gcc: undefined reference to
Solution 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.
Solution 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