Question

I am porting the ‘iw’ command to Android. This is wireless related. It appears that the iw tool has a set of commands that can be used, however from another user’s site:

A glance at the iw source revealed that iw sticks all that stuff into
an ELF section which mostly disappears when you link with
–gc-sections.

This person was porting ‘iw’ to Android also, but decided to do cross-compilation through make instead of building an Android.mk and building through android-ndk. He mentions that whenever --gc-sections is removed:

So with that exorcised from my linker command line, I finally
have a functioning Android iw

I am finding this to be true. Whenever I build ‘iw’ by constructing an Android.mk, and then running ‘iw’ on my Android device, all of these commands disappear.

Here is my Android.mk:

LOCAL_PATH:= $(call my-dir)

include $(CLEAR_VARS)

LOCAL_SRC_FILES:= \
  iw.c \
  genl.c \
  event.c \
  info.c \
  phy.c \
  interface.c \
  ibss.c \
  station.c \
  survey.c \
  util.c \
  mesh.c \
  mpath.c \
  scan.c \
  reg.c \
  version.c \
  reason.c \
  status.c \
  connect.c \
  link.c \
  offch.c \
  ps.c \
  cqm.c \
  bitrate.c \
  wowlan.c \
  roc.c \
  sections.c



LOCAL_C_INCLUDES += $(LOCAL_PATH)/../lowpan/include \
  $(LOCAL_PATH)/../libnl/include
  

LOCAL_CFLAGS +=  -g
LOCAL_CFLAGS += -fPIC -DPIC

LOCAL_STATIC_LIBRARIES := libnls libnl

ifeq ($(TARGET_BUILD_TYPE),release)
  LOCAL_CFLAGS += -g
endif

LOCAL_MODULE:= iw

include $(BUILD_EXECUTABLE)

If it is true that the android-ndk is somehow using --gc-sections and removing these commands from an ELF section, I cannot figure out how to stop it from doing this. Does anyone have any suggestions here?

---

Edit: It does look like the NDK is using this:

build/core/default-build-commands.mk:    -Wl,--gc-sections \
build/tools/toolchain-patches/build/0001-Options-brought-in-from-core-combo-for-IA.patch:+  -fexceptions -ffunction-sections -finline-functions \
build/tools/toolchain-patches/build/0001-Options-brought-in-from-core-combo-for-IA.patch:+  -Wl,-z,noexecstack -Wl,--gc-sections -nostdlib \
build/tools/toolchain-patches/build/0001-Options-brought-in-from-core-combo-for-IA.patch:+  -fexceptions -frtti -fstrict-aliasing -ffunction-sections -finline-functions  \
build/tools/toolchain-patches/build/0001-Options-brought-in-from-core-combo-for-IA.patch:+  -ffunction-sections -funwind-tables -fmessage-length=0 \
toolchains/arm-linux-androideabi-4.4.3/setup.mk:    -ffunction-sections \
toolchains/arm-linux-androideabi-4.4.3/setup.mk:    -Wl,--gc-sections \
toolchains/x86-4.4.3/setup.mk:    -ffunction-sections \
toolchains/x86-4.4.3/setup.mk:    -Wl,--gc-sections \

---

EDIT: I removed all instances of --gc-sections and -ffunction-sections from the android-ndk, and after rebuilding it works! However, I’d prefer not to modify the android-ndk, so there must be a modification to my local Android.mk to remove these two flags.

---

EDIT: I removed only --gc-sections from the above default android-ndk files, and that solved the problem. So, -ffunction-sections is not related. I just need to figure out how to disable --gc-sections in a local Android.mk

Answer 1

You can add line

LOCAL_LDLIBS += -Wl,--no-gc-sections

to your Android.mk file. But as result you will have both -gc-sections and --no-gc-sections specified at the same time because ndk-build always adds -gc-sections options for executables.

If this does not work then you have only way to hack setup.mk from your toolchain. Or you can build without ndk-build. As alternative variant I can suggest to use cmake for building Android projects. See android-cmake project for details