gcc: undefined reference to

Question

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?

Teddy

Answer 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

---

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 additional 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.

Gavelkind

And probably something like -L$HOME/ffmpeg/lib in this case, since ffmpeg is in the user's home directory rather than installed to system directories. – Malady

Hi alk. Running 'gcc -I$HOME/ffmpeg/include program.c -lavcodes' gave me '/usr/bin/ld: cannot find -lavcodes collect2: ld returned 1 exit status' – Teddy

Hi Wyzard. Running 'gcc -I$HOME/ffmpeg/include program.c -L$HOME/ffmpeg/lib' gave me the exact same error 'program.c:(.text+0x18): undefined reference to `avpicture_get_size' ' – Teddy

Thanks! Running 'gcc -I$HOME/ffmpeg/include program.c -L$HOME/ffmpeg/lib -lavcodec' gives 6 undefined reference errors. Some of the methods are from libavcodec and the others are from libavutil. So I added -lavutil at the end of the command and it compiled. – Teddy

This paragraph "Please also note that libraries need to be specified on the command line after the files needing them" saved my day, thanks. – Ashlaring

Shouldn't GCC be able to find libraries automatically by itself? Do we need to specify lib names when invoking make as well? – Margitmargo

For me, the hint about the order of the parameters was what solved my problem. Thanks a million! – Monserratemonsieur

Answer 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

Note: if unable / unwilling to modify the .h file(s) in question, you can surround their inclusion with extern "C":

extern "C" {
#include <abc.h>
} //extern

Endorse

Critical step, this. Although it didn't appear to be the original poster's problem, this must be considered in general. – Garin

@LimitedAtonement, I put this in because, while the OP provided a file with a .c extension, the "C++" tag has been set. – Endorse