Trouble is that in shared library context there is a huge class of data
which even if declared const will not be shared. Those are reference
relocations. For example consider this:
const struct { int id; const char *description; } table = {...};
Would the 'table' be shared? If compiled with PIC flag of course?
Unfortunately no, because 'description' member constitute a relocation,
which is to be resolved by *run-time* linker and can't be placed to
read-only segment. What would help in this situation? To split structure
members to two tables, so that at least id-s will be shared? To replace
'description' with offset in some other table computable at compile
time? Yes, yes, at the cost of code modification. But what about
pre-link? At least some contemporary Linuxen do offer shared object
pre-link, when relocations get re[!]-resolved by run-time linker only if
given shared object module can not be placed at pre-linked base address.
So that if the matter discussed in this ticket is of greater concern, do
use pre-link!
But above doesn't mean that we say no to constifications. Even more
elaborate methods are being implemented, see for example
http://cvs.openssl.org/chngview?cn=16578 and
http://cvs.openssl.org/chngview?cn=16522. Thanks for suggestions. A.
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