Re: Why does libca.so depend on libreadline.so?

[Andrew Johnson via Tech-talk <tech-talk at aps.anl.gov>]

Hi Dmitry,

On 5/10/23 3:06 AM, Dmitry Yu. Bolkhovityanov via Tech-talk wrote:

        The core of iocsh is a part of libCom.

Yes, this explains it all.

Albeit the design looks extremely surprising -- as if some IP implementation included an integrated ksh...


However, a brief investigation had found this had changed in the 7.0.x between 7.0.2 and 7.0.7: in 7.0.2 the libca.so.4.13.3 does have a dependency on libreadline, while 7.0.7's libca.so.4.14.2 does NOT.

So, if 7.0.x was refactored to get rid of this dependency, then this change can probably be back-ported to 3.15.x.  A quick inspection of https://epics.anl.gov/base/R7-0/7-docs/RELEASE_NOTES.html doesn't show any traces of this change; probably a "diff -Naur" between sources is required.  I'm not sure my skills are sufficient for this back-porting, but I'll try.

I suspect that your brief investigation missed the fact that there were changes to the build system's detection of whether the target has a usable copy of readline installed, and those changes are almost certainly why some of your builds linked the libCom shared library with it and others didn't.

If you don't want your builds of libCom and libCa to depend on readline, you should ensure that it doesn't get built in by setting COMMANDLINE_LIBRARY=EPICS in an appropriate CONFIG_SITE* file. Our default build configuration is likely to always try to make use of readline if it's available, since it makes the IOC's shell so much easier to interact with. That is especially true now that the command-line completion features that readline provides have been integrated into iocsh.

I'd be more than happy for us to apply the -Wl,--as-needed linker option in future releases if that resolves the issue, GitHub Pull Requests are welcome.

HTH,

- Andrew

On Mon, May 8, 2023 at 9:46?PM Dmitry Yu. Bolkhovityanov via Tech-talk <
tech-talk at aps.anl.gov> wrote:

Michael,

On Sun, 7 May 2023, Michael Davidsaver wrote:

On 5/6/23 23:12, Dmitry Yu. Bolkhovityanov via Tech-talk wrote:

 P.S. Yes, I'm aware of a "COMMANDLINE_LIBRARY=EPICS" flag (which took

some

 time to figure out :D), but the question remains -- why any commandline
 editing is needed for network communication libraries at all?

This is a side-effect of the convention we use to support both dynamic

and

static linking
with one configuration.  Since static libraries do not carry dependency
information, when
linking statically it is necessary to give a complete list of all

dependent

libraries
when linking any executable.  This includes libreadline when libCom

depends

on it.

One simple way to avoid this over-linking in the dynamic case by passing
"-Wl,--as-needed"
when linking.

(assumes GCC or clang on an ELF target)

Thanks for this -- I didn't know of this linker option!

However, I still don't understand: why libCom depends on libreadline (or
any commandline-editing library)?


With best regards,
Dmitry

-- 
Complexity comes for free, Simplicity you have to work for.