Null_directive (#)
A preprocessing directive of the form # <newline> is a null
directive and has no effect.
Conditional_Compilation
Conditional compilation is provided by the following directives:
#if constant-expression
Checks whether the constant expression is nonzero (true).
#ifdef identifier
Checks whether the identifier is defined.
#ifndef identifier
Checks whether the identifier is undefined.
#else
Introduces source lines to be compiled as an alternative to the
conditions tested by the previous directives.
#elif constant-expression
Delimits alternative source lines to be compiled if the constant
expression in the corresponding #if, #ifdef, or #ifndef
directive is false and if the additional constant expression
presented in the #elif directive is true. An #elif directive is
optional.
#endif
Ends the scope of the previous directives.
If the condition checked by #if, #ifdef, or #ifndef is true, then
all lines between the #else, #elif, and #endif are ignored. If the
condition is false, then any lines between the conditional
directive and the #else or #elif (if any) are ignored. If there is
no #else, then the lines between the conditional and the #endif are
ignored.
#define
The #define preprocessor directive has the form:
#define identifier token-string
The preprocessor substitutes the token string everywhere in the
program that it finds the identifier except within comments,
character constants, or string constants.
Macro replacements are defined in a #define directive of the
following form:
#define name([parm1[,parm2,...]]) token-string
Within the program, all macro references that have the following
form are replaced by the token string. The arguments in the macro
reference replace the corresponding parameters in the token string.
name([arg1[,arg2,...]])
#dictionary
The #dictionary directive is retained for compatibility with VAX C
and is supported only when running the DIGITAL C++ compiler in VAX C
mode (/STANDARD=VAXC).
#error
The #error directive issues an optional diagnostic message, and
ends compilation. This directive has the following form:
#error [message] <newline>
#include
The #include directive instructs the preprocessor to insert the
contents of the specified file or module into the program. An
#include directive can have one of three forms:
#include "filespec"
#include <filespec>
#include module-name
The first two forms are ANSI-compliant methods of file inclusion
and are therefore more portable. In these forms, .h is the default
file type, unless the compiler is instructed to supply no default
type (that is, a type of just ".") by the
/ASSUME=NOHEADER_TYPE_DEFAULT qualifier.
The third form is specific to OpenVMS systems for specifying the
inclusion of a module from a text library, and is not generally
needed or recommended because the ANSI forms also cause the text
libraries to be searched.
For the order of search, see /INCLUDE_DIRECTORY.
There is no defined limit to the nesting level of #include files
and modules.
#line
The #line directive applies a specified line number and optional
file specification to the next line of source text. This can be
useful for diagnostic messages. The #line directive has the
following forms:
#line integer-constant <newline>
#line integer-constant "filename" <newline>
#line pp-tokens <newline>
In the first two forms, the compiler gives the line following a
#line directive the number specified by the integer constant. The
optional filename in quotation marks indicates the name of the
source file that the compiler will provide in its diagnostic
messages. If the filename is omitted, the file name used is the
name of the current source file or the last filename specified in a
previous #line directive.
In the third form, macros in the #line directive are expanded
before it is interpreted. This allows a macro call to expand into
the integer-constant, filename, or both. The resulting #line
directive must match one of the other two forms, and is then
processed as appropriate.
#module
The #module directive is retained for compatibility with VAX C and
is supported only when running the DIGITAL C++ compiler in VAX C mode
(/STANDARD=VAXC). See also the ANSI C++ equivalent #pragma module
directive.
The #module directive passes information about an object module to
the compiler.
The #module directive can have one of the following forms:
#module identifier identifier
#module identifier string
The first argument of the directive is a DIGITAL C++ identifier or
macro that resolves to an identifier. It gives the system-recognized
(for example, internally recognized by the debugger and the
librarian) name of the module; the object file name remains the
same. The second argument specifies the optional identification
that appears on listings. This may be either a VAX C identifier, a
character-string constant with no more than 31 characters, or a
macro that resolves to one of these.
There can be only one #module directive per compilation. It can
appear anywhere before the C language text.
#pragma
The #pragma directive performs compiler-specific tasks as
designated by each implementation of the C language. DIGITAL
C++ for OpenVMS Systems supports the following pragmas:
#pragma [no]builtins
Enables the DIGITAL C++ built-in functions that directly access
processor instructions. If the pragma does not appear in your
program, the default is #pragma nobuiltins.
#pragma environment
Sets, saves, or restores the states of context pragmas. This
directive protects include files from contexts set by
encompassing programs, and protects encompassing programs from
contexts that could be set in header files that they include.
The #pragma environment directive affects the following
pragmas:
o #pragma extern_model
o #pragma extern_prefix
o #pragma member_alignment
o #pragma message
o #pragma pointer_size
o #pragma required_pointer_size
Syntax:
#pragma environment command_line
#pragma environment header_defaults
#pragma environment restore
#pragma environment save
command_line
Sets, as specified on the command line, the states of all
the context pragmas. You can use this pragma to protect
header files from environment pragmas that take effect
before the header file is included.
header_defaults
Sets the states of all the context pragmas to their
default values. This is almost equivalent to the
situation in which a program with no command-line options
and no pragmas is compiled, except that this pragma sets
the pragma message state to #pragma nostandard, as is
appropriate for header files.
save
Saves the current state of every pragma that has an
associated context.
restore
Restores the current state of every pragma that has an
associated context.
#pragma extern_model
Controls the compiler's interpretation of objects that have
external linkage. This pragma lets you choose the global
symbol model to be used for externs.
Syntax:
#pragma extern_model common_block [attr[,attr]...]
#pragma extern_model relaxed_refdef [attr[,attr]...]
#pragma extern_model strict_refdef "name" [attr[,attr]...]
#pragma extern_model strict_refdef
#pragma extern_model globalvalue
#pragma extern_model save
#pragma extern_model restore
The default model on DIGITAL C++ is #pragma relaxed_refdef noshr.
This is different from the model used by VAX C, which is common
block, shr.
The [attr[,attr]...] are optional psect attribute
specifications chosen from the following (at most one from each
line):
o gbl lcl (Not allowed with relaxed_refdef)
o shr noshr
o wrt nowrt
o pic nopic (Not meaningful for Alpha)
o ovr con
o rel abs
o exe noexe
o vec novec
o 0 byte 1 word 2 long 3 quad 4 octa 16 page
See Using DIGITAL C++ for OpenVMS Alpha Systems for more information
on the #pragma extern_model directive.
#pragma extern_prefix
Controls the compiler's synthesis of external names, which the
linker uses to resolve external name requests.
When you specify #pragma extern_prefix with a string argument,
the compiler prepends the string to all external names
produced by the declarations that follow the pragma
specification.
This pragma is useful for creating libraries where the
facility code can be attached to the external names in the
library.
Syntax:
#pragma extern_prefix "string"
#pragma extern_prefix save
#pragma extern_prefix restore
Where "string" prepends the quoted string to external names in
the declarations that follow the pragma specification.
The save and restore keywords can be used to save the current
pragma prefix string and to restore the previously saved pragma
prefix string, respectively.
The default external prefix, when none has been specified by a
pragma, is the null string.
#pragma function
Specifies that calls to the specified functions are not
intrinsic but are, in fact, function calls. This pragma has
the opposite effect of #pragma intrinsic.
Syntax:
#pragma function (function1[, function2, ...])
#pragma [no]inline
Expands function calls inline. The function call is replaced
with the function code itself.
Syntax:
#pragma inline (id,...)
#pragma noinline (id,...)
If a function is named in an inline directive, calls to that
function will be expanded as inline code, if possible.
If a function is named in a noinline directive, calls to that
function will not be expanded as inline code.
If a function is named in both an inline and a noinline
directive, an error message is issued.
For calls to functions named in neither an inline nor a
noinline directive, DIGITAL C++ expands the function as inline code
whenever appropriate as determined by a platform-specific
algorithm.
#pragma intrinsic
Specifies that calls to the specified functions are intrinsic
(that is, handled internally by the compiler, allowing it to
generate inline code, move or eliminate calls, or do various
other optimizations). This pragma is only valid for functions
that are known to the compiler.
Syntax:
#pragma intrinsic (function1[, function2, ...])
#pragma [no]member_alignment
Tells the compiler to align structure members on the next
boundary appropriate to the type of the member rather than the
next byte. For example, a long variable is aligned on the next
longword boundary; a short variable on the next word boundary.
Syntax:
#pragma nomember_alignment [base_alignment]
#pragma member_alignment [save | restore]
The optional base_alignment parameter can be used with #pragma
nomember_alignment to specify the base alignment of the
structure. Use one of the following keywords to specify the
base_alignment:
o BYTE (1 byte)
o WORD (2 bytes)
o LONGWORD (4 bytes)
o QUADWORD (8 bytes)
o OCTAWORD (16 bytes)
The optional save and restore keywords can be used to save the
current state of the member_alignment and to restore the
previous state, respectively. This feature is necessary for
writing header files that require member_alignment or
nomember_alignment, or that require inclusion in a
member_alignment that is already set.
#pragma message
Controls the issuance of individual diagnostic messages or
groups of messages. Use of this pragma overrides any
command-line options that may affect the issuance of messages.
Syntax:
#pragma message option1 message-list
#pragma message option2
where option1 is:
disable Suppresses the issuance of the indicated
messages.
Only messages of severity Warning (W) or
Information (I) can be disabled. If the
message has severity of Error (E) or
Fatal (F), it is issued regardless of
any attempt to disable it.
enable Enables the issuance of the indicated
messages.
errors Sets the severity of each message in the
message-list to Error.
fatals Sets the severity of each message on the
message-list to Fatal.
informationals Sets the severity of each message in the
message-list to Informational.
warnings Sets the severity of each message in the
message-list to Warning.
The message-list can be any one of the following:
o A single message identifier (within parentheses or not).
o A single message group name (within parentheses or not).
Message group name is:
all All the messages in the compiler
o A comma-separated list of message identifiers and group name,
freely mixed, enclosed in parentheses.
option2 is:
save -- saves the current state of which messages are
enabled and disabled.
restore -- restores the previous state of which messages
are enabled and disabled.
The save and restore options are useful primarily within header
fil #pragma module
The ANSI C compliant #pragma module directive is equivalent to
the VAX C compatible #module directive, but is supported in all
compiler modes. (The #module directive is retained for
compatibility and is supported only when compiling with the
/STANDARD=VAXC qualifier.) The #pragma module directive is
specific to DIGITAL C++ for OpenVMS Systems and is not portable.
Use the #pragma module directive to change the
system-recognized module name and version number. You can find
the module name and version number in the compiler listing file
and the linker load map.
Syntax:
#pragma module identifier identifier
#pragma module identifier string
The first parameter must be a valid DIGITAL C++ identifier. It
specifies the module name to be used by the linker. The second
parameter specifies the optional identification that appears on
listings and in the object file. It must be either a valid DEC
C identifier of 31 characters or less, or a character-string
constant of 31 characters or less.
Only one #pragma module directive can be processed per
compilation unit, and that directive must appear before any C
language text. The #pragma module directive can follow other
directives, such as #define, but it must precede any function
definitions or external data definitions.
#pragma pack
Specifies the byte boundary for packing members of C
structures.
Syntax:
#pragma pack [n]
The n specifies the new alignment restriction in bytes:
1 - align to byte
2 - align to word
4 - align to longword
8 - align to quadword
16 - align to octaword
A structure member is aligned to either the alignment specified
by #pragma pack or the alignment determined by the size of the
structure member, whichever is smaller. For example, a short
variable in a structure gets byte-aligned if #pragma pack 1 is
specified. If #pragma pack 2, 4, or 8 is specified, the short
variable in the structure gets aligned to word.
If #pragma pack is not used, or if it is specified without the
n, packing defaults to 16 on OpenVMS Alpha systems, and to 1
(byte alignment) on OpenVMS VAX systems.
#pragma pointer_size
Controls whether pointers are 32-bit pointers or 64-bit
pointers.
Syntax:
#pragma pointer_size keyword
Where keyword is one of the following:
o short -- 32-bit pointer
o long -- 64-bit pointer
o system_default -- 32-bit pointers on OpenVMS systems;
64-bit pointers on Digital UNIX systems
o save -- Saves the current pointer size
o restore -- Restores the current pointer size to its last
saved state
This directive is enabled only when the /POINTER_SIZE command-line
qualifier is specified. Otherwise, #pragma pointer_size has the
same effect as #pragma required_pointer_size.
#pragma required_pointer_size
Intended for use by developers of header files to control
pointer size within header files.
Syntax:
#pragma required_pointer_size keyword
Where keyword is one of the following:
o short -- 32-bit pointer
o long -- 64-bit pointer
o system_default -- 32-bit pointers on OpenVMS systems;
64-bit pointers on Digital UNIX systems
o save -- Saves the current pointer size
o restore -- Restores the current pointer size to its last
saved state
This directive is always enabled, even if the /POINTER_SIZE
command-line qualifier is omitted. Otherwise, #pragma
required_pointer_size has the same effect as #pragma pointer_size.
#pragma [no]standard
Directs the compiler to define regions of source code where
portability diagnostics are not to be issued.
Use #pragma nostandard to suppress diagnostics about non-ANSI C
extensions, regardless of the /STANDARD qualifier specified,
until a #pragma standard directive is encountered.
Use #pragma standard to reinstate the setting of the /STANDARD
qualifier that was in effect before before the last #pragma
nostandard was encountered.
Every #pragma standard directive must be preceded by a
corresponding #pragma nostandard directive.
Note that this pragma does not change the current mode of the
compiler or enable any extensions not already supported in that
mode.
#pragma use_linkage
Associates a special linkage, defined by the #pragma linkage
directive, with the specified functions.
Syntax:
#pragma use_linkage linkage-name (routine1, routine2, ...)
The linkage-name is the name of a linkage previously defined by
the #pragma linkage directive.
The parenthesized list contains the names of functions you want
to associated with the named linkage.
#undef
The #undef directive cancels a previously defined macro
replacement. Any other macro replacements that occurred before the
#undef directive remain.
The #undef directive has the following syntax:
#undef identifier
