DEC C
Language Reference Manual
B.39 The ## Operator (§3.8.3.3)
The ## operator within a macro
replacement list causes the two tokens on either side of the operator
to be concatenated into a single token.
In common C and VAX C compatibility mode, comments can also concatenate
two tokens because in these modes a comment is replaced by a null
string after macro invocations.
This behavior is not supported in strict ANSI or default mode, where
comments are replaced with a single space.
B.40 Error Directive (§3.8.5)
The #error directive causes an error
message to be issued and the compilation to cease.
B.41 Pragma Directive (§3.8.6)
The Standard's approved method of adding extensions to the language is
through the addition of pragmas. All unrecognized pragmas are diagnosed
with an informational message. Supported pragmas vary across platforms.
See your platform-specific DEC C documentation for more
information.
When only preprocessing a file, all pragmas recognized by DEC C
are written unaltered to the output.
B.42 Function Inline Expansion
Function inline expansion eliminates procedure-call overhead
and allows general optimization methods to apply across the expanded
code. Function inlining has advantages over macros in that arguments
are evaluated only once, parentheses need not be overused to avoid
problems with precedence, and the actual expansion can be controlled
from the command line.
The following pragmas are provided to control function inline expansion:
#pragma inline (function_name [,function_name....])
#pragma noinline (function_name [,function_name....])
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If a function is named in an inline
directive, calls to it are expanded as inline code, if the function has
the following properties:
- If a function is named in a noinline
directive, calls to it are not expanded as inline code.
- If a function is not named in an inline or a noinline directive, the compiler uses a
heuristic to perform inline expansion of calls where appropriate.
- The compiler issues an error if a function is named in both an
inline and a noinline directive.
If the noinline
compiler option is used, it overrides all inline pragma directives.
Inline functions have the following properties:
- An inline function can be recursive, but only one level of inline
expansion is performed if it is.
- Only calls from the source file containing the definition of the
inlined function are expanded inline.
- The address of an inline function can be taken and expressions
that imply the conversion of the inlined function name to an address
are allowed.
- The use of the varargs package
(allowing a function to take a variable number of arguments) is not
allowed for inline functions.
- An inline function cannot be declared with an ellipsis in its
argument list.
B.43 Linkage Pragmas
DEC C supports the #pragma
linkage and #pragma
use_linkage preprocessor directives on OpenVMS Alpha
systems.
These pragmas are used for defining special linkage characteristics and
to associate these linkage characteristics with functions. See your
platform-specific DEC C documentation for more information.
B.44 Other Pragmas
The following pragmas are provided for VAX C compatibility mode only:
#pragma dictionary CDD_path
#pragma module title ident
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These pragmas correspond to the #dictionary and #module directives, respectively.
See your platform-specific DEC C documentation for additional
pragmas supported on your system.
Appendix C
ASCII Equivalence Table
Figure C-1 shows the ASCII character set. Each character's octal,
decimal, and hexadecimal value is shown.
Figure C-1 ASCII Equivalence Chart
Appendix D
Common C Extensions Supported by DEC C
DEC C supports several common C (old-style C) extensions to
ANSI-standard C. These extensions are recognized only when the common C
compatibility option is used on the compiler command line. The common C
extensions allow you to use the c89 compiler to compile code originally
written for the portable C compiler (pcc).
The following sections describe the common C extensions available with
the common C compatibility option. Extensions to the ANSI-standard C
language are divided into two categories:
- Extensions compatible with ANSI C programs that produce diagnostic
messages when compiled without the common C compatibility option
- Extensions incompatible with ANSI C programs, which could produce
different compiler behavior when used without the common C
compatibility option
D.1 Extensions Compatible with ANSI C
- 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 object they point
to. Therefore, under the common C option, a pointer to float is compatible with a pointer to int.
- The digits 8 and 9 are valid in octal integer constants. (A warning
message is issued by the compiler, however.)
- Bit-field data types may include enum, short,
char, and long. The ANSI C Standard allows only int, unsigned
int, or signed int.
- long float is recognized as a synonym
for double.
- A third argument to the function main(), namely char
*envp[], is allowed.
1
When DEC C is run in common C compatibility mode, the main function can accept a third parameter, the
environment array envp. This array contains process
information such as the user name and controlling information, and has
no bearing on passing command-line arguments. Its primary use is during
exec and getenv library function calls.
See your
platform-specific DEC C documentation for more information
about invoking the main function within
your host environment.
- Text is allowed following the preprocessing directives #else and #endif.
- Address constants may be cast to int.
- Tentative definitions that exist at the completion of a compilation
remain tentative to the linker, in accordance with the traditional
model of definition resolution.
- Casts that do not cause a change in representation are legal as
lvalues.
- Implicit function declarations are created at file level, rather
than at block level.
- The types int and long are compatible.
- Taking the address of a variable with the register storage class is allowed.
- Block-level declarations of functions with static storage class are allowed.
- In array types, the element types are allowed to be incomplete.
- The type of a tentatively-defined variable is allowed to be
incomplete at the end of the compilation unit. A warning is issued for
this case.
- Values in case labels are allowed to have a pointer type.
- Trailing (extra) commas are allowed in enumeration lists.
- The semicolon following the last structure or union member may be
omitted.
- Carriage returns are accepted and treated as white space.
Note
1 Parameters to the function main() are only checked in strict ANSI
mode.
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D.2 Extensions Incompatible with ANSI C
- Unsigned preserving rules apply. (unsigned
char and unsigned short
promote to unsigned int.)
- Comments are converted to no spaces instead of a single space to
allow token concatenation. (The compiler attempts to concatenate the
two adjacent tokens.)
- All extern objects have file scope.
- Macro parameters are recognized and replaced within string or
character constants in the macro definition.
- During macro replacement, an argument's preprocessing tokens are
not macro replaced before the macro is expanded.
If the name of a
macro being replaced is found during the rescan of the replacement
list, it is macro replaced.
- Support for predefined macro names that do not conform to the ANSI
C Standard (that is, that do not start with two underscores or an
underscore followed by a capital letter).
- A preprocessor directive is only recognized as such if the
beginning # character occurs in the first
column of a line. Any preprocessor directives preceded by white space
are ignored.
- #ifdef is treated as "#if defined"
- #ifndef is treated as "#if !defined"
- Comments in macro replacement lists behave like ## operators when a valid token results after
concatenation, except that adjoining white space is not deleted. If the
resulting token is not valid, the comment in a macro replacement is
deleted.
- Trigraphs are not recognized and replaced.
Appendix E
VAX C Extensions Supported by DEC C
DEC C supports several VAX C extensions to ANSI-standard C.
These extensions are recognized only when the VAX C compatibility
option is used on the compiler command line. The VAX C extensions allow
you to use the DEC C compiler to compile code originally
written for the VAX C compiler.
The following sections describe the VAX C extensions available with the
VAX C compatibility option. Extensions to the ANSI-standard C language
are divided into two categories:
- Extensions compatible with ANSI C programs that produce diagnostic
messages when compiled without the VAX C compatibility option
- Extensions incompatible with ANSI C programs, which could produce
different compiler behavior when used without the VAX C compatibility
option
E.1 Extensions Compatible with ANSI C
- VAX C specific pragmas are recognized.
- 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 object they point
to. Therefore, under the VAX C option, a pointer to float is compatible with a pointer to int.
- The #module directive is allowed. (On
DIGITAL UNIX systems this directive produces a warning message and is
ignored.)
- The #dictionary directive is
allowed. (On DIGITAL UNIX systems this directive produces a warning
message and is ignored.)
- The module form of #include is
allowed. (On DIGITAL UNIX systems the module form of this directive
produces an error.)
- Specifying int for the type of a bit
field is equivalent to specifying signed
int in VAX C mode.
- Built-in functions are recognized.
- The main_program option may be used
to identify a particular function as the main function for a given
program.
When compiling in VAX C mode, another way to specify the
main function in a program is to include the following option in the
function definition:
This option is not a keyword, and it can be written uppercase or
lowercase. The main_program option is
useful for allowing a name other than main for the main program.
In a
prototype-style function definition, include main_program between the function declaration
part and the left brace, as in the following example:
char lower(int c_up)
main_program
{
.
.
.
}
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In an old-style function definition, include main_program in the same place as in the
prototype style, but before any parameter declarations, as in the
following example:
char lower(c_up)
main_program
int c_up;
{
.
.
.
}
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Both examples establish the function lower as the main function; execution begins
there, regardless of the order in which the functions are linked.
- Bit-field data types may include enum, short,
char, and long. The ANSI C standard allows only int, unsigned
int, or signed int.
- The last member of a structure may be an array with no size
specified.
- Two struct types or two union types are considered the same type if
their sizes are the same.
- Block-level declarations of functions with static storage class are allowed.
- The address of a constant may be passed to a function.
- Taking the address of a variable with register storage class is allowed.
- A third argument to the function main(), namely char
*envp[], is allowed.
When DEC C is run in VAX C compatibility mode, the main function can accept a third parameter, the
environment array envp. This array contains process
information such as the user name and controlling information, and has
no bearing on passing command-line arguments. Its primary use is during
exec and getenv library function calls.
1
See your platform-specific DEC C documentation
for more information about invoking the main function within your host environment.
- long float is recognized as a
synonym for double.
- Character constants containing multiple characters are packed in
little endian order. For example, 'AB' is treated as 'B' << 8 +
'A' instead of 'A' << 8 + 'B'.
- Trailing (extra) commas are allowed in enumeration lists.
- The element type of an array may be incomplete.
- Carriage returns are accepted and treated as white space.
Note
1 Parameters to the function main() are only checked in strict ANSI
mode.
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E.2 Extensions Incompatible with ANSI C
- Unsigned preserving rules apply. (unsigned
char and unsigned short
promote to unsigned int.)
- VAX C-specific predefined macros are recognized.
- VAX C-specific keywords are recognized.
- Macro parameters are recognized and are replaced as string or
character constants in the macro definition.
- Comments are converted to no spaces instead of a single space to
allow token concatenation. (The compiler attempts to concatenate the
two adjacent tokens.)
- Comments in macro replacement lists behave like ## operators when a valid token results after
concatenation, except that adjoining white space is not deleted. If the
resulting token is not valid, the comment in a macro replacement is
deleted.
- Trigraphs are not recognized or replaced.
- Variant structures and unions are allowed.
Variant structure and union declarations allow reference 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
DEC C copies 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 member must be declared in the variant aggregate
declaration, and it may not be declared as a pointer or an array.
Initialization of a variant structure or union is the same as that
of a normal structure or union.
With the VAX C compatibility
option, two structures or unions in an assignment operation need only
have the same size, rather than requiring the same members and member
types.
The following example shows the format of a variable
structure declaration, and how to reference members of a variant
structure:
#include <stdio.h>
enum packet_type {TEXT, INTEGER};
/* This structure can contain either a text_packet or an integer value.
It can only contain one of these at a time, since they share the same
storage. */
struct packet
{
enum packet_type type;
variant_union
{
variant_struct
{
int str_size;
char *text;
} text_packet;
variant_struct
{
int value;
} value_packet;
} text_or_int;
} packet = {TEXT, 24 ,"I love the color purple"};
main()
{
if (packet.type == TEXT)
printf(" %s. \n",packet.text);
else
printf(" %d \n", packet.value);
packet.type = INTEGER;
packet.value = 42;
printf(" The meaning of life, the universe, and everything is: %d. \n",
packet.value);
}
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