在Eclipse中使用gcc -shared找不到库来构建dll

Question

I am trying to use the FFMPEG library that I just compiled on Windows using MinGW in a C program in Eclipse. I have managed to build a simple HelloJNI.c file and run it:

gcc $(INCLUDES) -c -g -w HelloJNI.c
gcc -shared -o $(BIN_DIR)/hello.dll HelloJNI.o

I am now trying to compile a ffmpeg_native.c file that uses FFMPEG but I am getting errors running the following:

LIBRARY_PATH = -L:"c:/Dev/msys-1.0/local/lib" -L:"c:/Dev/msys-1.0/local"
INCLUDES = -I$(SRC_DIR) -I"c:/Dev/msys-1.0/local/include" -I"c:/Program Files/Java/jdk1.8.0_45/include" -I"c:/Program Files/Java/jdk1.8.0_45//include/win32"
BIN_DIR = ../bin

gcc $(INCLUDES) -c -g -w ffmpeg_native1.1.4.c
gcc -shared -o $(BIN_DIR)/exportnative.dll ffmpeg_native1.1.4.o $(LIBRARY_PATH) -lffmpeg -lavcodec -lx264 -lavformat -lavutil -lswscale

The first line runs fine but the second shows this output:

c:/Dev/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.9.3/../../../../x86_64-w64-mingw32/bin/ld.exe: cannot find -lavcodec
[etc for all libraries]

I am puzzled as I am able to run the following in MinGW with success:

ld -o /local/libffmpeg.so -L/local/lib -lavcodec -lx264 -lavformat -lavutil -lswscale

Answer 1

It cannot find library path since -L is missing before $(LIBRARY_PATH) . From gcc man:

-L dir
Add directory dir to the list of directories to be searched for -l .

So, if LIBRARY_PATH contains directory with needed libraries and everyting other is fine the following should work:

gcc -shared -o $(BIN_DIR)/exportnative.dll ffmpeg_native1.1.4.o -L$(LIBRARY_PATH) -lffmpeg -lavcodec -lx264 -lavformat -lavutil -lswscale

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It is not needed to put colon : between -L and the path. The following list should be added to the command line:

-L"c:/Dev/msys-1.0/local/lib" -L"c:/Dev/msys-1.0/local"`

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Note that it is better to use some other environment variable name for building the command line, since LIBRARY_PATH is used by gcc directly:

LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH . When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX . Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first).