Linux so export functions

How to list exported functions in a shared lib on Ubuntu

I have just built a shared lib on Ubuntu, and when I attempt to use the function, the application that loads the library is reporting ‘xxx’ symbol not found. I want to check (i.e. list) the functions that are exported by my library so I can investigate this issue further. Relevant details: OS: Ubuntu 9.10 compiler: gcc 4.4.1 linker: GNU ld 2.20

3 Answers 3

GNU nm lists the symbols from object files objfile. If no object files are listed as arguments, nm assumes the file a.out. [reference]

Thanks for that. It appears my symbol is exported, but undefined (it has a ‘U’ next to the function name). How can a function be exported and yet be undefined (IIRC, a linker option prevents this kind of absurbity). More importantly, what can I do to fix it?

By «Fixing it», I mean how can I ensure that my functions are exported AND DEFINED in the shared library?

Could it be that the undefined symbol is contained in another shared object? Check out Void’s advice.

Were you able to find the root cause for your problem? I am trying to link a FORTRAN-based library and it is showing a function as undefined in the output shared library

For exported functions add —defined-only option, otherwise this way you’ll get imports too. Also, to get only functions and not e.g. something like _edata , you should look for the symbols with T or t in the second column. So, something like this: nm -DCg —defined-only $MY_LIB | grep ‘^[0-9a-f]\+ [Tt] ‘

Is your shared library in the library load path or in the application’s run-time search path? It sounds like the dynamic linker can’t find your library. Try running ldd on your application to see if the library can be found at run-time, e.g.:

$ ldd /usr/bin/less linux-gate.so.1 => (0x0072a000) libncurses.so.5 => /lib/libncurses.so.5 (0x00c68000) libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0x007c7000) libdl.so.2 => /lib/tls/i686/cmov/libdl.so.2 (0x00286000) /lib/ld-linux.so.2 (0x002a1000) 

See the ld.so(8) man page for additional details on library search paths.

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c++ экспортировании функций из so

в линуксе kept simple — все символы экспортируются по-умолчанию, не надо ничего делать.
если Qt — лучше пользоваться Q_DECL_EXPORT и Q_DECL_IMPORT — меньше гемора будет с виндой. QtCreator генерит такой код по умолчанию при создании shared library проекта. Посмотри.

так у меня статическая либа не создаётся в code blocks для exporta из so

так у меня статическая либа не создаётся в code blocks для exporta из so

она не нужна, .so линкуется в программу напрямую

как мне её подключить к программе?

man gcc, ключи -l -L тебе в помощь

лан буду за компом попробую

как мне её подключить к программе?

gcc object_files path_to_lib.so -o program

gcc object_files -L/dierectory/with/lib -lname -o program

при этом .so файл должен иметь имя libname.so

#define API __attribute__ ((visibility("default"))) #ifdef __cplusplus extern "C" < #endif int API some_api_fun(); #ifdef __cplusplus >#endif 

компилятору скорми -fvisibility=hidden, тогда наружу будут торчать только те, что тебе надо. Это ускорит загрузку приложения.

How To Write Shared Libraries Ulrich Drepper Red Hat, Inc. drepper@redhat.com August 20, 2006

Нинада тибе никакая статическая либо. Под правильными платформами всё сделано без лишнего долбанутого гемора с либфайлами и сошки проверсиированы на уровне имени файла.

Про всякие dllMain, которые безпричинно крешат всё при запуске loadlibrary я вообще молчу. Такие дела.

И вообще, после портирования туда даже кутишных приложений у меня руки потом тресутся два дня. Такое чувство что ту платформу придумали садомазо, ибо на каждом шаге кажется что тебе специально палки в колёса вставляют.

Другое дело линукс. Всё работает из коробки.

nanoolinux ★★★★ ( 15.10.13 14:00:32 MSK )
Последнее исправление: nanoolinux 15.10.13 14:01:40 MSK (всего исправлений: 1)

How To Write Shared Libraries Ulrich Drepper Red Hat, Inc. drepper@redhat.com August 20, 2006

Плюсую Дреппера. Читать обязатлельно для полного понимания.

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Thread: howto export functions in *.so libraries

5 Cups of Ubuntu

howto export functions in *.so libraries

I am trying to convert one windows program to Linux.
The question is:
what is linux equivalent for

extern «C» int (__declspec(dllexport))someFunction(char* someparam)

Frothy Coffee!

Re: howto export functions in *.so libraries

Is just a mere macro to usually force the compiler to produce stdcall calling convention. In Linux AFAIK we only use cdecl ( http://en.wikipedia.org/wiki/X86_calling_conventions ).
Have fun!

5 Cups of Ubuntu

Re: howto export functions in *.so libraries

Frothy Coffee!

Re: howto export functions in *.so libraries

to be precise, the calling convention applies to i386 arch.
For x86-64 Windows and Linux follow 2 slightly different conventions: Linux follows AMD ABI calling convention, instead, as usual, M$ way is to slightly diverge from it from no apparent reason. sigh.
Anyway, given more registers in x86-64, a lot of variables are passed into them, instead of being put onto the stack then read (as happens with i386 convention). So even just for this reason (and believe me there are many more) x86-64 is faster than i386.

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