As programmers, we face it many times: a build fails on only our system. This is one such tale.
Conflicting declarations in zeromq
We build many system software into a custom FreeBSD platform. The first error message I got was while building zeromq. I had freshly synced a branch, merged some changes from another branch, and started a build.
The compiler complained:
CXX libzmq_la-gssapi_mechanism_base.lo In file included from gssapi_mechanism_base.hpp:30, from gssapi_mechanism_base.cpp:34: /usr/include/gssapi/gssapi.h:316: error: previous declaration of 'gss_OID_desc_struct* GSS_KRB5_NT_PRINCIPAL_NAME' with 'C++' linkage /usr/include/gssapi/gssapi_krb5.h:49: error: conflicts with new declaration with 'C' linkage
The error means that we are listing symbols two times, and the two listings conflict with each other. The error happened only on this branch. It built fine on the build server and on other branches I had synced on my system.
I wanted to make progress past this problem quickly. Google showed
gssapi.h on Mac
had an embedded
extern "C", but mine did not. I added this to my
/usr/include/gssapi/gssapi.h, and restarted the build. The error
Missing MD2 references in net-snmp
The build now stopped in a different package. This time, it was a
failure while running
configure to build net-snmp. The config.log
had this to say:
configure:5735: gcc -I../zeromq-4.1.0/include [...] -L../zeromq-4.1.0/src/.libs -L/usr/obj/.../secure/lib/libcrypto -lcrypto -R/usr/obj/.../secure/lib/libcrypto conftest.c -lzmq >&5 /usr/lib/libhx509.so.10: undefined reference to `MD2_Init' /usr/lib/libhx509.so.10: undefined reference to `MD2_Final' /usr/lib/libhx509.so.10: undefined reference to `MD2_Update'
The errors mean that we depend on MD2 encryption routines, but we cannot find them in any of the libraries specified. I took the conftest.c source from the same file and tried to compile it with the same options. I got the same error.
Notice that we link with a custom crypto library above. Turns out our custom library does not have MD2 support. What if I tried the standard library? When I used the standard crypto library, I didn’t get those errors! Because it had those routines (‘T’):
/usr/ports/security/openssl$ nm ./work/openssl-1.0.1e/libcrypto.so | grep MD2 000000000006ce20 T MD2 000000000006cc60 T MD2_Final 000000000006cdd0 T MD2_Init 000000000006cce0 T MD2_Update 000000000006cb00 T MD2_options 000000000015eea0 R MD2_version
But, but, it is working for others! Everyone is using the same custom library! What went wrong for me?
Notice that the other library it depends on above is the zeromq
-lzmq). I replaced its location with my working branch,
and that worked too!
gcc [...] -L../zeromq-4.1.0/src/.libs test.o -lzmq [...]
gcc [...] -L../working_branch/zeromq-4.1.0/src/.libs test.o -lzmq [...]
Back to zeromq
So is something fishy in my zeromq library? Maybe the error I saw earlier that I “fixed” had to do something about it?
I got rid of the changes I made earlier in
/usr/include/gssapi/gssapi.h and started a build again. It failed
with the same declaration conflict error as earlier.
Now, I built zeromq on the other branch where it worked and compared the build output.
At first glance, they looked the same.
To understand the compilation more than the bare
CXX, I checked if
there was a way.
./configure --help showed that I could do
V=1. I did that, and noticed that there was a
option. This option means there is a separate config file with
“discovered”, i.e. configured features. A search brought up
config.hpp, and sure enough, I found this in the broken build:
/* BREAKS BUILD */ /* Define to 1 if you have the `gssapi_krb5' library (-lgssapi_krb5). */ #define HAVE_LIBGSSAPI_KRB5 1
How did this happen? I looked at
config.log in both branches:
configure:17985: checking for gss_init_sec_context in -lgssapi_krb5 configure:18010: gcc -std=gnu99 -o conftest -g -O2 -D__BSD_VISIBLE -D_REENTRANT -D_THREAD_SAFE conftest.c -lgssapi_krb5 -lrt -lpthread >&5 configure:18010: $? = 0 configure:18019: result: yes
configure:17967: checking for gss_init_sec_context in -lgssapi_krb5 configure:17992: gcc -std=gnu99 -o conftest -g -O2 -D__BSD_VISIBLE -D_REENTRANT -D_THREAD_SAFE conftest.c -lgssapi_krb5 -lrt -lpthread >&5 configure:17992: $? = 0 configure:18004: result: no configure:18014: WARNING: libgssapi_krb5 is needed for GSSAPI security
The same command returns “yes” in one branch, but “no” in another!? What gives?
Never edit generated files
So, I opened up the configure script that was running the command. And this is what I found in the working branch:
# XXX ac_cv_lib_gssapi_krb5_gss_init_sec_context=no # XXX
This explains everything. Someone had explicitly “disabled” this
library check in the
configure script. But this is a generated
autoconf generates it from configure.ac and configure.in.
For some reason, this file was re-generated by my autoconf during the
build. When that happened, the customization was dropped!
Why this did not happen to me on the other branches, I still do not know. Maybe the timestamps were wrong only on this particular branch? I tried another clean sync and build, but the error did not happen again. I do not understand all the regeneration criteria yet.
Anyway, the fix was to comment a line in
configure.ac instead. It’s
been a very interesting evening.