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OpenVMS Programming Concepts Manual

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11.7.2 Obtaining 64-Bit Pointers to Memory

The Compaq C RTL has many functions that return pointers to newly allocated memory. In each of these functions, the application owns the memory pointed to and is responsible for freeing that memory.

Functions that allocate memory are:


Each of these functions has a 32-bit and 64-bit implementation. When you use the /POINTER_SIZE qualifier, the following functions can also be called:

_malloc32 , _malloc64
_calloc32 , _calloc64
_realloc32 , _realloc64
_strdup32 , _strdup64

When you specify /POINTER_SIZE=32, all malloc calls default to _malloc32 .

When you specify /POINTER_SIZE=64, all malloc calls default to _malloc64 .

Regardless of whether the application calls a 32-bit or 64-bit memory allocation routine, there is still a single free function. This function accepts either pointer size.

Note that the memory allocation functions are the only ones that return pointers to 64-bit memory. All Compaq C RTL structure pointers returned to the calling application (such as a FILE, WINDOW, or DIR) are always 32-bit pointers. This allows both 32-bit and 64-bit callers to pass these structure pointers within the application.

11.7.3 Compaq C Header Files

The header files distributed with Compaq C Version 5.2 and higher support 64-bit pointers. Each function prototype whose signature contains a pointer is constructed to indicate the size of the pointer accepted.

A 32-bit pointer can be passed as an argument to functions that accept either a 32-bit or 64-bit pointer for that argument.

A 64-bit pointer, however, cannot be passed as an argument to a function that accepts a 32-bit pointer. Attempts to do this are diagnosed by the compiler with a MAYLOSEDATA message. The diagnostic message IMPLICITFUNC means the compiler can do no additional pointer-size validation for calls to that function.

You might find the following pointer-size compiler diagnostics useful:

    A function prototype was not found before using the specified function. The compiler and run-time system rely on prototype definitions to detect incorrect pointer-size usage. Failure to include the proper header files can lead to incorrect results or pointer truncation.
    A truncation is necessary to do this operation. The operation could be passing a 64-bit pointer to a function that does not support a 64-bit pointer in the given context. It could also be a function returning a 64-bit pointer to a calling application that is trying to store that return value in a 32-bit pointer.
    This message (when enabled) helps expose real MAYLOSEDATA messages that are being suppressed because of a cast operation.

11.7.4 Functions Affected

The Compaq C RTL shipped with OpenVMS Alpha Version 7.0 accommodates applications that use only 32-bit pointers, only 64-bit pointers, or a combination of both. To use 64-bit memory, you must, at a minimum, recompile and relink an application. The amount of source code change required depends on the application itself, calls to other run-time libraries, and the combinations of pointer sizes used.

With respect to 64-bit pointer support, the functions in the Compaq C RTL fall into four categories:

  • Functions not impacted by choice of pointer size
  • Functions enhanced to accept either pointer size
  • Functions having a 32-bit and 64-bit implementation
  • Functions that accept only 32-bit pointers

From an application developer's perspective, the first two types of functions are the easiest to use in either a single-pointer or mixed-pointer mode.

The third type requires no modifications when used in a single-pointer compilation but might require source code changes when used in a mixed-pointer mode.

The fourth type requires careful attention whenever 64-bit pointers are used. No Pointer-Size Impact

The choice of pointer-size has no impact on a function if its prototype contains no pointer-related parameters or return values. The mathematical functions are good examples of this.

Even some functions in this category that do have pointers in their prototype are not impacted by pointer size. For example, strerror has the prototype:

char * strerror (int error_number);

This function returns a pointer to a character string, but this string is allocated by the Compaq C RTL. As a result, to support both 32-bit and 64-bit applications, these types of pointers are guaranteed to fit in a 32-bit pointer. Functions Accepting Both Pointer Sizes

The Alpha architecture supports 64-bit pointers. The OpenVMS Alpha Calling Standard specifies that all arguments are actually passed as 64-bit values. Before OpenVMS Alpha Version 7.0, all 32-bit addresses passed to procedures were sign-extended into this 64-bit parameter. The called function declared the parameters as 32-bit addresses, which caused the compiler to generate 32-bit instructions (such as LDL) to manipulate these parameters.

Many functions in the Compaq C RTL are enhanced to receive the full 64-bit address. For example, consider strlen :

size_t strlen (const char *string);

The only pointer in this function is the character-string pointer. If the caller passes a 32-bit pointer, the function works with the sign-extended 64-bit address. If the caller passes a 64-bit address, the function works with that address directly.

The Compaq C RTL continues to have only a single entry point for functions in this category. There are no source-code changes required to add any of the four pointer-size options for functions of this type. The OpenVMS documentation refers to these functions as 64-bit friendly. Functions with Two Implementations

There are many reasons why a function might need two implementations---one for 32-bit pointers, the other for 64-bit pointers. Some of these reasons include:

  • The pointer size of the return value is the same size as the pointer size of one of the arguments. If the argument is 32 bits, the return value is 32 bits. If the argument is 64 bits, the return value is 64 bits.
  • One of the arguments is a pointer to an object whose size is pointer-size sensitive. To know how many bytes are being pointed to, the function must know whether the code was compiled in 32-bit or 64-bit pointer-size mode.
  • The function returns the address of dynamically allocated memory. The memory is allocated in 32-bit space when compiled for 32-bit pointers and in 64-bit space when compiled for 64-bit pointers.

From the application developer's point of view, there are three function prototypes for each of these functions. The <string.h> header file contains many functions whose return value is dependent upon the pointer size used as the first argument to the function call. For example, consider the memset function. The header file defines three entry points for this function:

void * memset   (void *memory_pointer, int character, size_t size);
void *_memset32 (void *memory_pointer, int character, size_t size);
void *_memset64 (void *memory_pointer, int character, size_t size);

The first prototype is the function that your application would currently call if using this function. The compiler changes a call to memset into a call to either _memset32 when compiled /POINTER_SIZE=32, or _memset64 when compiled /POINTER_SIZE=64.

You can override this default behavior by directly calling either the 32-bit or the 64-bit form of the function. This accommodates applications using mixed pointer sizes, regardless of the default pointer size specified with the /POINTER_SIZE qualifier.

Note that if the application is compiled without specifying the /POINTER_SIZE qualifier, neither the 32-bit specific nor the 64-bit specific function prototypes are defined. In this case, the compiler automatically calls the 32-bit interface for all interfaces having dual implementations.

Table 11-4 shows the Compaq C RTL functions that have dual implementations in support of 64-bit pointer size. When compiling with the /POINTER_SIZE qualifier, calls to the unmodified function names are changed to calls to the function interface that matches the pointer size specified with the qualifier.

Table 11-4 Functions with Dual Implementations
basename malloc strpbrk wcsncat
bsearch mbsrtowcs strptime wcsncpy
calloc memccpy strrchr wcspbrk
catgets memchr strsep wcsrchr
ctermid memcpy strstr wcsrtombs
cuserid memmove strtod wcsstr
dirname memset strtok wcstok
fgetname mktemp strtol wcstol
fgets mmap strtoll wcstoul
fgetws qsort strtoq wcswcs
fullname realloc strtoul wmemchr
gcvt rindex strtoull wmemcpy
getcap strcat strtouq wmemmove
getcwd strchr tgetstr wmemset
getname strcpy tmpnam  
gets strdup wcscat  
index strncat wcschr  
longname strncpy wcscpy Functions Restricted to 32-Bit Pointers

Some functions in the Compaq C RTL do not support 64-bit pointers. If you try to pass a 64-bit pointer to one of these functions, the compiler generates a %CC-W-MAYLOSEDATA warning. Applications compiled with /POINTER_SIZE=64 might need to be modified to avoid passing 64-bit pointers to these functions.

Table 11-5 shows the functions restricted to using 32-bit pointers. The Compaq C RTL offers no 64-bit support for these functions. You must ensure that only 32-bit pointers are used with these functions.

Table 11-5 Functions Restricted to 32-Bit Pointers
atexit getopt modf setstate
execve iconv recvmsg setvbuf
execvp initstate sendmsg  
frexp ioctl setbuf  

Table 11-6 shows functions that make callbacks to user-supplied functions as part of processing that function call. The callback procedures are not passed 64-bit pointers.

Table 11-6 Callbacks That Pass Only 32-Bit Pointers
from_vms to_vms
ftw tputs

11.7.5 Reading Header Files

This section introduces the pointer-size manipulations used in the Compaq C RTL header files. Use the following examples to become more comfortable reading these header files and to help modify your own header files.



#   if (VMS__VER < 70000000) || !defined __ALPHA (2)
#      error " Pointer size usage not permitted before OpenVMS Alpha V7.0"
#   endif
#   pragma __pointer_size __save (3)
#   pragma __pointer_size 32 (4)
#   pragma __pointer_size 64
#   pragma __pointer_size __restore


All Compaq C compilers that support the /POINTER_SIZE qualifier predefine the macro __INITIAL_POINTER_SIZE . The Compaq C RTL header files take advantage of the ANSI rule that if a macro is not defined, it has an implicit value of 0.

The macro is defined as 32 or 64 when the /POINTER_SIZE qualifier is used. It is defined as 0 if the qualifier is not used. The statement shown as (1) can be read as "if the user has specified either /POINTER_SIZE=32 or /POINTER_SIZE=64 on the command line."

Compaq C Version 5.2 and higher is supported on many OpenVMS platforms. The lines shown as (2) generate an error message if the target of the compilation is one that does not support 64-bit pointers.

A header file cannot assume anything about the actual pointer-size context in effect at the time the header file is included. Furthermore, the Compaq C compiler offers only the __INITIAL_POINTER_SIZE macro and a mechanism to change the pointer size, but no way to determine the current pointer size.

All header files that have a dependency on pointer sizes are responsible for saving (3), initializing (4), altering (5), and restoring (6) the pointer-size context.


#ifndef __CHAR_PTR32 (1)
#   define __CHAR_PTR32 1
    typedef char * __char_ptr32;
    typedef const char * __const_char_ptr32;
#   pragma __pointer_size 64
#ifndef __CHAR_PTR64 (2)
#   define __CHAR_PTR64 1
    typedef char *__ char_ptr64;
    typedef const char * __const_char_ptr64;


Some function prototypes need to refer to a 32-bit pointer when in a 64-bit pointer-size context. Other function prototypes need to refer to a 64-bit pointer when in a 32-bit pointer-size context.

Compaq C binds the pointer size used in a typedef at the time the typedef is made. The typedef declaration of __char_ptr32 (1) is made in a 32-bit context. The typedef declaration of __char_ptr64 (2) is made in a 64-bit context.


#   if (__VMS_VER < 70000000) || !defined __ALPHA
#      error " Pointer size usage not permitted before OpenVMS Alpha V7.0"
#   endif
#   pragma __pointer_size save
#   pragma __pointer_size 32
#   pragma __pointer_size 64
int abs (int __j); (4)
__char_ptr32 strerror (int __errnum); (5)


Before declaring function prototypes that support 64-bit pointers, the pointer context is changed (2) from 32-bit pointers to 64-bit pointers.

Functions restricted to 32-bit pointers are placed in the 32-bit pointer context section (1) of the header file. All other functions are placed in the 64-bit context section (3) of the header file.

Functions that have no pointer-size impact ((4) and (5)) are located in the 64-bit section. Functions that have no pointer-size impact, except for a 32-bit address return value (5), are also in the 64-bit section, and use the 32-bit specific typedef s previously discussed.


#   pragma __pointer_size 64
#if __INITIAL_POINTER_SIZE == 32 (1)
#   pragma __pointer_size 32
char *strcat (char *__s1, __const_char_ptr64 __s2); (2)
#   pragma __pointer_size 32
    char *_strcat32  (char *__s1, __const_char_ptr64 __s2); (3)
#   pragma __pointer_size 64
    char *_strcat64  (char *__s1, const char *__s2); (4)


This example shows declarations of functions that have both a 32-bit and 64-bit implementation. These declarations are located in the 64-bit section of the header file.

The normal interface to the function (2) is declared using the pointer size specified on the /POINTER_SIZE qualifier. Because the header file is in 64-bit pointer context and because of the statements at (1), the declaration at (2) is made using the same pointer size context as the /POINTER_SIZE qualifier.

The 32-bit specific interface (3) and the 64-bit specific interface (4) are declared in 32-bit and 64-bit pointer-size context, respectively.

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