问题
I've a program that I must build. The program depends on libA, and libA depends on libB. Both libs are in the same folder but ldd libA.so does not include libB.so so I must add it while linking it.
This is my gcc command:
gcc -L/path/to/libraries/lib -lA -lB -I/path/to/libraries/include main.cpp
The program builds and links, but it does not start. It gives me following error:
./a.out: symbol lookup error: /path/to/libraries/lib/libA.so: undefined symbol: symbol_used_in_libA_but_defined_in_libB
With ldd I can see that libB.so is not included in my binary:
linux-vdso.so.1 => (0x00007fffaecd9000)
libA.so => /path/to/libraries/lib/libA.so (0x00007effc02a4000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007effbfebb000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007effbfca5000)
/lib64/ld-linux-x86-64.so.2 (0x00007effc05cb000)
I have these conditions:
/path/to/librariesis insideLD_LIBRARY_PATH- running
ldconfigis ok andldconfig -pfind bothlibA.soandlibB.so - If in gcc command I change
-lBwith-lBBit gives me a linker error, so I think thatgccfind correctlylibB.soeven if it does not link it inside the executable.
What I'm doing wrong? What I can do in order to link the executable to both libraries?
回答1:
Most Linux distributions (I assume you are using Linux based on the output of ldd) seem to configure gcc as to pass --as-needed to ld by default (e.g., see here for Debian). This means the final library/executable will only depend on a library (i.e., have a DT_NEEDED tag for that library) if some symbol of that library is actually used by the library/executable.
In your case, main.cpp does not use any functions of libB so the linker does not add libB as a dependency of the final executable. You can work around it by passing the --no-as-needed flag to the linker. E.g.,
gcc -Wl,--no-as-needed ...
Of course, the proper fix is to relink libA and make sure it lists libB as a dependency.
来源：https://stackoverflow.com/questions/28088100/gcc-build-links-but-shared-library-does-not-appear-with-ldd