage-class code number from Tables 4-4, 4-5, and 4-6.

Table 4-7 shows the psect name and psect attributes for the storage-class code numbers from Table 4-4, Table 4-5, and Table 4-6. Where name is used for the psect name in Table 4-7, the name of the psect is the same as name in the declarations or pragmas in Table 4-4, or the quoted brace-enclosed names in Tables 4-5 and 4-6.

The combined use of the readonly and noshare modifiers is ignored by the compiler in the following declarations:

readonly  noshare  static  int  x;
readonly  noshare  globaldef  int  x;

When it encounters a situation as shown in the previous example, the compiler ignores the noshare modifier and accepts readonly . The order of the storage-class specifier, the storage-class modifier, and the data-type keyword within a declaration is not significant.

The Compaq C compiler does static (global) initialization of pointers by using the .ADDRESS directive. By using this mechanism, the compiler efficiently generates position-independent code. The linker makes image sections that contain such initialization nonshareable.