=;The OpenVMS Frequently Asked Questions (FAQ)D

The OpenVMS Frequently Asked Questions (FAQ)



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HPermit structure members to be naturally aligned whenever possible, and ?avoid using /NOMEMBER_ALIGNMENT. If you need to disable member @alignment, use the equivilent #pragma to designate the specific Hstructures. The alignment of structure members normally only comes into Fplay with specific unaligned data structures---such as the sys$creprc Gquota itemlist---and with data structures that are using data that was ?organized by a system using byte or other non-member alignment.

DVersions of HP C such as V6.0 include the capability to extract the Ccontents of the standard header libraries into directories such as FSYS$SYSROOT:[DECC$LIB...], and provide various logical names that can Cbe defined to control library searches. With HP C versions such as GV6.0, the default operations of the compiler match the expectations of ?most OpenVMS programmers, without requiring any definitions of Esite-specific library-related logical names. (And logical names left Dfrom older DEC C versions can sometimes cause the compiler troubles locating header files.)

HHP C V5.6 and later include a backport library, a mechanism by which HP AC running on older OpenVMS releases can gain access to newer RTL Hroutines added to the RTL in later OpenVMS releases---the language RTLs 5ship with OpenVMS itself, and not with the compilers.

EExample C code is available in SYS$EXAMPLES:, in DECW$EXAMPLES (when Gthe DECwindows examples are installed), in TCPIP$SERVICES (or on older Freleases, UCX$EXAMPLES) when HP TCP/IP Services is installed), on the *Freeware CD-ROMs, and at web sites such as



HFor additional information on the OpenVMS Ask The Wizard (ATW) area and Bfor a pointer to the available ATW Wizard.zip archive, please see 0Section 3.8.<

10.22.1 Other common C issues



BThe localtime() function and various other functions maintain the Hnumber of years since 1900 in the "struct tm" structure member tm_year. GThis field will contain a value of 100 in the year 2000, 101 for 2001, Aetc., and the yearly incrementation of this field is expected to continue.

GThe C epoch typically uses a longword (known as time_t) to contain the Gnumber of seconds since midnight on 1-Jan-1970. At the current rate of Dconsumption of seconds, this longword is expected to overflow (when Ginterpreted as a signed longword) circa 03:14:07 on 19-Jan-2038 (GMT), Eas this time is circa 0x7FFFFFFF seconds since the C base date. (The >most common solution is to ensure that time_t is an unsigned.)

EIf C does not correctly handle the display of the local system time, Fthen check the UTC configuration on OpenVMS---the most common symptom Eof this is a skew of one hour (or whatever the local daylight saving Htime change might be). This skew can be caused by incorrect handling of Dthe "is_dst" setting in the application program, or by an incorrect <OpenVMS UTC configuration on the local system. (See section 1Section 4.4.)

FFloating point is prohibited in OpenVMS Alpha inner-mode (privileged) Gcode, and in any process or other execution context that does not have Bfloating point enabled. C programmers developing and working with DOpenVMS Alpha high-IPL kernel-mode code such as device drivers will Hwant to become familiar with the floating point processing available in 3the environment, and with the C compiler qualifier F/INSTRUCTION_SET=[NO]FLOATING_POINT. Device drivers and other similar )kernel-mode C code must be compiled with @/INSTRUCTION_SET=FLOATING_POINT and /EXTERN_MODEL=STRICT_REFDEF.

FAdditionally, the SYS$LIBRARY:SYS$LIB_C.TLB/LIBRARY parameter will be Hneeded to be appended to the module specification or declared via the C Fcompiler's include library logical name mechanism when the C compiler Gis resolving kernel-mode data structures and definitions. This library Acontains OpenVMS kernel-mode and other system declaractions, and Eparticularly a mixture of undocumented definitions and declarations, Band particularly definitions and declarations that are subject to >change (and that can accordingly lead to requirements for the #recompilation of application code).

GIn addition to the user-mode C Run-Time Library (RTL) mentioned in the lOpenVMS C RTL documentation and referenced over in Section 3.9, there Fis a second and parallel kernel-mode RTL accessable to device drivers Hand other kernel code on OpenVMS Alpha and OpenVMS I64. The most common Ftime this second C library is noticed is when C code is (erroneously) Glinked with /SYSEXE/SYSLIB, and duplicate symbol errors typically then Aarise. As code running in supervisor-, executive- or kernel-mode Hcontext cannot call out a user-mode RTL or other user-mode library, you Gwill want to respecify the command as LINK /SYSEXE/NOSYSLIB. This will Beliminate the duplicate symbol errors, since only the kernel-mode Hlibrary will be referenced, and it will also avoid calling out into the user-mode libraries.

GWhen sharing variables with other languages, here is some example HP C code...

 

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      ...        #pragma extern_model save )      #pragma extern_model strict_refdef "      extern int   VMS$GL_FLAVOR; #      #pragma extern_model restore 
      ... 




1and here is some associated example Bliss code...

 

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      ...       EXTERNAL          VMS$GL_FLAVOR,       .... 


?

10.22.2 Other common C++ issues



EHP C++ (a separate compiler from HP C) provides both symbol mangling Band symbol decoration. Some of the details of working with longer Fsymbol names and the resulting symbol name mangling in mixed language Henvironments are listed in the shareable image cookbook, and in the C++ Adocumentation. Symbol name decoration permits the overloading of Hfunctions (by adding characters to the external symbol for the function Ato indicate the function return type and the argument data types Hinvolved), and mixed-language external references can and often do need Dto disable this decoration via the extern "C" declaration mechanism:

 

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      extern "C" 
        { )        extern int ExternSymbol(void *); .        extern int OtherExternSymbol(void *); 
        } 




aAlso see Section 14.7 for information on /ARCHITECTURE and /OPTIMIZE=TUNE.

iSee Section 10.15 for information on the C system and the lib$spawn call in CAPTIVE environments.

EConstructs such as the order of incrementation or decrementation and Athe order of argument processing within an argument list are all Dimplementation-defined. This means that C coding constructs such as:

 

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    i = i++;     a[i] = i++;     foo( i, i++, --i); 




Eare undefined and can have (adverse) implications when porting the C Ccode to another C compiler or to another platform. In general, any Hcombination of ++, --, =, +=, -=, *=, etc operators that will cause the Fsame value to be modified multiple times (between what the ANSI/ISO C 8standard calls "sequence points") produce undefined and implementation-specific results.

FWithin C, the following are the "sequence points": the ";" at the end Dof a C statment, the ||, &&, ?:, and comma operators, and a Fcall to a function. Note specifically that = is NOT a sequence point, Gand that the individual arguments contained within a function argument Hlist can be processed from right to left, from left to right, or at any random whim.

AHP C for OpenVMS VAX (formerly DEC C) and VAX C do differ in the related processing.

DSo you are looking for OpenVMS-specific definitions (include files)?

FUCBDEF.H, PCBDEF.H and other OpenVMS-specific definitions---these are Dconsidered part of OpenVMS and not part of the C compiler kit---are /available on all recent OpenVMS Alpha releases.

2To reference the version-dependent symbol library Dsys$share:sys$lib_c.tlb, use a command similar to the following for compilation:

 

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'$ CC sourcea+SYS$LIBRARY:SYS$LIB_C/LIB 




?You can also define DECC$TEXT_LIBRARY to reference the library.

=You will want to review the Programming Concepts manual, and -specifically take a quick look at Chapter 21.

CAnd some general background: the STARLET definitions (and thus the Hsys$starlet_c.tlb library) contain the symbols and the definitions that Fare independent of the OpenVMS version. The LIB definitions (and thus Hsys$lib_c) contain symbols and definitions that can be dependent on the FOpenVMS version. You won't need to rebuild your code after an OpenVMS Gupgrade if you have included definitions from STARLET. The same cannot Gbe said for some of the definitions in LIB---you might need to rebuild Byour code. (The UCB structure can and has changed from release to release, for instance.)

FRecent versions of C automatically search sys$starlet_c.tlb. Explicit +specification of sys$lib_c.tlb is required.

FAlso see the Ask The Wizard website topics (2486), (3803), and (1661):



HFor additional information on the OpenVMS Ask The Wizard (ATW) area and Bfor a pointer to the available ATW Wizard.zip archive, please see 0Section 3.8.

iSee Section 9.5 for information on the C off_t limitations, resolved Cin OpenVMS V7.3-1 and later and in ECO kits available for specific <OpenVMS releases. The use of a longword for off_t restricts Happlications using native C I/O to file sizes of two gigabytes or less, Dor these applications must use native RMS or XQP calls for specific operations.j

10.23 Status of Programming Tools on OpenVMS VAX?



HDECthreads V7.3 and the HP C compiler (also known as Compaq C and DEC C)@V6.4 are presently expected to be the last updates and the last Creleases of these development packages for use on OpenVMS VAX. The Erun-time support for both DECthreads (CMA$RTL) and for C (DECC$CRTL) will continue to beHmaintained, and will continue to be available on OpenVMS VAX. The VAX C GV3.2 compiler is the final VAX C compiler release for OpenVMS VAX, and Hthe VAX C Run-Time Library (VAXCRTL) will also continue to be available.

GNew development and new features and product enhancements continue for Cthe OpenVMS Alpha and the OpenVMS IA-64 DECthreads and C compilers.s

10.24 Choosing a Version Number for Application Code?



GOne of the common rules-of-thumb used for choosing a displayed version Hnumber string for a new version of a layered product or an application, Fits implications, and its expected effects on client applications and users, follows:



BFor additional version-numbering materials and for information on <assigning module generation numbers, please see the OpenVMS G(POLYCENTER) Software Product Installation Utility---variously refered Dto by acronyms including PCSI and SPIA---reference manual available %within the OpenVMS documentation set.

?Of course, all of this is obviously subject to interpretation, Hparticularly around the distinction between large and small changes and Hsuch. The scale of the application is also a factor, as larger and more Gcomplex applications will tend toward smaller increments and will tend Ato see the maintenance number incremented, while new releases of Dsmaller applications will tend to see the minor version incremented somewhat more frequently.

HThe goal of all this is to provide a guide to relative scale of changes Eand the associated effort involved in an upgrade for the user and/or for the application programmer.e

10.25 Selecting a Process Dump Directory?



CYou can customize the device and directory for the process dump by Fdefining the logical names SYS$PROCDMP and SYS$PROTECTED_PROCDMP. The Eformer is for non-privileged dumps, while the latter is the location Awhere privileged image dumps are written, and preferably an area 0protected against untrusted access. For example:

 

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#$ define SYS$PROCDMP SYS$ERRORLOG: 3$ define /exec SYS$PROTECTED_PROCDMP SYS$ERRORLOG: 




CThe abouve presumes that the SYS$ERRORLOG logical name points to a valid location.

EThere is presently no means to change the name of the generated dump ;file from IMAGENAME.DMP to something else. Accordingly, you@will want to use different target directories for this purpose, >particularly if there is more than one application or process "potentially writing process dumps.^

10.26 Access to Itanium Assembler?



FIf you are interested in accessing the native Intel Itanium assembler =within the OpenVMS I64 GNV environment---and since the iasi64G assembler is a Unix program and GNV is a Unix environment for OpenVMS G I64---you can simply copy iasi64.ext into your gnu:[bin] directory in 5 place of "as.", and of "AS.EXE".

GAlternately and probably also better, you can write an "as." Cscript to invoke the iasi64.exe image from its particular prefered location on the local system.

1A typical "as." script looks like this:

 

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path/iasi64.exe $1 $2 $3 $4 $5 


b

10.27 Kernel-mode coding restrictions?



FFloating point is prohibited in OpenVMS Alpha inner-mode (privileged) Fcode, and within any process or other execution context that does not *have floating point enabled and available.

?Programmers developing and working with OpenVMS Alpha high-IPL Fkernel-mode code, such as device drivers, will further want to become Dfamiliar with the floating-point processing and the instruction set Hemulation available in the particular target environment (if any). When Dworking with C, inner-mode programmers will want to become familiar Bwith the C compiler qualifier /INSTRUCTION_SET=[NO]FLOATING_POINT.

EDevice drivers and other similar kernel-mode C code must be compiled Ewith /INSTRUCTION_SET=FLOATING_POINT and /EXTERN_MODEL=STRICT_REFDEF.

HAdditionally, inner-mode code cannot call out to the user-mode language Hrun-time libraries nor to any of the OpenVMS system run-time libraries. GIn particular, this prohibition prevents pages of inner-mode-protected Gmemory from being allocated and interspersed within the user-mode heap (or other such user-mode data structures.

EThe prohibtion on user libraries also generally means that such code Amust be linked with LINK /NOSYSLIB, and quite probably also with D/SYSEXE. The former causes the linker to avoid searching the system Fshareable image libraries (via IMAGELIB.OLB), while the latter brings Din symbols typically only known to or otherwise accessable from the OpenVMS executuve.

DTo include kernel-mode C programming definitions, macros and system *constants within a C compilation, include DSYS$LIBRARY:SYS$LIB_C.TLB/LIBRARY on the C compilation. (Constructs /defined within the system macro library LIB.MLB(or within its C equivalent SYS$LIB_C.TLBGtend to be version-dependent, or undocumented, or both.) As an example ?of the compilation, the following is a typical C device driver compilation command:

 

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M$   CC /STANDARD=RELAXED_ANSI89/INSTRUCTION=NOFLOATING_POINT/EXTERN=STRICT - G        'DEBUG_CC_DQ_OPT' 'ARCH_CC_OPT' 'CHECK_CC_OPT' 'SHOW_CC_OPT' - <        /LIS=LIS$:xxDRIVER/MACHINE_CODE/OBJ=OBJ$:xxDRIVER - :        SRC$:xxDRIVER.C+SYS$LIBRARY:SYS$LIB_C.TLB/LIBRARY 




DAdditionally, code running in executive mode in an AST or in kernel Hmode cannot call RMS services, or routines which directly or indirectly call RMS.

HFor related kernel-mode programming materials and driver documentation, Gplease see the Writing OpenVMS Alpha Device Driversin C book, ISBN 1-55558-133-1.n

10.28 Decoding an Access Violation (ACCVIO) Error?



FTo decode the virtual addresses returned by an access violation or by Ganother similar OpenVMS display, you need to have created and retained >a listings file---preferably one with machine code generation enabled---and a full link map.

FStarting with the virtual address reported by the error, use the link Cmap to find the module that contributed the code that contains the Hvirtual address range. Calculate the offset from the base of the range, Gby subtracting the base of the range from teh failing virtual address. AThen use the compiler listings for the particular component that Econtributed the code to locate the offset of the failing instruction.

@If the map and listings information was not maintained, working Abackwards is far more difficult---you are left to use the binary Dinstruction data around the failure to locate the associated source Ccode, and this process is far more involved. This usually involves Gmatching up blocks of decoded instructions around the failing code, or Hthe direct analog involving matching up ranges of decoded instructions. 7Keep the maps and listing files around, in other words.

GRather easier than an approach based on virtual address arithmetic and Hfar easier than working backwards from the instruction stream is to use ?integrated debugging---this inclusion is arguably an essential Bcomponent of any non-trivial application---and to use the OpenVMS Debugger.

EThe OpenVMS Debugger in particular can be used to examine the source ?code, to examine the stack, and can even be programmed to wait @patiently for the incidence of a particular value or failure or Bcondition, and this is far easier than working backwards from the Finstruction stream is to use integrated debugging---this inclusion is Harguably an essential component of any non-trivial application---and to Buse the OpenVMS Debugger. The debugger can also be activated from Ewithin a signal handler, and commands to generate a traceback can be Hgenerated directly, or through the invocation of a procedure containing a series of debugger commands.

EDetails on the debugger are in the OpenVMS Debugger Manual, and also =see the discussion of dyanmically activating the Debugger in 3Section 10.19.c

10.29 Generating an AUTODIN-II CRC32?



8The following code can be used to generate an AUTODIN-IIB32-bit Cyclic Redundency Check (CRC32) value from an input string >descriptor, similar to that used by the HP C compiler for its H/NAMES=SHORTENED mechanism, and by various other applications requiring a CRC32.

GThe routine uses the OpenVMS library routine lib$crc_table to generate Ga sixteen longword array of data from the specified encoded polynomial Ecoefficient (AUTODIN-II, in this case), and then lib$crc to generate 2the CRC32 value from the array and the input data.

 

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?static int CreateCRC32( struct dsc$descriptor *InputDataDesc )   {   uint32 AUTODIN2;   uint32 Seed = ~0UL; &  uint32 Coefficient = 0x0EDB88320UL;   uint32 CRCArray[16];  #  lib$establish( lib$sig_to_ret );  A  lib$crc_table( (void *) &Coefficient, (void *) CRCArray ); N  AUTODIN2 = lib$crc( (void *) CRCArray, (void *) &Seed, InputDataDesc );   AUTODIN2 ^= Seed;    return AUTODIN2;   } 


X

10.30 Enabling built-in tracing?



 

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$ RUN SYS$SYSTEM:SYSMAN =SYSMAN> SYS_LOAD ADD TR$DEBUG TR$DEBUG/LOAD_STEP=INIT/LOG SYSMAN> Exit $$ @SYS$UPDATE:VMS$SYSTEM_IMAGES.COM 




%To stop it from loading early in boot

 

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$  RUN SYS$SYSTEM:SYSMAN 1SYSMAN> SYS_LOAD REMOVE TR$DEBUG TR$DEBUG/LOG SYSMAN> Exit $$ @SYS$UPDATE:VMS$SYSTEM_IMAGES.COM 




?The first occurance of the name TR$DEBUG within the command is Dconsidered the "product" and the second is considered the ?"image" that should exist within SYS$LOADABLE_IMAGES.

EWhen TR$DEBUG loads in the init phase, it will automatically turn on tracing.

Also see the SDA TR extension.




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