-LdirAdd directory dir to the list of directories to be searched for -l. -llibrary
-l librarySearch the library named library when linking. (The secondalternative with the library as a separate argument is only for POSIXcompliance and is not recommended.)It makes a difference where in the command you write this option;the linker searches and processes libraries and object files inthe order they are specified. Thus, `foo.o -lz bar.o' searcheslibrary `z' after file foo.o but before bar.o. If bar.o refers tofunctions in `z', those functions may not be loaded.The linker searches a standard list of directories for thelibrary, which is actually a file named liblibrary.a. The linkerthen uses this file as if it had been specified precisely by name.The directories searched include several standard systemdirectories plus any that you specify with -L.Normally the files found this way are library files—archive fileswhose members are object files. The linker handles an archive fileby scanning through it for members which define symbols that haveso far been referenced but not defined. But if the file that isfound is an ordinary object file, it is linked in the usualfashion. The only difference between using an -l option andspecifying a file name is that -l surrounds library with `lib' and`.a' and searches several directories.    以上来源于gcc手册

--as-needed
--no-as-neededThis option affects ELF DT_NEEDED tags for dynamic libraries mentioned on the command line after the --as-neededoption.  Normally the linker will add a DT_NEEDED tag for each dynamic library mentioned on the command line,regardless of whether the library is actually needed or not.  --as-needed causes a DT_NEEDED tag to only be emitted fora library that satisfies an undefined symbol reference from a regular object file or, if the library is not found inthe DT_NEEDED lists of other libraries linked up to that point, an undefined symbol reference from another dynamiclibrary.  --no-as-needed restores the default behaviour.--add-needed
--no-add-neededThese two options have been deprecated because of the similarity of their names to the --as-needed and --no-as-neededoptions.  They have been replaced by --copy-dt-needed-entries and --no-copy-dt-needed-entries.--copy-dt-needed-entries
--no-copy-dt-needed-entriesThis option affects the treatment of dynamic libraries referred to by DT_NEEDED tags inside ELF dynamic librariesmentioned on the command line.  Normally the linker won't add a DT_NEEDED tag to the output binary for each librarymentioned in a DT_NEEDED tag in an input dynamic library.  With --copy-dt-needed-entries specified on the command linehowever any dynamic libraries that follow it will have their DT_NEEDED entries added.  The default behaviour can berestored with --no-copy-dt-needed-entries.This option also has an effect on the resolution of symbols in dynamic libraries.  With --copy-dt-needed-entriesdynamic libraries mentioned on the command line will be recursively searched, following their DT_NEEDED tags to otherlibraries, in order to resolve symbols required by the output binary.  With the default setting however the searchingof dynamic libraries that follow it will stop with the dynamic library itself.  No DT_NEEDED links will be traversed toresolve symbols.--以上来源于man手册

--add-needed                Set DT_NEEDED tags for DT_NEEDED entries in   following dynamic libs
--no-add-needed             Do not set DT_NEEDED tags for DT_NEEDED entries   in following dynamic libs
--as-needed                 Only set DT_NEEDED for following dynamic libs if used
--no-as-needed              Always set DT_NEEDED for following dynamic libs

g++ -shared a.o -ltest1 -lxxx -lrt -o libtest2.so

g++ -Wl,--as-needed -ltest1 -lc -lm -ldl -lpthread -L/home/ocaml/lib/ -lrt -o app main.o

g++ -Wl,--as-needed main.o -ltest1 -lc -lm -ldl -lpthread   -L/home/ocaml/lib/  -lrt -o app

g++ -shared -Wl,--as-needed -lGalaxyParser -lc -lm -ldl -lpthread -L/home/ocaml/lib/ -lrt -o PyGalaxy.so PyGalaxy.o

g++ -shared -Wl,--as-needed PyGalaxy.o -lGalaxyParser -lc -lm -ldl -lpthread -L/home/ocaml/lib/ -lrt -o PyGalaxy.so