链接和编译后C执行错误

Question

I'm getting some strange results trying to link to an external library. If I run the following from the command line:

gcc fftwTest.c -I../extlib/fftw-3.3.5-dll32 -L../extlib/fftw-3.3.5-dll32 -lfftw3-3 -lm -o test

test.exe compiles, but when I run it I get an error message saying the program can't start because libfftw3-3.dll is missing from my computer. Despite the fact that the linker found it when compiling.

If, instead, I place libfftw3-3.dll into the same directory as my fftwTest.c file and do -L to there, it compiles and runs OK. So I guess this suggests I'm doing something wrong with my -L command, but as it linked and compiled I don't know quite what.

Any help would be appreciated.

Thanks!

Answer 1

Nothing is wrong with your link command, and yes, the linker found it while compiling because you specified the path to it, but that's a different story when running the program (the linker doesn't /cannot hardcode the path for the DLL, it could be located somewhere else on the deployment machine(s)).

The DLL must be in the system path or in the same directory as the executable. In your case you could:

• copy the DLLs in the .exe directory (you found that yourself)
• or add: set PATH=%PATH%;../extlib before starting your executable in a batch file (or set the full absolute path in the system path using Windows environment variable management)

Copying the DLLs in the executable directory is better when you distribute the package to a third party so no configuration is needed, however, if you use/upgrade the DLL many times and on a lot of small programs, the best solution on your development machine is to keep it in one place and set the system PATH to run it (that's the tradeoff)