Compaq C for OpenVMS Systems provides support for the storage-class modifiers noshare , readonly , and _align as VAX C keywords. The recognition of these three storage-class modifiers as keywords (along with the other VAX C specific keywords) is controlled by a combination of the compiler mode and the /ACCEPT command-line qualifier. The default behavior on OpenVMS systems is for the compiler to recognize these storage-class modifiers as keywords in the VAX C compatibility mode and relaxed ANSI C mode (assuming that /ACCEPT=NOVAX_KEYWORDS is not also specified.) Conversely, they are not recognized by default in all other modes unless overridden by /ACCEPT=VAX_KEYWORDS.

Compaq C also provides the __inline , __forceinline and __align storage-class modifiers. These are recognized as valid keywords in all compiler modes on all platforms. They are in the namespace reserved to the C implementation, so it is not necessary to allow them to be treated as user-declared identifiers. They have the same effects on all platforms, except that on OpenVMS VAX systems, the __forceinline modifier does not cause any more inlining than the __inline modifier does.

Compaq C also provides the inline storage-class modifier. This modifier is supported in relaxed ANSI C mode (/STANDARD=RELAXED_ANSI89) or if the /ACCEPT=C99_KEYWORDS or /ACCEPT=GCCINLINE qualifier is specified.

For additional information about the __inline , __forceinline , __align , and inline storage-class modifiers, see the Compaq C Language Reference Manual.

You can use a storage-class specifier and a storage-class modifier in any order; usually, the modifier is placed after the specifier in the source code. For example:

extern noshare int x; /* Or, equivalently...*/ int noshare extern x;

The following sections describe each of the Compaq C storage-class modifiers.

4.4.1 The noshare Modifier

The noshare storage-class modifier assigns the attribute NOSHR to the program section of the variable. Use this modifier to allow other programs, used as shareable images, to have a copy of the variable's psect without the shareable images changing the variable's value in the original psect.

When a variable is declared with the noshare modifier and a shared image that has been linked to your program refers to that variable, a copy is made of the variable's original psect to a new psect in the other image. The other program may alter the value of that variable within the local psect without changing the value still stored in the psect of the original program.

For example, if you need to establish a set of data that will be used by several programs to initialize local data sets, then declare the external variables using the noshare specifier in a Compaq C program. Each program receives a copy of the original data set to manipulate, but the original data set remains for the next program to use. If you define the data as extern without the noshare modifier, a copy of the psect of that variable is not made; each program would be allowed access to the original data set, and the initial values would be lost as each program stores the values for the data in the psect. If the data is declared as const or readonly , each program is able to access the original data set, but none of the programs can then change the values.

You can use the noshare modifier with the static , extern , globaldef , and globaldef {"name"} storage-class specifiers. For more information about the possible combinations of specifiers and modifiers, and the effects of the storage-class modifiers on program-section attributes, see Section 4.8.

You can use noshare alone, which implies an external definition of storage class extern . Also, when declaring variables using the extern and globaldef {"name"} storage-class specifiers, you can use noshare , const , and readonly , together, in the declaration. If you declare variables using the static or the globaldef specifiers, and you use both of the modifiers in the declaration, the compiler ignores noshare and accepts const or readonly .

4.4.2 The readonly Modifier

The readonly storage-class modifier, like the const data-type qualifier, assigns the NOWRT attribute to the variable's program section; if used with the static or globaldef specifier, the variable is stored in the $CODE psect, which has the NOWRT attribute by default.

You can use both the readonly and const modifiers with the static , extern , globaldef , and globaldef {"psect"} storage-class specifiers.

In addition, both the readonly modifier and the const modifier can be used alone. When you specify these modifiers alone, an external definition of storage class extern is implied.

The const modifier restricts access to data in the same manner as the readonly modifier. However, in the declaration of a pointer, the readonly modifier cannot appear between the asterisk and the pointer variable to which it applies.

The following example shows the similarity between the const and readonly modifiers. In both instances, the point variable represents a constant pointer to a nonconstant integer.

readonly int * point; int * const point;

The _align and __align storage-class modifiers have the same semantic meaning. The difference is that __align is a keyword in all compiler modes while _align is a keyword only in modes that recognize VAX C keywords. For new programs, using __align is recommended.

The _align and __align storage-class modifiers align objects of any of the Compaq C data types on a specified storage boundary. Use these modifiers in a data declaration or definition.

See the Compaq C Language Reference Manual for a detailed description of __align and _align .

4.5 Floating-Point Numbers (float, double, long double)

When declaring floating-point variables, you determine the amount of precision needed for the stored object. In Compaq C, you can have single-precision, double-precision, and extended double-precision variables.

The float keyword declares a single-precision, floating-point variable. A float variable is represented internally in the VAX compatible, F_floating-point binary format.

For double-precision variables, you can choose D_floating or G_floating. On Alpha systems, you can also choose single- and double-precision IEEE formats (S_floating and T_floating, respectively), and extended double-precision format (X_floating).

The double keyword declares a double-precision, floating-point variable. Compaq C provides two VAX C compatible formats for specifying double variables: D_floating or G_floating.

The G_floating precision of approximately 15 digits is less than that of variables represented in D_floating format. Although there are more bits allocated to the exponent in G_floating precision, fewer bits are allocated to the mantissa, which determines precision (see Table 4-2 ).

In VAX C, the default representation of double variables is D_floating. To select the G_floating representation, you compiled with the /G_FLOAT qualifier.

In Compaq C, the /FLOAT qualifier replaces the /G_FLOAT qualifier, but /G_FLOAT is retained for compatibility.

When compiling with Compaq C on OpenVMS VAX systems, if you omit both /G_FLOAT and /FLOAT, the default representation of double variables is D_floating (unless /MIA is specified, in which case the default is G_floating).

When compiling with Compaq C on OpenVMS Alpha systems, if you omit both /G_FLOAT and /FLOAT, the default representation of double variables is G_floating.

For OpenVMS Alpha systems, the /FLOAT qualifier accepts the additional option IEEE_FLOAT. If you specify /FLOAT=IEEE_FLOAT, single and double variables are represented in IEEE_floating format (S_floating for single float, and T_floating for double float).

You cannot specify both the /FLOAT and /G_FLOAT qualifiers on the command line.

Modules compiled with the D_floating representation should not be linked with modules compiled with the G_floating representation. Since there are no functions in the Compaq C Run-Time Library (RTL) that perform floating-point format conversions on files, use the OpenVMS RTL functions MTH$CVT_D_G, MTH$CVT_G_D, MTH$CVT_DA_GA, and MTH$CVT_GA_DA if you do not wish to recompile the program. For more information about using the OpenVMS RTL, see the VMS Run-Time Library Routines Volume.

On OpenVMS VAX systems, Compaq C maps the ANSI C defined long double type to the G_floating or D_floating format.

On OpenVMS Alpha systems, long double variables are represented by default in the software-emulated X_floating format. If you specify /L_DOUBLE_SIZE=64, long double variables are represented as G_floating, D_floating, or T_floating, depending on the value of the /FLOAT or /G_FLOAT qualifier.

Table 4-2 shows the supported floating-point formats, and their approximate sizes and range of values.

Table 4-2 Floating-Point Formats

Data type	Floating-Point Format	Length of Variable	Range of Values	Precision (decimal digits)
float	F_floating	32-bit	2.9 * 10 -39 to 1.7 * 10 38	6
double	D_floating	64-bit	2.9 * 10 -39 to 1.7 * 10 38	16
double	G_floating	64-bit	5.6 * 10 -309 to 9.0 * 10 307	15
float	S_floating (ALPHA ONLY)	32-bit	1.2 * 10 -38 to 3.4 * 10 38	6
double	T_floating (ALPHA ONLY)	64-bit	2.2 * 10 -308 to 1.8 * 10 308	15
long double	X_floating (ALPHA ONLY)	128-bit	3.4 * 10 -4932 to 1.2 * 10 4932	33

4.6 Pointer Conversions

When running the compiler in VAX C mode, relaxed pointer and pointer/integer compatibility is allowed. That is, all pointer and integer types are compatible, and pointer types are compatible with each other regardless of the type of the object they point to. Therefore, in VAX C mode, a pointer to float is compatible with a pointer to int . This is not true in ANSI C mode.

Although pointer conversions do not involve a representation change when compiling in VAX C mode, because of alignment restrictions on some machines, access through an unaligned pointer can result in much slower access time, a machine exception, or unpredictable results.

4.7 Structure Alignment

The alignment and size of a structure is affected by the alignment requirements and sizes of the structure components for each Compaq C platform. A structure can begin on any byte boundary and occupy any integral number of bytes. However, individual architectures or operating systems can specify particular alignment and padding requirements.

Compaq C on VAX processors does not require that structures or structure members be aligned on any particular boundaries.

The components of a structure are laid out in memory in the order they are declared. The first component has the same address as the entire structure. On VAX processors, each additional component follows its predecessor in the immediately following byte.

For example, the following type is aligned as shown in Figure 4-1:

struct {char c1; short s1; float f; char c2; }

Figure 4-1 OpenVMS VAX Structure Alignment

The alignment of the entire structure can occur on any byte boundary, and no padding is introduced. The float variable f may span longwords, and the short variable s1 may span words.

The following pragma can be used to force specific alignments:

#pragma member_alignment

Structure alignment for Compaq C for OpenVMS Systems on VAX processors is achieved by the default, #pragma nomember_alignment , which causes data structure members to be byte-aligned (with the exception of bit-field members).

Structure alignment for Compaq C for OpenVMS Systems on Alpha processors is achieved by the default, #pragma member_alignment , which causes data structure members to be naturally aligned. This means that data structure members are aligned on the next boundary appropriate to the type of the member, rather than on the next byte.

For more information on the #pragma member_alignment preprocessor directive, see Section 5.4.12.

4.7.1 Bit-Field Alignment

Bit fields can have any integral type. However, the compiler issues a warning if /STANDARD=ANSI89 is specified, and the type is something other than int , unsigned int , or signed int . Bit fields are allocated within the unit from low order to high order. If a bit field immediately follows another bit field, the bits are packed into adjacent space, even if this overflows into another byte. However, if an unnamed bit field is specified to have length 0, filler is added so the bit field immediately following starts on the next byte boundary.

For example, the following type is aligned as shown in Figure 4-2:

struct {int i:2; int ii:2; unsigned int ui: 30; }

Figure 4-2 OpenVMS Bit-Field Alignment

Bit field ii is positioned immediately following bit field i . Because there are only 28 bit positions remaining and ui requires 30 bits, the first 28 bits of ui are put into the first longword, and the remaining two bits overflow into the next longword.

4.7.2 Bit-Field Initialization

The Compaq C compiler initializes bit fields in struct s differently than VAX C does. The following program compiles without error using both compilers but the results are different. Compaq C skips over unnamed bits but VAX C does not.

#include <stdio.h> int t() { static struct bar {unsigned :1; unsigned one : 1; unsigned two : 1; }; struct bar foo = {1,0}; printf("%d %d\n",foo.one,foo.two); return 1; }

When compiled with Compaq C, this example produces the following output:

1 0

When compiled with VAX C, this example produces the following output:

0 0

4.7.3 Variant Structures and Unions

Variant structures and unions are Compaq C extensions available in VAX C compatibility mode only, and they are not portable.

Variant structure and union declarations allow you to refer to members of nested aggregates without having to refer to intermediate structure or union identifiers. When a variant structure or union declaration is nested within another structure or union declaration, the enclosed variant aggregate ceases to exist as a separate aggregate, and Compaq C propagates its members to the enclosing aggregate.

Variant structures and unions are declared using the variant_struct and variant_union keywords. The format of these declarations is the same as that for regular structures or unions, with the following exceptions:

• Variant aggregates must be nested within other valid structure or union declarations.
• A tag cannot be used in a variant aggregate declaration.
• At least one declarator must be declared in the variant aggregate declaration, and it must not be declared as a pointer or an array.

Initialization of a variant structure or union is the same as that for a normal structure or union.

As with regular structures and unions, in VAX C compatibility mode, variant structures and unions in an assignment operation need only have the same size in bits, rather than requiring the same members and member types.

To show the use of variant aggregates, consider the following code example that does not use variant aggregates:

/* The numbers to the right of the code represent the byte offset * * from the enclosing structure or union declaration. */ struct TAG_1 { int a; /* 0-byte enclosing_struct offset */ char *b; /* 4-byte enclosing_struct offset */ union TAG_2 /* 8-byte enclosing_struct offset */ { int c; /* 0-byte nested_union offset */ struct TAG_3 /* 0-byte nested_union offset */ { int d; /* 0-byte nested_struct offset */ int e; /* 4-byte nested_struct offset */ } nested_struct; } nested_union; } enclosing_struct;

If you want to access nested member d , then you need to specify all the intermediate aggregate identifiers:

enclosing_struct.nested_union.nested_struct.d

If you try to access member d without specifying the intermediate identifiers, then you would access the incorrect offset from the incorrect structure. Consider the following example:

enclosing_struct.d

The compiler uses the address of the original structure ( enclosing_struct ), and adds to it the assigned offset value for member d (0 bytes), even though the offset value for d was calculated according to the nested structure ( nested_struct ). Consequently, the compiler accesses member a (0-byte offset from enclosing_struct ) instead of member d .

The following code example shows the same code using variant aggregates:

/* The numbers to the right of the code present the byte offset * * from enclosing_struct. */ 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 */ variant_struct { int d; /* 8-byte enclosing_struct offset */ int e; /* 12-byte enclosing_struct offset */ } nested_struct; } nested_union; } enclosing_struct;

The members of the nested_union and nested_struct variant aggregates are propagated to the immediately enclosing aggregate ( enclosing_struct ). The variant aggregates cease to exist as individual aggregates.

Since the nested_union and nested_struct variant aggregates do not exist as individual aggregates, you cannot use tags in their declarations, and you cannot use their identifiers ( nested_union , nested_struct ) in any reference to their members. However, you are free to use the identifiers in other declarations and definitions within your program.

To access member d , use the following notation:

enclosing_struct.d

Using the following notation causes unpredictable results:

enclosing_struct.nested_union.nested_struct.d

If you use normal structure or union declarations within a variant aggregate declaration, the compiler propagates the structure or union to the enclosing aggregate, but the members remain a part of the nested aggregate. For example, if the nested structure in the last example was of type struct , the following offsets would be in effect: