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HP C
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This chapter presents the following topics concerning HP C data storage and representation on OpenVMS systems:
When you define a HP C variable, the storage class determines not only its scope but also its location and lifetime. The lifetime of a variable is the length of time for which storage is allocated. For OpenVMS systems, storage for a HP C variable can be allocated in the following locations:
Variables that are placed on the stack or in a register are temporary. For example, variables of the auto and register storage classes are temporary. Their lifetimes are limited to the execution of a single block or function. All declarations of the internal storage classes (auto and register) are also definitions; the compiler generates code to establish storage at this point in the program.
Program sections, or psects, are used for permanent variables; the lifetime of identifiers extends through the course of the entire program. A psect represents an area of virtual memory that has a name, a size, and a series of attributes that describe the intended or permitted usage of that portion of memory. For example, the compiler places variables of the static, external, and global storage classes in psects; you have some control as to which psects contain which identifiers. All declarations of the static storage class are also definitions; the compiler creates the psect at that point in the program. In HP C, the first declaration of the external storage class is also a definition; the linker initializes the psect at that point in the program.
Table 4-1 shows the location and lifetime of a variable when you use each of the storage-class keywords.
For a comparison between the global and external storage classes, see Section 4.3.2.
For more information about psects, see Section 4.8.
Sections 4.3 and 4.4 describe the following external linkage storage-class specifiers and modifiers that are specific to HP C for OpenVMS Systems:
These keywords are supported by the HP C compiler for compatibility purposes, and are available only in VAX C mode (/STANDARD=VAXC) and relaxed mode (/STANDARD=RELAXED).
However, the HP C compiler also provides an alternative, standard-conforming method of controlling objects that have external linkage. To take advantage of this method, use the #pragma extern_model preprocessor directive and the /EXTERN_MODEL and /[NO]SHARE_GLOBALS command-line qualifiers.
The pragma and command-line qualifiers replace the VAX C mode storage-class specifiers (globaldef, globalref, globalvalue) and storage-class modifiers (noshare and readonly). They allow you to select the implementation model of external data and control the psect usage of your programs. The _align storage-class modifier is still used to ensure object alignment.
The pragma and command-line qualifier approach also has these advantages:
For a description of the #pragma extern_model preprocessor directive and its relationship to the external storage classes it replaces, see Section 5.4.5.
For a description of the _align storage-class modifier, see Section 4.4.3.
For a description of the /EXTERN_MODEL and /[NO]SHARE_GLOBALS command-line qualifiers, see Section 1.3.4.
In addition to the storage-class specifiers described in the HP C Language Reference Manual, the VAX C compatibility mode of HP C provides the globaldef, globalref, and globalvalue storage-class specifiers. These specifiers allow you to assign the global storage classes to identifiers. The global storage classes are specific to HP C for OpenVMS Systems and are not portable.
Use the globaldef specifier to define a global variable. Use the globalref specifier to refer to a global variable defined elsewhere in the program.
When you use the globaldef specifier to define a global symbol, the symbol is placed in one of three program sections: the $DATA (VAX ONLY) or $DATA$ (ALPHA, I64) psect using globaldef alone, the $CODE (VAX ONLY) or $READONLY$ (ALPHA, I64) psect using globaldef with readonly or const, or a user-named psect. You can create a user-named psect by specifying the psect name as a string constant in braces immediately following the globaldef keyword, as shown in the following definition:
globaldef{"psect_name"} int x = 2; |
This definition creates a program section called psect_name and allocates the variable x in that psect. You can add any number of global variables to this psect by specifying the same psect name in other globaldef declarations. In addition, you can specify the noshare modifier to create the psect with the NOSHR attribute. Similarly, you can specify the readonly or const modifier to create the psect with the NOWRT attribute. 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.
Variables declared with globaldef can be initialized; variables declared with globalref cannot, because these declarations refer to variables defined, and possibly initialized, elsewhere in the program. Initialization is possible only when storage is allocated for an object. This distinction is especially important when the readonly or const modifier is used; unless the global variable is initialized when the variable is defined, its permanent value is 0.
In the VAX MACRO programming language, it is possible to give a global variable more than one name. However, in HP C, only one global name can be used for a particular variable. HP C assumes that distinct global variable names denote distinct objects; the storage associated with different names must not overlap. |
Example 4-1 shows the use of global variables.
Example 4-1 Using Global Variables |
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/* This example shows how global variables are used * * in HP C programs. */ #include <stdlib.h> #include <stdio.h> extern void fn(); (1)int ex_counter = 0; (2)globaldef double velocity = 3.0e10; (3)globaldef {"distance"} long miles = 100; int main() { printf(" *** FIRST COMP UNIT ***\n"); printf("counter:\t%d\n", ex_counter); printf("velocity:\t%g\n", velocity); printf("miles:\t\t%d\n\n", miles); fn(); printf(" *** FIRST COMP UNIT ***\n"); printf("counter:\t%d\n", ex_counter); |
(4) printf("velocity:\t%g\n", velocity); printf("miles:\t\t%d\n\n", miles); exit (EXIT_SUCCESS); } /* ---------------------------------------------------- * * The following code is contained in a separate * * compilation unit. * * ---------------------------------------------------- */ #include <stdio.h> static ex_counter; (5)globalref double velocity; globalref long miles; fn(void) { ++ex_counter; printf(" *** SECOND COMP UNIT ***\n"); if ( miles > 50 ) velocity = miles * 3.1 / 200 ; printf("counter:\t%d\n", ex_counter); printf("velocity:\t%g\n", velocity); printf("miles:\t\t%d\n", miles); } |
Key to Example 4-1:
Sample output from Example 4-1 is as follows:
$ RUN EXAMPLE.EXE[Return] *** FIRST COMP UNIT *** counter: 0 velocity: 3.000000e+10 miles: 100 *** SECOND COMP UNIT *** counter: 1 velocity: 1.55 miles: 100 *** FIRST COMP UNIT *** counter: 0 velocity: 1.55 miles: 100 |
The global storage-class specifiers define and declare objects that differ from external variables both in their storage allocation and in their correspondence to elements of other languages. Global variables provide a convenient and efficient way for a HP C function to communicate with assembly language programs, with OpenVMS system services and data structures, and with other high-level languages that support global symbol definition, such as HP PL/I. For more information about multilanguage programming, see Chapter 3.
HP C imposes no limit on the number of external variables in a single program.
There are other functional differences between the external and global variables. For example:
#ifdef __DECC #define EXPORT globaldef #define IMPORT globalref #else #define EXPORT #define IMPORT extern #endif . . . IMPORT int foo; EXPORT int foo = 53; |
One similarity between the external and global storage classes is in the way the compiler recognizes these variables internally. External and global identifiers are not case-sensitive. No matter how the external and global identifiers appear in the source code, the compiler converts them to uppercase letters. For ease in debugging programs, express all global and external variable identifiers in uppercase letters.
Another similarity between the external and global storage classes is that you can place the external variables and the global variables (optionally) in psects with a user-defined name and, to some degree, user-defined attributes. The compiler places external variables in psects of the same name as the variable identifier, viewed by the linker in uppercase letters. The compiler places globaldef{"name"} variables in psects with names specified in quotation marks, delimited by braces, and located directly after the globaldef specifier in a declaration. Again, the linker considers the psect name to be in uppercase letters.
The compiler places a variable declared using only the globaldef specifier and a data-type keyword into the $DATA (VAX ONLY) or $DATA$ (ALPHA, I64) psect. 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.
A global value is an integral value whose identifier is a global symbol. Global values are useful because they allow many programmers in the same environment to refer to values by identifier, without regard to the actual value associated with the identifier. The actual values can change, as dictated by general system requirements, without requiring changes in all the programs that refer to them. If you make changes to the global value, you only have to recompile the defining compilation unit (unless it is defined in an object library), not all of the compilation units in the program that refer to those definitions.
You can use the globalvalue specifier only with identifiers of type enum, int, or with pointer variables. |
An identifier declared with globalvalue does not require storage. Instead, the linker resolves all references to the value. If an initializer appears with globalvalue, the name defines a global symbol for the given initial value. If no initializer appears, the globalvalue construct is considered a reference to some previously defined global value.
Predefined global values serve many purposes in OpenVMS system programming, such as defining status values. It is customary in OpenVMS system programming to avoid explicit references to such values as those returned by system services, and to use instead the global names for those values.
HP 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 mode (assuming that /ACCEPT=NOVAXC_KEYWORDS is not also specified.) Conversely, they are not recognized by default in all other modes unless overridden by /ACCEPT=VAXC_KEYWORDS.
HP 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 VAX systems, the __forceinline modifier does not cause any more inlining than the __inline modifier does.
HP C also provides the inline storage-class modifier. This modifier is supported in relaxed mode (/STANDARD=RELAXED) 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 HP 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 HP C storage-class modifiers.
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 HP 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.
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