Ever since I upgraded to debian 9, osmo-hnbgw failed to link in my build: /usr/bin/ld: hnbgw.o: undefined reference to symbol 'sctp_send@@VERS_1' I noticed that adding -lsctp to the build fixed that, yet noticed that no build slaves nor anyone else seemed to need that. Some events coincided my re-install of deb9: - There has always been an -lsctp in the Makefile.am for osmo-hnbgw, but it was recently dropped in http://git.osmocom.org/osmo-iuh/commit/?id=7235ea02d78299471d58f4202d8c8d68… - I adopted the jenkins.sh -Werror flags in some builds to get fatal warnings. Passed by './configure CFLAGS=-Werror'. Someone else added this in jenkins.sh and I assumed that was a good way to do things. - I noticed that I couldn't see debug symbols in gdb, so added 'CLFAGS=-g' in my builds. read on... Comparing with a docker build kindly provided by laforge, I noticed that a plain 'clone; ./configure; make' does actually succeed on my system! It is a convolution of CFLAGS causing the error. By passing a 'CFLAGS=foo' to the osmo-iuh configure script, not only do I add 'foo', but I override CFLAGS defaults, and actually *remove* other CFLAGS. Turns out, whether I pass CFLAGS=-g or not, -g is always part of the build command lines: '-g -O2' is the default. Funnily enough, with CFLAGS=-g passed, I end up removing the -O2 option. A conclusion here is that in jenkins.sh, we should *not* pass CFLAGS to configure, so that we don't override any default CFLAGS. A configure option like --with-strict-warnings can add to CFLAGS in configure.ac instead So now I guess because I added CFLAGS=-Werror before, I ended up removing -g. Hence I saw a need to add -g again, goof. Did so in all builds while I was at it, hence -g ended up in osmo-iuh as well. The -g stuck around, and building osmo-iuh with CFLAGS=-g does, as I already said, effectively remove the -O2 option for all things built in osmo-iuh. Now I'm confused. Why does a lack of -O2 break a linkage. It's not the osmo-hnbgw linkage per se that seems to be the cause. I can link that last step with or without -O2 successfully. It must be a combination of building a depending library without -O2 and then linking that to osmo-iuh. I diffed the entire build log between a succeeding and a failing build of osmo-iuh, and really the only difference everywhere is that the succeeding build uses -O2 everywhere. The only differences in config.log are the same: most conftest programs have -O2 removed, few have a -g added. The conftest conclusions are all identical. My big question now is: sctp_send() is actually called from hnbgw.c directly. So IIUC, we should in fact be required to pass -lsctp to the linker for osmo-hnbgw. If that is correct, how am I allowed to skip that by passing -O2? That's all I figured out so far. I still would like to find out why this can happen, any ideas would be appreciated. Anyone should be able to reproduce this by building osmo-iuh with ./configure CFLAGS=-g ~N