先看看與動態庫連結相關的幾個選項的man說明：

--as-needed
--no-as-needed
    This option affects ELF DT_NEEDED tags for dynamic libraries mentioned on the command line after the --as-needed
    option.  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 for
    a library that satisfies an undefined symbol reference from a regular object file or, if the library is not found in
    the DT_NEEDED lists of other libraries linked up to that point, an undefined symbol reference from another dynamic
    library.  --no-as-needed restores the default behaviour.

--add-needed
--no-add-needed
    These two options have been deprecated because of the similarity of their names to the --as-needed and --no-as-needed
    options.  They have been replaced by --copy-dt-needed-entries and --no-copy-dt-needed-entries.

--copy-dt-needed-entries
--no-copy-dt-needed-entries
    This option affects the treatment of dynamic libraries referred to by DT_NEEDED tags inside ELF dynamic libraries
    mentioned on the command line.  Normally the linker won't add a DT_NEEDED tag to the output binary for each library
    mentioned in a DT_NEEDED tag in an input dynamic library.  With --copy-dt-needed-entries specified on the command line
    however any dynamic libraries that follow it will have their DT_NEEDED entries added.  The default behaviour can be
    restored with --no-copy-dt-needed-entries.

    This option also has an effect on the resolution of symbols in dynamic libraries.  With --copy-dt-needed-entries
    dynamic libraries mentioned on the command line will be recursively searched, following their DT_NEEDED tags to other
    libraries, in order to resolve symbols required by the output binary.  With the default setting however the searching
    of dynamic libraries that follow it will stop with the dynamic library itself.  No DT_NEEDED links will be traversed to
    resolve symbols.

再看看ld的幫助

--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

關鍵的看–as-needed，意思是說：只給用到的動態庫設定DT_NEEDED。比如：

g++ -shared  PyGalaxy.o -lGalaxyParser -lxxx  -lrt  -o PyGalaxy.so

像這樣連結一個PyGalaxy.so的時候，假設PyGalaxy.so裡面用到了libGalaxyParser.so但是沒 有用到libxxx.so。檢視依賴關係如下：

[email protected]:~$ readelf -d PyGalaxy.so 

Dynamic section at offset 0x7dd8 contains 26 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libGalaxyParser.so]
 0x0000000000000001 (NEEDED)             Shared library: [libxxx.so]
 ...

當開啟–as-needed的時候，像

g++ -shared  -Wl,--as-needed PyGalaxy.o -lGalaxyParser -lxxx  -lrt  -o PyGalaxy.so

這樣連結PyGalaxy.so的時候，檢視依賴關係如下：

[email protected]:~$ readelf -d PyGalaxy.so 

Dynamic section at offset 0x7dd8 contains 26 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libGalaxyParser.so]
 ...

–as-needed就是忽略連結時沒有用到的動態庫，只將用到的動態庫set NEEDED。

開啟–as-needed的一些常見的問題：