Dynamic load and call in BSD Unix

[Gordon Cross]

In article <5635 at thorin.cs.unc.edu>, alexande at unc.cs.unc.edu (Geoffrey D. Alexander) writes:
> I would to dynamically load and call a function from C.  The name of the
> function would be specified by the user.  I am running under SUN  BSD Unix.
> Can this be done?  The function I am looking for is similiar to the function
> offered by the LOAD macro of IBM's MVS operating system.

I wrote a dynamic loader that we use here at this company as a debugging
tool since the load time is MUCH MUCH less than the time to re-link a very
large application.  If you do not have some sort of kernel support that
allows one to modify program text (which was the case for us), dynamic loading
has certain restrictions.  First and most importantly, it must be possible
to execute instructions in the data section.  Secondly if the function you
are loading is already bound into your process, any direct (ie not through
pointers) calls cannot be made to reference the new version.  The /proc file
system (coming soon to System V) should solve this problem elegantly.  It may
be possible to use the existing "ptrace" system call as well but this requires
a parent/child relationship in which the child has agreed to be traced.
Since our applications are based on an object-oriented system in which the
majority of function calls get translated into indirection through pointers,
this restriction did not pose serious problems.   We only needed to load the
new function address into the corresponding pointer.  However there should be
no problem with loading new functions.  If you find that dynamic loading as
I've just described should work for you, let me know.  Perhaps we could port
it to your system...


Gordon Cross
Intergraph Corp.  Huntsville, AL
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