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Compaq C

Compaq C
User's Guide for OpenVMS Systems

Contents

Index

The following sections describe these options in greater detail.

[NO]ACCURACY_SENSITIVE (ALPHA ONLY)

The default is ACCURACY_SENSITIVE.

If you specify NOACCURACY_SENSITIVE, the compiler is free to reorder floating-point operations based on algebraic identities (inverses, associativity, and distribution). This allows the compiler to move divide operations outside of loops, which improves performance.

The default, ACCURACY_SENSITIVE, directs the compiler to use only certain scalar rules for calculations. This setting can prevent some optimizations.

If you use the /ASSUME=NOACCURACY_SENSITIVE qualifier, Compaq C might reorder code (based on algebraic identities) to improve performance. The results can be different from the default (/ASSUME=ACCURACY_SENSITIVE) because of how the intermediate results are rounded. However, the NOACCURACY_SENSITIVE results are not categorically less accurate than those gained by the default.

[NO]ALIGNED_OBJECTS (ALPHA ONLY)

The default is /ASSUME=ALIGNED_OBJECTS.

On OpenVMS Alpha systems, dereferencing a pointer to a longword- or quadword-aligned object is more efficient than dereferencing a pointer to a byte- or word-aligned object. Therefore, the compiler can generate more optimized code if it makes the assumption that a pointer object of an aligned pointer type does point to an aligned object.

Since the compiler determines the alignment of the dereferenced object from the type of the pointer, and the program is allowed to compute a pointer that references an unaligned object (even though the pointer type indicates that it references an aligned object), the compiler must assume that the dereferenced object's alignment matches or exceeds the alignment indicated by the pointer type. Specifying /ASSUME=ALIGNED_OBJECTS (the default) allows the compiler to make such an assumption. With this assumption made, the compiler can generate more efficient code for pointer dereferences of aligned pointer types.

To prevent the compiler from assuming the pointer type's alignment for objects that it points to, use the /ASSUME=NOALIGNED_OBJECTS qualifier.

Before deciding whether to specify /ASSUME=NOALIGNED_OBJECTS or /ASSUME=ALIGNED_OBJECTS, you need to know what programming practices will affect your decision.

The compiler assumes that pointers point to objects that are aligned at least as much as the alignment of the pointer type. For example:

A pointer of type short points to objects that are at least short -aligned.
A pointer of type int points to objects that are at least int -aligned.
A pointer of type struct foo points to objects that have an alignment of struct foo (that is, the alignment of the strictest member alignment, or byte alignment if you have specified #pragma nomember_alignment for struct foo ).

If your module breaks this rule, your program will suffer alignment faults at runtime that can seriously degrade performance. If you can identify the places in your code where the rule is broken, use the __unaligned type qualifier. Otherwise, the /ASSUME=NOALIGNED_OBJECTS qualifier effectively treats all dereferences as if they were unaligned.

Compaq C for OpenVMS Alpha Systems aligns all nonmember declarations on natural boundaries, so by default all objects do comply with the previous assumption. Also, the standard library routine malloc on OpenVMS systems returns quadword-aligned heap memory.

A program can violate the previous assumption in any of the following ways:

By explicitly specifying a lesser alignment for an object than the pointer type's alignment
By casting a pointer to a pointer type of stricter alignment
By enclosing a member-aligned object inside a nonmember-aligned object

The following example explicitly specifies a lesser alignment for an object than the pointer type's alignment, which occurs when the address of an unaligned int member of a struct with #pragma nomember_alignment is used in a pointer dereference:

#pragma nomember_alignment struct foo { char C; int i; /* i is unaligned because of char C */ }; struct foo st; int *i_p; i_p = &st.i; ... *i_p ... /* An expression containing a dereferenced i_p */

This example casts a pointer to a pointer type with stricter alignment:

int *i_p; char *c_p; ....... ....... i_p = (int *)c_p; ... *i_p ... /* An expression containing a dereferenced i_p */

The following example encloses a member-aligned object inside a nonmember-aligned object:

#pragma member_alignment struct inside { int i; /* this type asserts that its objects have at least longword alignment (int is a longword)... */ }; #pragma nomember_alignment struct outside { char C; struct inside s; /* ...but foo_ptr -> s is only byte-aligned! */ } *foo_ptr;

The expression foo_ptr -> s has a type whose alignment is explicitly specified to be longword (because longword is the strictest alignment of the structure's members), but the expression type is only guaranteed to be byte-aligned.

Also note that just as the pointer type information can direct the compiler to generate the appropriate code to dereference the pointer (code that does not cause alignment faults), it can also direct the compiler to generate even better code if it indicates that the object is at least longword-aligned.

[NO]CLEAN_PARAMETERS (ALPHA ONLY)

The default is /ASSUME=CLEAN_PARAMETERS.

The Alpha Calling Standard requires integers less than 64 bits long that are passed by value to have their upper bits either zeroed or sign-extended to make full 64-bit values. These are referred to as clean parameters. Some old code does not follow this convention. This can cause problems if the called program assumes that the caller followed the Calling Standard by passing only clean parameters.

Specifying /ASSUME=NOCLEAN_PARAMETERS allows a program to be called by old code that might pass unclean integer parameters. It directs the compiler to generate run-time code to clean the short integers so they comply with the Calling Standard.

[NO]EXACT_CDD_OFFSETS

The default is /ASSUME=NOEXACT_CDD_OFFSETS.

If /ASSUME=EXACT_CDD_OFFSETS is specified, the records input from the CDD are given the exact alignment (relative to the start of the record) specified by the CDD definition. This alignment is independent of the current compiler member-alignment setting.

If /ASSUME=NOEXACT_CDD_OFFSETS is specified, the compiler may modify the offsets specified in a CDD record according to the current member-alignment setting.

[NO]HEADER_TYPE_DEFAULT

The default is /ASSUME=HEADER_TYPE_DEFAULT.

In past versions of the C compiler, the #include directive always supplied a default file type of .h for C compilations. Similarly, the C++ compiler supplied a default file type of .hxx for C++ compilations.

However, the ANSI C++ standard requires that, for example, #include <iostream> be distinguishable from #include <iostream.hxx> . This is not possible with the header file-type default mechanism in effect.

You can disable the type default mechanism for either Compaq C or DIGITAL C++ by specifying /ASSUME=NOHEADER_TYPE_DEFAULT.

With /ASSUME=NOHEADER_TYPE_DEFAULT specified, an #include directive written with the standard syntax for header name (enclosed in quotes or angle brackets) will use the filename as specified, without supplying a default file type. More precisely stated, the default file type will be empty (just ".").

For example, a directory might contain three files named IOSTREAM., IOSTREAM.HXX, and IOSTREAM.H. By default, the C++ compiler processes #include <iostream> such that the file IOSTREAM.HXX is found, while the C compiler would find IOSTREAM.H.

However, if /ASSUME=NOHEADER_TYPE_DEFAULT is specified, the same directive causes the file IOSTREAM. to be found by both compilers, and the only way to include the file named IOSTREAM.HXX or IOSTREAM.H is to specify the .hxx or .h file type explicitly in the #include directive. Be aware that while the OpenVMS operating system treats filenames as case-insensitive and normally displays them in uppercase, filenames in #include directives should use lowercase for best portability. This is more in keeping with other C and C++ implementations.

[NO]MATH_ERRNO (ALPHA ONLY)

The default is /ASSUME=MATH_ERRNO, which does not allow intrinsic code for such math functions to be generated, even if /OPTIMIZE=INTRINSICS is in effect. Their prototypes and call formats, however, are still checked.

[NO]POINTERS_TO_GLOBALS (ALPHA ONLY)

The default is /ASSUME=POINTER_TO_GLOBALS, which directs the compiler to assume that global variables have had their addresses taken in separately compiled modules and that, in general, any pointer dereference could be accessing the same memory as any global variable. This is often a significant barrier to optimization.

The /ANSI_ALIAS command-line qualifier allows some resolution based on data type, but /ASSUME=NOPOINTER_TO_GLOBALS provides significant additional resolution and improved optimization in many cases.

/ASSUME=NOPOINTER_TO_GLOBALS tells the compiler that any global variable accessed through a pointer in the compilation must have had its address taken within that compilation. The compiler can see any code that takes the address of an extern variable. If it does not see the address of the variable being taken, the compiler can assume that no pointer points to the variable.

Consider the following code sequence:

extern int x; ... int *p; ... *p = 3;

Under /ASSUME=NOPOINTERS_TO_GLOBALS, the compiler can assume that x is not changed by the assignment through p when generating code. This can lead to faster code.

In combination with the /PLUS_LIST_OPTIMIZE qualifier, several source modules can be treated as a single compilation for the purpose of this analysis. Because run-time libraries such as the Compaq C RTL do not take the addresses of global variables defined in user programs, source modules can often be combined into a single compilation that allows /ASSUME=NOPOINTER_TO_GLOBALS to be used effectively.

Be aware that /ASSUME=NOPOINTERS_TO_GLOBALS does not tell the compiler that the compilation never uses pointers to access global variables (which is seldom true of real C programs).

[NO]WEAK_VOLATILE (ALPHA ONLY)

This option affects the generation of code for assignments to objects that are less than or equal to 16 bits in size (for example: char, short) that have been declared as volatile.

Specifying /ASSUME=WEAK_VOLATILE directs the compiler to generate code for volatile assignments to single bytes or words without using the load-locked store-conditional sequences that, in general, are required to assure volatile data integrity when direct byte or word memory-access instructions are not being used.

This option is intended for use in special I/O hardware access situations, and should not generally be used.

The default is /ASSUME=NOWEAK_VOLATILE, which uses interlocked instructions for sub-longword volatile accesses when byte or word instructions are not enabled.

[NO]WHOLE_PROGRAM (ALPHA ONLY)

The default is /ASSUME=NOWHOLE_PROGRAM.

The optimizations enabled by /ASSUME=WHOLE_PROGRAM include all those enabled by /ASSUME=NOPOINTER_TO_GLOBALS, and possibly additional optimizations as well.

[NO]WRITABLE_STRING_LITERALS

For /STANDARD=VAXC or /STANDARD=COMMON, the default is /ASSUME=WRITABLE_STRING_LITERALS.

For all other compiler modes, the default is /ASSUME=NOWRITABLE_STRING_LITERALS.

/[NO]CHECK[=([NO]UNINITIALIZED_VARIABLES, [NO]BOUNDS (ALPHA ONLY) [NO]POINTER_SIZE[=(option,...)])] (ALPHA ONLY)

This qualifier is for use as a debugging aid.

/CHECK=UNINITIALIZED_VARIABLES

Use /CHECK=UNINITIALIZED_VARIABLES to initialize all automatic variables to the value 0xfffa5a5afffa5a5a. This value is a floating NaN and, if used, causes a floating-point trap. If used as a pointer, this value is likely to cause an ACCVIO.

/CHECK=BOUNDS (ALPHA ONLY)

Use /CHECK=BOUNDS to enable run-time checking of array bounds. Array-bounds processing is performed in the following way:

Checks are done only when accessing an array.
Checks are not done when accessing a pointer, even if that access is done using the subscript operator. This means that checks are not done on arrays declared as formal parameters because they are considered pointers in the C language. If a formal parameter is a multi-dimension array, all bounds except the first are checked.
If an array is accessed using the subscript operator (as either the left or right operand), and the subscript operator is not the operand of an address-of operator, the check is for the index to be between 0 and the number of array elements minus one, inclusive.
If an array is accessed using the subscript operator (as either the left or right operand), and the subscript operator is the operand of the address-of operator, the check is for the index to be between 0 and the number of elements in the array, inclusive.
The reason for treating the address-of case differently is that it is common programming practice to have a loop such as:
int a[10]; int *b; for (b = a ; b < &a[10] ; b++) { .... }
In this case, access to &a[10] is allowed even though it is outside the range of the array.
If the array is being accessed using pointer addition, the check is for the value being added to be between 0 and the number of elements in the array, inclusive.
If the array is being accessed using pointer subtraction (that is, the subraction of an integer value from a pointer, not the subtraction of one pointer from another), the check is for the value being subtracted to be between the negation of the number of elements in the array and 0, inclusive.
In the previous three cases, an optional compile-time message (ident SUBSCRBOUNDS2) can be enabled to detect the case where an array has been accessed using either a constant subscript or constant pointer arithmetic, and the element accessed is exactly one past the end of the array.
Bounds checking is not done for arrays declared with one element. (Because ANSI C does not allow arrays without dimensions inside struct s, it is common practice to declare such arrays with a bounds specifier of 1.)
In this case, an optional compile-time message (ident SUBSCRBOUNDS1) can be enabled to detect the case where an array declared with a single element is accessed using either a constant subscript or constant pointer arithmetic, and the element accessed is not part of the array.
Compaq C emits run-time checks for arrays indexed by constants, even though the compiler can and does detect this situation at compile-time. An exeption is that no run-time check is made if the compiler can determine that the access is valid.

Here are examples of some array references:

int a[10]; int *b; int c; int *d; int vla[c]; int one[1]; a[c] = 1; // check c is from 0-9 b[c] = 1; // no check c[a] = 1; // check c is from 0-9 b = &a[c] // check c is from 0-10 *(a + c) = 1; // check c is from 0-10 *(a - c) = 1; // check c is from -10 to 0 d = a + c; // check that c is from 0-10 d = b + c; // no check a[1] = 1; // no run-time check - know access is valid vla[1] = 1; // run-time check a[10] = 1; // run-time check (and compiler diagnostic) d = a + 10; // no run-time check, optional SUBSCRBOUNDS2 // message can be enabled c = one[5]; // no run-time check, optional SUBSCRBOUNDS1 // message can be enabled

If a multi-dimension array is accessed, the compiler performs checks on each of the subscript expressions, making sure each is within the corresponding bound. So for the following code, the compiler checks that both x and y are between 0 and 9. It does not check that 10 * x + y is between 0 and 99:
int a[10][10]; int x,y,z; x = a[x][y];

Notes

Because of operating system differences, the behavior of the run-time array-bounds checking is different on Tru64 UNIX systems than on OpenVMS systems.

If there is no handler, an OpenVMS program fails with:
%SYSTEM-F-SUBRNG, arithmetic trap, subscript out of range at PC=xxx, PS=xxx %TRACE-F-TRACEBACK, symbolic stack dump follows
On Tru64 UNIX systems, the output would be:
Trace/BPT trap (core dumped)
Furthermore, to trap the error on OpenVMS systems, a user needs to write:
signal(SIGFPE, handler);
While on Tru64 UNIX systems, the equivalent line would be:
signal(SIGTRAP, handler);
When run-time checking is enabled, the Compaq C compiler emits a bad check in certain cases. These cases arise when an array is accessed using pointer arithmetic and run-time array-bounds checking is enabled. In such a case, the compiler can output only the checking code for the first pointer-arithmetic operation performed on the array. This can result in an incorrect check if the resulting pointer value is again operated on by pointer arithmetic.

Consider the following expression where a is a pointer, b is an array, and c and d are integers:
a = b + c - d;
When bounds checking is enabled, the compiler outputs a check to verify that c is within the bounds of the array. This leads to an incorrect run-time trap in cases where c is outside the bounds of the array and c - d is not.

In these cases, the compiler outputs a diagnostic noting that the check code it produced is bad. You can then recode the pointer expression so that the integer part is in parentheses. In this way, the expression will contain only one pointer-arithmetic operation, and the compiler will output the correct check. In the previous example, the expression would be changed to:
a = b + (c - d);

/CHECK=POINTER_SIZE (ALPHA ONLY)

Use /CHECK=POINTER_SIZE to direct the compiler to generate code that checks 64-bit pointer values (used in certain contexts where 32-bit pointers are also present) to make sure they will fit in a 32-bit pointer. If such a value cannot be represented by a 32-bit pointer, the run-time code signals a range error (SS$_RANGEERR).

To control the types of pointer-size checks you want made, use one or more of the POINTER_SIZE option keywords shown in Table 1-4.

Table 1-4 /CHECK =POINTER_SIZE Qualifier Options
Option Usage

[NO]ASSIGNMENT Check whenever a 64-bit pointer is assigned to a 32-bit pointer (including use as an actual argument).

[NO]CAST Check whenever a 64-bit pointer is cast to a 32-bit pointer.

[NO]INTEGER_CAST Check whenever a long pointer is cast to a 32-bit integer.

[NO]PARAMETER Check all formal parameters at function startup to make sure that all formal parameters declared to be 32-bit pointers are 32-bit values.

ALL Do all checks.

NONE Do no checks.

**Table 1-4 /CHECK =POINTER_SIZE Qualifier Options**
Option	Usage
[NO]ASSIGNMENT	Check whenever a 64-bit pointer is assigned to a 32-bit pointer (including use as an actual argument).
[NO]CAST	Check whenever a 64-bit pointer is cast to a 32-bit pointer.
[NO]INTEGER_CAST	Check whenever a long pointer is cast to a 32-bit integer.
[NO]PARAMETER	Check all formal parameters at function startup to make sure that all formal parameters declared to be 32-bit pointers are 32-bit values.
ALL	Do all checks.
NONE	Do no checks.

Specifying /CHECK=POINTER_SIZE defaults to /CHECK=POINTER_SIZE=(ASSIGNMENT,PARAMETER).

For information about compiler features that affect pointer size, see the following:

/POINTER_SIZE
#pragma pointer_size
#pragma required_pointer_size
__initial_pointer_size predefined macro

The following contrived program contains a number of pointer assignments. The comment on each line indicates what /CHECK=POINTER_SIZE keyword to specify to enable checking for that line.

#pragma required_pointer_size long int *a; char *b; typedef char * l_char_ptr; #pragma required_pointer_size short char *c; int *d; foo(int * e) /* Check e if PARAMETER is specified. */ { d = a; /* Check a if ASSIGNMENT is specified. */ c = (char *) a; /* Check a if CAST is specified. */ c = (char *) d; /* No checking ever. */ foo( a ); /* Check a if ASSIGNMENT is specified. */ bar( a ); /* No checking ever - no prototype */ b = (l_char_ptr) a; /* No checking ever. */ c = (l_char_ptr) a; /* Check a if ASSIGNMENT is specified */ b = (char *) a; /* Check if CAST is specified. */ }

Defaults

Omitting this qualifier defaults to /NOCHECK, which equates to /CHECK=(NOUNINITIALIZED_VARIABLE,NOBOUNDS,NOPOINTER_SIZE).

Specifying /CHECK defaults to /CHECK=(UNINITIALIZED_VARIABLES, BOUNDS, POINTER_SIZE), which equates to /CHECK=(UNINITIALIZED_VARIABLES, BOUNDS, POINTER_SIZE=(ASSIGNMENT,PARAMETER)).

/[NO]COMMENTS=option

Governs whether or not comments appear in preprocess output files and, if they are to appear, whether they appear themselves or are replaced by a single space.

Table 1-5 shows the /COMMENTS qualifier options.

Table 1-5 /COMMENTS Qualifier Options
Option Usage

AS_IS Specifies that the comment appears in the output file.

SPACE Specifies that a single space replaces the comment in the output file.

**Table 1-5 /COMMENTS Qualifier Options**
Option	Usage
AS_IS	Specifies that the comment appears in the output file.
SPACE	Specifies that a single space replaces the comment in the output file.

/NOCOMMENTS specifies that nothing replaces the comment in the output file. This can result in inadvertent token pasting.

The Compaq C preprocessor might replace a comment at the end of a line or on a line by itself with nothing, even if /COMMENTS=SPACE is specified. Doing so does not change the meaning of the program.

The default is /COMMENTS=SPACE for the ANSI89, RELAXED_ANSI89, and MIA modes of the compiler. The default is /NOCOMMENTS for all other compiler modes.

Specifying /COMMENTS on the command line defaults to /COMMENTS=AS_IS.

/[NO]CROSS_REFERENCE

Specifies whether the compiler generates cross-references for variable names.

If you specify /CROSS_REFERENCE, the compiler lists, for each variable referenced in the procedure, the line numbers of the lines on which the variable is referenced.

This qualifier has no effect unless you also specify /LIST and either /SHOW=SYMBOLS or /SHOW=BRIEF. The default is /NOCROSS_REFERENCE.

/[NO]DEBUG[=(option[,...])]

Includes information in the object module for use by the OpenVMS Debugger.

If the /DEBUG qualifier is not specified, the default is:

/DEBUG=(TRACEBACK,NOSYMBOLS) on Alpha systems.
/DEBUG=(TRACEBACK,NOINLINE,NOSYMBOLS) on VAX systems.

Specifying /DEBUG with no keywords is equivalent to specifying /DEBUG=ALL.

Table 1-6 describes the debugger options.

Table 1-6 Debugger Compilation Options
Option Usage

ALL Includes symbol table records and traceback records for both VAX and Alpha systems. On VAX systems, this also selects the behavior of the INLINE keyword.
On Alpha systems, /DEBUG=ALL is equivalent to /DEBUG=(TRACEBACK,SYMBOLS).
On VAX systems, /DEBUG=ALL is equivalent to /DEBUG=(TRACEBACK,SYMBOLS,INLINE).

INLINE (VAX ONLY) Generates debug information to cause a STEP command to STEP/INTO an inlined function call.

NOINLINE (VAX ONLY) Generates debug information to cause a STEP command to STEP/OVER an inlined function call.

NONE Does not include any debugging information. This is equivalent to /NODEBUG.

NOTRACEBACK Suppresses generation of traceback records.

NOSYMBOLS Suppresses generation of symbol table records.

SYMBOLS Generates symbol table records.

TRACEBACK Generates traceback records.

**Table 1-6 Debugger Compilation Options**
Option	Usage
ALL	Includes symbol table records and traceback records for both VAX and Alpha systems. On VAX systems, this also selects the behavior of the INLINE keyword. On Alpha systems, /DEBUG=ALL is equivalent to /DEBUG=(TRACEBACK,SYMBOLS). On VAX systems, /DEBUG=ALL is equivalent to /DEBUG=(TRACEBACK,SYMBOLS,INLINE).
INLINE (VAX ONLY)	Generates debug information to cause a STEP command to STEP/INTO an inlined function call.
NOINLINE (VAX ONLY)	Generates debug information to cause a STEP command to STEP/OVER an inlined function call.
NONE	Does not include any debugging information. This is equivalent to /NODEBUG.
NOTRACEBACK	Suppresses generation of traceback records.
NOSYMBOLS	Suppresses generation of symbol table records.
SYMBOLS	Generates symbol table records.
TRACEBACK	Generates traceback records.

/DECC

Invokes the Compaq C compiler.