如何在Linux中链接和编译.so文件

Question

I am having .c and .so file. I tried by using the following compilation: gcc main.c -ldl . In that .c file i linked to .so file through dlsym() . How to compile using .so file with .c.

Answer 1

Probably you can do this:

when linking do:

g++ -o prog prog.o -ldllname

If libdllname.so is not in the system directory then add its directory to the library path:

g++ -o prog prog.o -L/path/to/my/library/folder -ldllname

Answer 2

This is based on your further comments. First guard the declarations of your header file.

#ifndef HEADER_PROTECT
#define HEADER_PROTECT

---------- Here is the content of header

#endif

Next, check in your code, are you defining multiple definitions. Or are you re-defining the standard functions again? Can you please post your code to guide you better? Looks like you have re-defined Close_Comm(), can you check it? Error says that the definition is there in main.c also.

The following is the general way to compile shared object and link it. To compile shared objects.

-g : for debug information
fPIC: for position independent code
$gcc -fPIC -g myfile

The following will create the shared  object libmyfile.so

$gcc -shared -o libymyfile.so myfile.o

Now,In order to link it with your main.c.
I assume that the libmyfile.so is in your current path, thus -L./

$gcc main.c -o main.out -L./ -lmyfile

Now, you need to export the LD_LIBRARY_PATH on the bash; in order to execute the binary.

$LD_LIBRARAY_PATH=$LD_LIBRARAY_PATH:./
$./main.out

The dlsym is to load the symbol from the shared object at the run-time. If you want to load the shared object at run time, this can be used. The following is one of the example of dlsym Hack the standard function in library and call the native library function afterwards

Answer 3

dlsym() is used to find a symbol in an open library file. you first need to use dlopen() in order to open the file, and only then use dlsym()