gcc: undefined reference to

问题

I would like to compile this.

program.c

#include <libavcodec/avcodec.h>

int main(){
    int i = avpicture_get_size(AV_PIX_FMT_RGB24,300,300);
}

Running this

gcc -I$HOME/ffmpeg/include program.c

gives error

/tmp/ccxMLBme.o: In function `main':
program.c:(.text+0x18): undefined reference to `avpicture_get_size'
collect2: ld returned 1 exit status

However, avpicture_get_size is defined. Why is this happening?

回答1:

However, avpicture_get_size is defined.

No, as the header (<libavcodec/avcodec.h>) just declares it.

The definition is in the library itself.

So you might like to add the linker option to link libavcodec when invoking gcc:

-lavcodec

Please also note that libraries need to be specified on the command line after the files needing them:

gcc -I$HOME/ffmpeg/include program.c -lavcodec

Not like this:

gcc -lavcodec -I$HOME/ffmpeg/include program.c

Update:

Referring to Wyzard's comment the complete command might look like this:

gcc -I$HOME/ffmpeg/include program.c -L$HOME/ffmpeg/lib -lavcodec

For libraries not stored in the linkers standard location the option -L specifies an addtional search path to lookup libraries specified using the -l option, that is libavcodec.x.y.z in this case.

For a detailed reference on gcc's linker option, please read here.

回答2:

Are you mixing C and C++? One issue that can occur is that the declarations in the .h file for a .c file need to be surrounded by:

#if defined(__cplusplus)
  extern "C" {                 // Make sure we have C-declarations in C++ programs
#endif

and:

#if defined(__cplusplus)
  }
#endif

来源：https://stackoverflow.com/questions/22426574/gcc-undefined-reference-to

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