HP C Programmer's Guide (92434-90009)

150 Chapter5
Programming for Portability
Porting between HP C and VMS C
Porting between HP C and VMS C
The C language itself is easy to port from VMS to HP-UX for two main reasons:
• There is a high degree of compatibility between HP C and other common industry
implementations of C as well as within the HP-UX family.
• The C language itself does not consider file manipulation or input/output to be part of
the core language. These issues are handled via libraries. Thus, C avoids some of the
thorniest issues of portability.
In most cases, HP C (in compatibility mode) is a superset of VMS C. Therefore, porting
from VMS to HP-UX is easier than porting in the other direction. The next several
subsections describe features of C that can cause problems in porting.
Core Language Features
• Basic data types in VMS have the same general sizes as their counterparts on HP-UX.
In particular, all integral and floating-point types have the same number of bits.
structs and unions do not necessarily have the same size because of different
alignment rules.
• Basic data types are aligned on arbitrary byte boundaries in VMS C. HP-UX
counterparts generally have more restrictive alignments.
• Type char is signed by default on both VMS and HP-UX.
• The unsigned adjective is recognized by both systems and is usable on char, short,
int, and long. It can also be used alone to refer to unsigned int.
• Both VMS and HP-UX support void and enum data types although the allowable uses of
enum vary between the two systems. HP-UX is generally less restrictive.
• The VMS C storage class specifiers globaldef, globalref, and globalvalue have no
direct counterparts on HP-UX or other implementations of UNIX. On HP-UX, variables
are either local or global, based strictly on scope or static class specifiers.
• The VMS C class modifiers readonly and noshare have no direct counterparts on
HP-UX.
• structs are packed differently on the two systems. All elements are byte aligned in
VMS whereas they are aligned more restrictively on the different HP-UX architectures
based upon their type. Organization of fields within the struct differs as well.
• Bit-fields within structs are more general on HP-UX than on VMS. VMS requires that
they be of type int or unsigned whereas they may be any integral type on HP-UX.
• Assignment of one struct to another is supported on both systems. However, VMS
permits assignment of structs provided the types of both sides have the same size.
HP-UX is more restrictive because it requires that the two sides be of the same type.
• VMS C stores floating-point data in memory using a proprietary scheme. Floats are
stored in F_floating format. Doubles are stored either in D_floating format or
G_floating format. D_floating format is the default. HP-UX uses IEEE standard