One of the last tasks in preparing to use the DEC C RTL, if you are going to port your source programs across systems, is to be aware of specific differences between the DEC C RTL and the run-time libraries of other implementations of the C language. This section describes some of the problems that you might encounter when porting programs to and from an OpenVMS system. Although portability is closely tied to the implementation of the DEC C RTL, this section also contains information on the portability of other DEC C for OpenVMS constructs.
The DEC C RTL provides ANSI C defined library functions as well as many commonly available APIs and a few OpenVMS extensions. See Section 1.5 for specific standards, portions of which are implemented by the DEC C RTL. Attempts have been made to maintain complete portability in functionality whenever possible. Many of the Standard I/O and UNIX I/O functions and macros contained in the DEC C RTL are functionally equivalent to those of other implementations.
The RTL function and macro descriptions elaborate on issues presented in this section and describe concerns not documented here.
The following list documents issues of concern if you wish to port C programs to the OpenVMS environment:
For example, if a program tries to reference location 0 on an OpenVMS system, a hardware error (ACCVIO) is returned and the program terminates abnormally. OpenVMS systems reserve the first page of address space to catch incorrect pointer references, such as a reference to a location pointed to by a null pointer. For this reason, some existing programs that run on some UNIX systems may fail and you should modify them, as necessary. (Digital UNIX and OpenVMS, however, are compatible in this regard.)
int foo 123;
Programs using this form of initialization must be changed.
int a, b, c;
extern variables in a program may be treated
differently using DEC C on OpenVMS systems
than they would on UNIX systems. See the DEC C User's Guide for OpenVMS Systems for more
information.
a[i] = i++;
x = func_y() + func_z();
f(p++, p++)
Neither DEC C nor any other C compiler can guarantee that such expressions evaluate in the same order on all C compilers.
The DEC C RTL supports an improved and enhanced reentrancy. The following types of reentrancy are supported:
Failure to specify AST reentrancy might cause I/O routines to fail, setting errno to EALREADY.
The default reentrancy type is TOLERANT.
You can set the reentrancy type by compiling with the /REENTRANCY command-line qualifier or by calling the decc$set_reentrancy function. This function must be called exclusively from non-AST level.
When programming an application using multiple threads or ASTs, you should consider three classes of functions:
Most functions have no internal data at all. For these functions, synchronization is necessary only if the parameter is used by the application in multiple threads or in both AST and non-AST context. For example, although the strcat function is ordinarily safe, the following is an example of unsafe usage:
extern char buffer[100];
void routine1(char *data) {
strcat( buffer, data );
}
If routine1 executed concurrently in multiple threads, or if routine1 is interrupted by an AST routine that calls it, the results of the strcat call are unpredictable.
The second class of functions are those that have thread-local static data. Typically, these are routines in the library that return a string where the application is not permitted to free the storage for the string. These routines are thread-safe but not AST-reentrant. This means they can safely be called concurrently, and each thread will have its own copy of the data. They cannot be called from AST routines if it is possible that the same routine was executing in non-AST context. The routines in this class are:
asctime perror clock strerror ctermid strtok ctime times cuserid VAXC$ESTABLISH gmtime the errno variable localtime wcstok
All the socket functions are also included in this list if the TCP /IP product in use is thread-safe.
The third class of functions are those that affect process-wide data. These functions are neither thread-safe nor AST-reentrant. For example, sigsetmask establishes the process-wide signal mask. Consider a routine like the following:
void update_data
base()
{
int old_mask;
old_mask = sigsetmask( 1 << (SIGINT - 1));
/* Do work here that should not be aborted. */
sigsetmask( old_mask );
}
If update_database was called concurrently in multiple threads, thread 1 might unblock SIGINT while thread 2 was still performing work that should not be aborted.
The routines in this class are:
Mixing the multithread programming model and the OpenVMS AST programming model in the same application is not recommended. The application has no mechanism to control which thread gets interrupted by an AST. This can result in a resource deadlock if the thread holds a resource that is also needed by the AST routine. The following routines use mutexes. To avoid a potential resource deadlock, do not call them from AST routines in a multithreaded application.
This section is for application developers who need to use 64-bit virtual memory addressing on OpenVMS Alpha Version 7.0 or higher.
OpenVMS Alpha 64-bit virtual addressing support makes the 64-bit virtual address space defined by the Alpha architecture available to both the OpenVMS operating system and its users. It also allows per-process virtual addressing for accessing dynamically mapped data beyond traditional 32-bit limits.
The DEC C Run-Time Library on OpenVMS Alpha Version 7.0 systems and higher includes the following features in support of 64-bit pointers:
The DEC C Run-Time library on OpenVMS Alpha Version 7.0 systems and higher can generate and accept 64-bit pointers. Functions that require a second interface to be used with 64-bit pointers reside in the same object libraries and shareable images as their 32- bit counterparts. No new object libraries or shareable images are introduced. Using 64-bit pointers does not require changes to your link command or link options files.
The DEC C 64-bit environment allows an application to use both 32-bit and 64-bit addresses. For more information about how to manipulate pointer sizes, see the /POINTER_SIZE qualifier and #pragma pointer_size and #pragma required_pointer_size preprocessor directives in the DEC C User's Guide for OpenVMS Systems.
The /POINTER_SIZE qualifier requires you to specify a value of 32 or 64. This value is used as the default pointer size within the compilation unit. As an application programmer, you can compile one set of modules by using 32-bit pointers and another set by using 64- bit pointers. Care must be taken when these two separate groups of modules call each other.
Use of the /POINTER_SIZE qualifier also influences the processing of DEC C RTL header files. For those functions that have a 32-bit and 64-bit implementation, specifying /POINTER_SIZE enables function prototypes to access both functions, regardless of the actual value supplied to the qualifier. In addition, the value specified to the qualifier determines the default implementation to call during that compilation unit.
The #pragma pointer_size and #pragma required_pointer_size preprocessor directives can be used to change the pointer size in effect within a compilation unit. You can default pointers to 32-bit pointers and then declare specific pointers within the module as 64- bit pointers. You would also need to specifically call the _malloc64 form of malloc to obtain memory from the 64-bit memory area.
The DEC 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 have a 32-bit and a 64-bit implementation. When the /POINTER_SIZE qualifier is used, the following functions can also be called:
When /POINTER_SIZE=32 is specified, all malloc calls default to _ malloc32.
When /POINTER_SIZE=64 is specified, 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.
Be aware that the memory allocation functions are the only ones that return pointers to 64-bit memory. All DEC 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.
The header files distributed with DEC 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. If this function is a DEC C RTL function, refer to the reference section of this manual for the name of the header file that defines 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 and /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. To enable this warning, compile with the qualifier /WARNINGS=ENABLE=MAYHIDELOSS.
The DEC C RTL shipped with OpenVMS Alpha Version 7.0 accommodates applications that use only 32-bit pointers, only 64-bit pointers, or combinations 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 runtime libraries, and the combinations of pointer sizes used.
With respect to 64-bit pointer support, the functions in the DEC C RTL fall into four categories:
From an application developer's perspective, the first two types of functions are the easiest to use in either a single 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.
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 DEC 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.
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 DEC 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 DEC 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.
There are many reasons why a function might need one implementation for 32-bit pointers, and another for 64-bit pointers. Some of these reasons include:
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.
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 1-5 shows the DEC 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 on the qualifier.
| 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 |
A few functions in the DEC 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 1-6 shows the functions restricted to using 32-bit pointers. The DEC C RTL offers no 64-bit support for these functions. You must ensure that only 32-bit pointers are used with these functions.
| atexit | getopt | modf | setbuf |
| execve | iconv | putenv | setstate |
| execvp | initstate | recvmsg | setvbuf |
| frexp | ioctl | sendmsg |
Table 1-7 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.
| decc$from_vms | decc$to_vms |
| ftw | tputs |
This section introduces the pointer-size manipulations used in the DEC C RTL header files. Use the following examples to become more proficient in reading these header files and to help modify your own header files.
: #if __INITIAL_POINTER_SIZE 1 # 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 #endif : : #if __INITIAL_POINTER_SIZE 5 # pragma __pointer_size 64 #endif : : #if __INITIAL_POINTER_SIZE 6 # pragma __pointer_size __restore #endif :All DEC C compilers that support the /POINTER_SIZE qualifier predefine the macro __INITIAL_POINTER_SIZE. The DEC 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 at 1 can be read as "if the user has specified either /POINTER_SIZE=32 or /POINTER_SIZE=64 on the command line".
DEC C Version 5.2 and higher is supported on many OpenVMS platforms. The lines at 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 DEC C compiler offers only the __INITIAL_ POINTER_SIZE macro and a mechanism to change the pointer size, but not a 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;
#endif
:
:
#if __INITIAL_POINTER_SIZE
# pragma __pointer_size 64
#endif
:
:
#ifndef __CHAR_PTR64 2
# define __CHAR_PTR64 1
typedef char * __char_ptr64;
typedef const char * __const_char_ptr64;
#endif
:
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.
DEC 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 __INITIAL_POINTER_SIZE # 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 #endif : 1 : #if __INITIAL_POINTER_SIZE 2 # pragma __pointer_size 64 #endif : 3 : 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 typedefs previously discussed.
:
#if __INITIAL_POINTER_SIZE
# pragma __pointer_size 64
#endif
:
:
#if __INITIAL_POINTER_SIZE == 32 1
# pragma __pointer_size 32
#endif
:
char *strcat (char *__s1, __const_char_ptr64 __s2); 2
:
#if __INITIAL_POINTER_SIZE
# 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
:
#endif
:
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.