I am trying to use __thread in this small program without luck. Any idea if this TLS is supported in ndk 10c clang 3.4/3.5? The same program compiles fine with ndk gcc 4.8/4.9 and native clang/gcc compilers.
Here is the program and compile line -
__thread int counter;
int main () { counter=20; return 0; }
[armeabi] Compile++ thumb: test <= test.cpp
/Users/padlar/android/android-ndk-r10c/toolchains/llvm-3.5/prebuilt/darwin-x86/bin/clang++ -MMD -MP -MF ./obj/local/armeabi/objs/test/test.o.d -gcc-toolchain /Users/padlar/android/android-ndk-r10c/toolchains/arm-linux-androideabi-4.8/prebuilt/darwin-x86 -fpic -ffunction-sections -funwind-tables -fstack-protector -Wno-invalid-command-line-argument -Wno-unused-command-line-argument -no-canonical-prefixes -fno-integrated-as -target armv5te-none-linux-androideabi -march=armv5te -mtune=xscale -msoft-float -fno-exceptions -fno-rtti -mthumb -Os -g -DNDEBUG -fomit-frame-pointer -fno-strict-aliasing -I/Users/padlar/android/android-ndk-r10c/sources/cxx-stl/stlport/stlport -I/Users/padlar/android/android-ndk-r10c/sources/cxx-stl//gabi++/include -Ijni -DANDROID -Wa,--noexecstack -Wformat -Werror=format-security -fPIE -frtti -I/Users/padlar/android/android-ndk-r10c/platforms/android-19/arch-arm/usr/include -c jni/test.cpp -o ./obj/local/armeabi/objs/test/test.o
[armeabi] Executable : test
/Users/padlar/android/android-ndk-r10c/toolchains/llvm-3.5/prebuilt/darwin-x86/bin/clang++ -Wl,--gc-sections -Wl,-z,nocopyreloc --sysroot=/Users/padlar/android/android-ndk-r10c/platforms/android-19/arch-arm -Wl,-rpath-link=/Users/padlar/android/android-ndk-r10c/platforms/android-19/arch-arm/usr/lib -Wl,-rpath-link=./obj/local/armeabi ./obj/local/armeabi/objs/test/test.o /Users/padlar/android/android-ndk-r10c/sources/cxx-stl/stlport/libs/armeabi/thumb/libstlport_static.a -lgcc -gcc-toolchain /Users/padlar/android/android-ndk-r10c/toolchains/arm-linux-androideabi-4.8/prebuilt/darwin-x86 -no-canonical-prefixes -target armv5te-none-linux-androideabi -Wl,--no-undefined -Wl,-z,noexecstack -Wl,-z,relro -Wl,-z,now -fPIE -pie -L/Users/padlar/android/android-ndk-r10c/platforms/android-19/arch-arm/usr/lib -llog -lc -lm -o ./obj/local/armeabi/test
jni/test.cpp:2: error: undefined reference to '__aeabi_read_tp'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [obj/local/armeabi/test] Error 1
ndk-which objdump -S obj/local/armeabi/objs/test/test.o
obj/local/armeabi/objs/test/test.o: file format elf32-littlearm
Disassembly of section .text.main:
00000000 <main>:
__thread int counter;
int main () { counter=20; return 0; }
0: b580 push {r7, lr}
2: 4904 ldr r1, [pc, #16] ; (14 <main+0x14>)
4: f7ff fffe bl 0 <__aeabi_read_tp>
8: 1840 adds r0, r0, r1
a: 2114 movs r1, #20
c: 6001 str r1, [r0, #0]
e: 2000 movs r0, #0
10: bd80 pop {r7, pc}
12: 46c0 nop ; (mov r8, r8)
14: 00000000 .word 0x00000000
gcc4.6 disass of the same program
ndk-which objdump -S obj/local/armeabi/objs/test/test.o
obj/local/armeabi/objs/test/test.o: file format elf32-littlearm
Disassembly of section .text.startup.main:
00000000 <main>:
__thread int counter;
int main () { counter=20; return 0; }
0: b508 push {r3, lr}
2: 4804 ldr r0, [pc, #16] ; (14 <main+0x14>)
4: 4478 add r0, pc
6: f7ff fffe bl 0 <__emutls_get_address>
a: 2314 movs r3, #20
c: 6003 str r3, [r0, #0]
e: 2000 movs r0, #0
10: bd08 pop {r3, pc}
12: 46c0 nop ; (mov r8, r8)
14: 0000000c .word 0x0000000c
__thread style TLS (similarly C++11 style thread_local) is not currently supported by the Android linker (we're working on it though). For now you'll have to use pthread_getspecific and pthread_setspecific.
EDIT: Starting with NDK r12, Clang has support for emulated TLS. While the above is still technically true (the linker on the device does not support this), the compiler can replace builtin TLS support with pthread calls on your behalf.
NB: Android still only supports thread_local for objects with trivial destructors until r14. See discussions on https://github.com/android-ndk/ndk/issues/156#issuecomment-231878690 and https://github.com/android-ndk/ndk/issues/216.