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$(SPEC_S Portability Guide,
$(P It's good software engineering practice to minimize gratuitous
portability problems in the code.
Techniques to minimize potential portability problems are:
$(LI The integral and floating type sizes should be considered as
Algorithms should be designed to continue to work properly if the
type size increases.)
$(LI Floating point computations can be carried out at a higher
precision than the size of the floating point variable can hold.
Floating point algorithms should continue to work properly if
precision is arbitrarily increased.)
$(LI Avoid depending on the order of side effects in a computation
that may get reordered by the compiler. For example:
a + b + c
$(P can be evaluated as (a + b) + c, a + (b + c), (a + c) + b, (c + b) + a,
etc. Parentheses control operator precedence, parentheses do not
control order of evaluation.
$(P Function parameters can be evaluated either left to right
or right to left, depending on the particular calling conventions
$(P If the operands of an associative operator + or * are floating
point values, the expression is not reordered.
$(LI Avoid dependence on byte order; i.e. whether the CPU
is big-endian or little-endian.)
$(LI Avoid dependence on the size of a pointer or reference being
the same size as a particular integral type.)
$(LI If size dependencies are inevitable, put an $(TT assert) in
the code to verify it:
assert(int.sizeof == (int*).sizeof);
<h2>32 to 64 Bit Portability</h2>
$(P 64 bit processors and operating systems are here.
With that in mind:
$(LI Integral types will remain the same sizes between
32 and 64 bit code.)
$(LI Pointers and object references will increase in size
from 4 bytes to 8 bytes going from 32 to 64 bit code.)
$(LI Use $(B size_t) as an alias for an unsigned integral
type that can span the address space.
Array indices should be of type $(B size_t).)
$(LI Use $(B ptrdiff_t) as an alias for a signed integral
type that can span the address space.
A type representing the difference between two pointers
should be of type $(B ptrdiff_t).)
$(LI The $(B .length), $(B .size), $(B .sizeof), $(B .offsetof)
and $(B .alignof)
properties will be of type $(B size_t).)
$(P Endianness refers to the order in which multibyte types
are stored. The two main orders are $(I big endian) and
$(I little endian).
The compiler predefines the version identifier
$(B BigEndian) or $(B LittleEndian) depending on the order
of the target system.
The x86 systems are all little endian.
$(P The times when endianness matters are:)
$(LI When reading data from an external source (like a file)
written in a different
endian format.)
$(LI When reading or writing individual bytes of a multibyte
type like $(B long)s or $(B double)s.)
<h2>OS Specific Code</h2>
$(P System specific code is handled by isolating the differences into
separate modules. At compile time, the correct system specific
module is imported.
$(P Minor differences can be handled by constant defined in a system
specific import, and then using that constant in an
$(I IfStatement) or $(I StaticIfStatement).