[Libav-user] 答复: 答复: Are there any ways to use a lib (built from C++ OpenCV) in Ffmpeg filter?

[R n]

By the way, I have also been playing with the compiling flags for FFmpeg, but got different errors. For example:


I used '--extra-libs=-lmylibrary -L/usr/lib64 -lstdc++', and got:


.../binutils/2.25/centos6-native/da39a3e/bin/ld: .../mylibrary/0.1/gcc-4.9-glibc-2.20-fb/80414d5/lib/libmylibrary.a(mylibrary_file.cpp.o): relocation R_X86_64_32 against `.bss' can not be used when making a shared object; recompile with -fPIC

.../mylibrary/0.1/gcc-4.9-glibc-2.20-fb/80414d5/lib/libmylibrary.a: error adding symbols: Bad value



Thanks.


Rich

________________________________
发件人: Libav-user <libav-user-bounces at ffmpeg.org> 代表 R n <flynewdream at hotmail.com>
发送时间: 2016年9月3日 3:54
收件人: This list is about using libavcodec, libavformat, libavutil, libavdevice and libavfilter.
主题: [Libav-user] 答复: Are there any ways to use a lib (built from C++ OpenCV) in Ffmpeg filter?


Thanks Perette.


I did exactly what you suggested for the C and C++ files. (I have two header files, one for C and another is for C++, and the C++ header file includes the C header file. I have one cpp file in which I put the C and C++ source code. I can also split them into two cpp files for clarity). I guess perhaps the problem is in the how I compile the library and how I compile FFmpeg when I call the library from FFmpeg.


Any suggestions?


Thanks.


Rich




________________________________
发件人: Libav-user <libav-user-bounces at ffmpeg.org> 代表 Perette Barella <perette at barella.org>
发送时间: 2016年9月3日 3:38
收件人: This list is about using libavcodec, libavformat, libavutil, libavdevice and libavfilter.
主题: Re: [Libav-user] Are there any ways to use a lib (built from C++ OpenCV) in Ffmpeg filter?

On 2016年09月02日, at 22:32, R n <flynewdream at hotmail.com<mailto:flynewdream at hotmail.com>> wrote:
I am writing a C++ library which is based on OpenCV. I want to use it in a Ffmpeg filter (written in C), so I write a wrapper of the library using C. The wrapper consists of a C header file and some C functions which call the C++ functions (in the C++ files). I expect the FFmpeg filter to only include the C header and call the C functions.
…
However, I get the following error when compiling FFmpeg:
.../binutils/2.25/centos6-native/da39a3e/bin/ld: .../mylibrary/0.1/gcc-4.9-glibc-2.20/80414d5/lib/libmylibrary.a(mylibrary_file.cpp.o): undefined reference to symbol '_ZdlPv@@GLIBCXX_3.4'
.../libgcc/4.9.x/gcc-4.9-glibc-2.20/024dbc3/lib/libstdc++.so: error adding symbols: DSO missing from command line

Your approach is correct, but C++ “mangles” function names to make them unique when considering parameters and such.  You need to specify C linkage, so in your .h file, wrap it in extern “C”:

#ifdef __cplusplus
#warning is c++
extern "C" {
#endif
char foo (int);

#ifdef __cplusplus
}
#endif

You need to #include the .h file in the corresponding .cpp file, so the C++ compiler knows not to mangle those particular function names when they’re compiled.

Perette


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