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?

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.

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