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The Question is: Is it possible to build an image linked with a VAXC OLB and execute it on a VAX running VMS 5.5-2H4 with DECC RTL installed? We have tried this and have found that global sections mapped by the VAXC - bound application are not aligned properly. We are trying to avoid rebuilding the application with the DECC compiler. The Answer is :
VAX C and Compaq (DEC) C are fully compatible, and can be installed
coresident, and can operate on the same OpenVMS VAX system. Compaq
(DEC) C can be installed on OpenVMS VAX V5.5-2 through V6.0 (inclusive),
with the installation of the Compaq (DEC) C run-time library kit provided
with the compiler. The Compaq (DEC) C run-time kit is not required on
OpenVMS VAX V6.1 and later, nor on OpenVMS Alpha V6.1 and later.
The Wizard would strongly encourage the use of Compaq (DEC) C over VAX C,
as it is far better at finding latent problems in C source code, and it
makes for far easier porting of C code to Compaq (DEC) C on OpenVMS Alpha
or to other ANSI C platforms. Compaq (DEC) C also supports a VAX C
compatibility mode.
As for alignment questions involving installed images, that does not
appear to be directly tied to the use of an object library (OLB). The
Wizard would recommed the use of shareable images for VAX C or Compaq
(DEC) C, over the use of a compiler object library. For information on
creating shareable images -- and why you might want to -- please see the
pointer to the shareable image cookbook included in the OpenVMS Frequently
Asked Questions (FAQ).
The initial Compaq (DEC) C compiler command first used when porting many
VAX C programs to Compaq (DEC) C is:
$ CC/DECC/STANDARD=VAXC/PREFIX=ALL
For additional information on using global sections from C, see the
global section programming example available at the OpenVMS Ask The
Wizard website.
The Wizard could infer from the question that the application may be
overmapping a program data structure with a global section -- this is
similar to a Fortran COMMON, and is not an approach that the Wizard
would recommend. For reasons of flexibility and adaptability, and
to avoid frequent relinking when constituent shareable images change
sizes, the Wizard prefers to use dynamic, run-time, position-independent
global section over a link-time position-dependent COMMON -- global
sections can be mapped into the next available memory, and the program
can control the size of the global section as required. COMMONs are
static, and image(s) must be periodically relinked. (Be aware that
position independence means that you cannot store virtual addresses
in a section, you must store base-relative offsets -- offsets from the
base address of the global section -- in any data structures present
in the section. This "referencing" and "dereferencing" is required
as the global section is position-independent; the section may not be
mapped into the same portion of virtual address space in each process
accessing it.) When working with commons and particularly with extern
externals, also see the /SHARE_GLOBAL and /EXTERN_MODE=COMMON_BLOCK
Compaq (DEC) C command qualifiers.
Also see the member alignment pragmas and the /MEMBER_ALIGNMENT compiler
qualifier. Compaq (DEC) on OpenVMS VAX uses byte alignment, while
Compaq (DEC) C on OpenVMS Alpha uses natural alignment. (OpenVMS Alpha
is particularly sensitive to memory aligment.) This difference is not
usually a problem, save for specific cases such as sharing of data
structures between OpenVMS VAX and OpenVMS Alpha systems (eg: network
packets or RMS file records), or for OpenVMS Alpha structures that
must explicitly be unaligned (eg: the process quota list array on the
sys$creprc system service). When the data structure must have a
particular member alignment, use the #pragma [no]member_alignment.
Compaq (DEC) C also assumes that string literals are not writeable,
where some C programs assume these "constants" can be written. To
assist in porting this code, see the /ASSUME=WRITABLE_STRING_LITERALS
qualifier.
For specific information on the "not aligned properly" problem that is
being reported, please contact the customer support center -- there is
insufficient information on the error and on the application included
with the report to provide an answer with any degree of certainty.
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