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Date: Thu, 20 Sep 2012 12:41:52 -0700 (PDT)
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Subject: Dynamic linking with undefined symbol giving runtime error
From: aun...@gmail.com
Injection-Date: Thu, 20 Sep 2012 19:41:52 +0000
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I am trying to use a lib(mongodb C driver) in my application but stuck badl=
y in a linking problem. The libs are 4 objects (libbson.a + libbson.so + li=
bmongoc.a + libmongoc.so). My Makefile has libraries added like this. The r=
elevant ones are -lmongoc and -lbson

srv_la_LIBADD =3D -lrt -lcre2 -lre2 -lcurl -lpthread -lmongoc \
                -lbson @MODULES_LIBADD@

The driver libs are compiled and installed successfully at /usr/local/lib. =
I have even made links to these in /lib and /lib64 directories

The problem is that my application gets compiled and linked successfully  b=
ut the final srv.so contains UNDEFINED symbols from the mongo C driver(libm=
ongoc.a) which give runtime errors.

Error loading module srv.so:/usr/local/c_icap/lib/c_icap/srv.so:       unde=
fined symbol: mongo_cursor_set_query

The linker line from Make process is:

libtool: link: gcc -shared  -fPIC -DPIC  .libs/srv_la-srv.o
 .libs/srv_la-erd.o=20
 .libs/srv_la-erd_list.o=20
 .libs/srv_la-str_util.o=20
 .libs/srv_la-url_util.o=20
 .libs/srv_la-easyzlib.o=20
 .libs/srv_la-state_db.o=20
 .libs/srv_la-data_access_api.o=20
 .libs/srv_la-cJSON.o =20
 .libs/srv_la-clib_es.o  =20
 -lrt -lcre2 -lre2 /usr/lib/x86_64-linux-gnu/libcurl.so=20
 -lpthread -lmongoc -lbson  -O2 -O2   -Wl,-soname -Wl
 ,srv.so -o .libs/srv.so


Specifically the objdump of the srv.so says:

root@talha:/webproxy# objdump -t services/.libs/srv.so|grep "mongo"
0000000000000000         *UND*  0000000000000000              mongo_cursor_=
set_query
0000000000000000         *UND*  0000000000000000              mongo_destroy
0000000000000000         *UND*  0000000000000000              mongo_cmd_aut=
henticate
0000000000000000         *UND*  0000000000000000              mongo_create_=
index
0000000000000000         *UND*  0000000000000000              mongo_cursor_=
destroy
0000000000000000         *UND*  0000000000000000              mongo_update
0000000000000000         *UND*  0000000000000000              mongo_cursor_=
next
0000000000000000         *UND*  0000000000000000              mongo_cursor_=
init
0000000000000000         *UND*  0000000000000000              mongo_remove
0000000000000000         *UND*  0000000000000000              mongo_connect
0000000000000000         *UND*  0000000000000000              mongo_cursor_=
bson


Whereas the bson symbols are added correctly. This is strange as both the l=
ibs are installed at the same location and have been added in the Makefile.

I tried to link libmongoc.a(static) statically but that results in the link=
er giving a portability warning of srv.so linking against libmongoc.a and p=
roduces final srv.a instead of src.so(but this kind of makes sense). Even t=
hen, the symbols are not present in srv.a. Which is even more strange to me=
.

The the libs were built with -fPIC flag so that part is catered. Any helpfu=
l hints? Why is it not linking properly and why is the linker not giving an=
y errors?