Compaq C
Compaq 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 Compaq C documentation for more
information.
When only preprocessing a file, all pragmas recognized by Compaq 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
Compaq 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 Compaq 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 Compaq 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 Compaq C
Compaq 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 Compaq 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 Compaq 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 Compaq C
Compaq 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 Compaq 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 Tru64 UNIX systems this directive produces a
warning message and is ignored.)
- The
#dictionary
directive is allowed. (On Tru64 UNIX systems this directive produces a
warning message and is ignored.)
- The module form of
#include
is allowed. (On Tru64 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 Compaq 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 Compaq 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
Compaq 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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