gcc: Linking an External Library

Question

In program.c, I would like to use a method 'avcodec_register_all()' defined in libavcodec/avcodec.h.

Running

gcc program.c -L$HOME/ffmpeg/lib/libavfilter.a -L$HOME/ffmpeg/lib/libavcodec.a

Gives me an error

/tmp/ccNeQywU.o: In function `main':
program.c:(.text+0x3f): undefined reference to `avcodec_register_all'
collect2: ld returned 1 exit status

Spelling is correct and the function is defined. Why is this happening?

program.c

#include <stdio.h>

int main (int args, char *argv[])
{
 avcodec_register_all();
}

Answer 1

It is like this

gcc -static -lavfilter -lavcodec -L/HOME/ffmpeg/lib/

You need to include the file avcodec.h and also add the path to that file in your include path flags to gcc.

gcc -static -lavfilter -lavcodec -L$HOME/ffmpeg/lib/ -I$HOME/ffmpeg/include

The static is given because your are trying to use a *.a library and not *.so (dynamic).

Answer 2

使用-lavcodec和现有参数来gcc

Answer 3

One of the reasons may be that the function avcodec_register_all() is defined in a C++ file. It should be defined as extern "C" in avcodec.h and the library should be compiled with this proper avcodec.h .