CMake finds fftw library but cannot link - linker

Question

I have problem linking the fftw library using cmake. I use a findFFTW.cmake file in order to find the library. I know this is successfully finding the library because I set the REQUIRED flag to be true for finding the library and the make process goes through fine.
Despite linking it with my executable, I am still getting undefined reference errors. Some related posts, whose solutions I have tried.
Undefined reference to "function name from external library"
http://answers.ros.org/question/171326/catkin-linking-order-undefined-reference-to-symbol/
Updates
Thanks to ComicSansMS, the CMake below should now model the dependencies correctly.
CMake file of project (updated 3/7)
cmake_minimum_required(VERSION 2.8.3)
project(gist_extractor)
## Find catkin macros and libraries
find_package(catkin REQUIRED COMPONENTS
roscpp
rospy
std_msgs
image_transport
cv_bridge
sensor_msgs
cmake_modules
)
find_package(OpenCV REQUIRED)
find_package(Eigen REQUIRED)
find_package(FFTW REQUIRED)
###########
## Build ##
###########
## Set GIST variables for building library
set(GIST_PATH /home/andy/Development/lear_gist-1.2)
## Specify additional locations of header files
include_directories(include ${catkin_INCLUDE_DIRS} ${GIST_PATH} ${FFTW_INCLUDES})
## Declare a gist library
add_library(gist SHARED ${GIST_PATH}/standalone_image.c ${GIST_PATH}/gist.c) # THIS IS NOT BEING BUILT
target_link_libraries(gist ${FFTW_LIBRARIES})
## Add cmake target dependencies of the library
#MESSAGE( STATUS "GIST_LIBRARY_PATH: " ${GIST_PATH})
## Declare a C++ executable
add_executable(gist_extractor src/gist_extractor.cpp)
target_link_libraries(gist_extractor ${catkin_LIBRARIES} gist)
EDITS 2
We now have a linking error if we use the above CMake file. Specifically the make process fails when I try running
target_link_libraries(gist_extractor ${catkin_LIBRARIES} gist)
I have a couple of observations. Firstly, my gist library is being built correctly based on the below console messages.
Linking C shared library /home/andy/Projects/ROS/robot_ws/devel/lib/libgist.so
[ 80%] Built target gist
Scanning dependencies of target gist_extractor
[100%] Building CXX object
`gist_extractor/CMakeFiles/gist_extractor.dir/src/gist_extractor.cpp.o
But we can see that there are undefined reference errors when we try to link our executable with the gist library.
Linking CXX executable gist_extractor
: undefined reference to `color_gist_scaletab'
Here's why I don't understand why this is occurring. In gist_extractor.cpp, I have included the header file that includes the 'color_gist_scaletab' function. Specifically, this 'color_gist_scaletab is in defined in 'gist.h' and implemented in 'gist.c'. I would think that building my library gist should give me access to 'color_gist_scaletab'. I have posted the relevant files below.
gist.h
#ifndef GIST_H_INCLUDED
#define GIST_H_INCLUDED
#include "standalone_image.h"
/*! Graylevel GIST for various scales. Based on Torralba's Matlab
* implementation. http://people.csail.mit.edu/torralba/code/spatialenvelope/
*
* Descriptor size is w*w*sum(n_orientations[i],i=0..n_scale-1)
*
* #param src Source image
* #param w Number of bins in x and y axis
*/
float *bw_gist_scaletab(image_t *src, int nblocks, int n_scale, const int *n_orientations);
/*! #brief implementation of grayscale GIST descriptor.
* Descriptor size is w*w*(a+b+c)
*
* #param src Source image
* #param w Number of bins in x and y axis
*/
float *bw_gist(image_t *scr, int nblocks, int a, int b, int c);
/*! #brief implementation of color GIST descriptor.
*
* #param src Source image
* #param w Number of bins in x and y axis
*/
float *color_gist(color_image_t *src, int nblocks, int a, int b, int c);
/*! Color GIST for various scales. Based on Torralba's Matlab
* implementation. http://people.csail.mit.edu/torralba/code/spatialenvelope/ */
float *color_gist_scaletab(color_image_t *src, int nblocks, int n_scale, const int *n_orientations);
#endif
gist_extractor.cpp
// color_gist_scaletab is defined in gist.h
// I'm including relevant header file
#include "/home/andy/Development/lear_gist-1.2/gist.h"
//SOME MORE STUFF
// This is where I call the function
float *gist_descriptor = color_gist_scaletab(im, nblocks, n_scale, orientations_per_scale);

Answer 1

It seems you did not model your dependencies correctly.
You use fftw from inside gist.c, which is part of the gist library target.
However, that target does not have a dependency to fftw, only the downstream target gist_extractor has. This is a recipe for trouble, especially in static builds, where toolchains are often picky about the order in which libraries appear on the linker command line.
Add fftw as a dependency to gist:
target_link_libraries(gist ${FFTW_LIBRARIES})
Also, there is no need to do this:
add_dependencies(gist_extractor gist)
target_link_libraries(gist_extractor ${GIST_PATH}/libleargist.a)
If both targets are built as part of the same project, simply do:
target_link_libraries(gist_extractor gist)
By using the target name instead of hardcoding the output file, you not only make your build script more portable, but also allow CMake to better track the inter-target dependencies for you.