交叉编译中的 --sysroot 等等在编译时的作用

--sysroot=dir 的作用

如果在编译时指定了-sysroot=dir 就是为编译时指定了逻辑目录。编译过程中需要引用的库，头文件，如果要到/usr/include目录下去找的情况下，则会在前面加上逻辑目录。

如此处我们指定 -sysroot=/home/shell.albert/tools/toolschain_arm/4.4.3/arm-none-linux-gnueabi/sys-root
将dir作为逻辑根目录(搜索头文件和库文件)。比如编译器通常会在 /usr/include 和 /usr/lib 中搜索头文件和库，使用这个选项后将在 dir/usr/include 和 dir/usr/lib 目录中搜索。如果使用这个选项的同时又使用了 -isysroot 选项，则此选项仅作用于库文件的搜索路径，而 -isysroot 选项将作用于头文件的搜索路径。这个选项与优化无关，但是在 CLFS 中有着神奇的作用。

LD时候的-rpath,-rpath-link 的意思:

-rpath=dir

Add a directory to the runtime library search path. This is used when linking an ELF executable with shared objects. All -rpath arguments are concatenated and passed to the runtime linker, which uses them to locate shared objects at runtime. The -rpath option is also used when locating shared objects which are needed by shared objects explicitly included in the link; see the description of the -rpath-link option. If -rpath is not used when linking an ELF executable, the contents of the environment variable LD_RUN_PATH will be used if it is defined.

-rpath-link=dir

When
using ELF or SunOS, one shared library may require another. This
happens when an ld -shared link includes a shared library as one of the
input files.

When
the linker encounters such a dependency when doing a non-shared,
non-relocatable link, it will automatically try to locate the required
shared library and include it in the link, if it is not included
explicitly. In such a case, the -rpath-link option specifies the first
set of directories to search. The -rpath-link option may specify a
sequence of directory names either by specifying a list of names
separated by colons, or by appearing multiple times.

This
option should be used with caution as it overrides the search path that
may have been hard compiled into a shared library. In such a case it is
possible to use unintentionally a different search path than the
runtime linker would do.

The
difference between -rpath and -rpath-link is that directories specified
by -rpath options are included in the executable and used at runtime,
whereas the -rpath-link option is only effective at link time. Searching
-rpath in this way is only supported by native linkers and cross
linkers which have been configured with the --with-sysroot option.

-L: “链接”的时候，去找的目录，也就是所有的 -lFOO 选项里的库，都会先从 -L 指定的目录去找，然后是默认的地方。
-rpath: “运行”的时候，去找的目录。运行的时候，要找 .so 文件，会从这个选项里指定的地方去找。对于交叉编译，只有配合 --sysroot 选项才能起作用。
-rpath_link
（或者 -rpath-link）：这个也是用于“链接”的时候的，例如你显示指定的需要 FOO.so，但是 FOO.so 本身是需要
BAR.so 的，后者你并没有指定，而是 FOO.so 引用到它，这个时候，会先从 -rpath-link 给的路径里找。

也就是说，-rpath指定的路径会被记录在生成的可执行程序中，用于运行时。
-rpath-link 则只用于链接时。

rpath-link的格式：

./configure --cross-prefix=/home/zhou/android/android_adt/ndk/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/arm-linux-androideabi- --sysroot=/home/zhou/android/android_adt/ndk/platforms/android-9/arch-arm --disable-shared --disable-symver --disable-everything --enable-gpl --enable-runtime-cpudetect --enable-decoder=h263 --enable-encoder=h263 --enable-encoder=libx264 --enable-parser=h264 --enable-libx264 --enable-decoder=h264 --prefix=/home/hzh/temp/vpu/build/ffmpeg/ --extra-cflags= -DANDROID -D__thumb__ -mthumb -I../build/x264//include --extra-ldflags= -L../build/x264//lib -Wl,-rpath-link,../build/x264//lib --extra-cxxflags=-Wno-multichar -fno-exceptions -fno-rtti

如果有多个rpath, 则：

-Wl,-rpath=/home/oracle/db_1/lib -Wl,-rpath=/home/app/oracle
也可以简写，注意是”:”，和PATH的路径分隔符一样，还要注意中英文字符：
-Wl,-rpath=/home/oracle/db_1/lib:/home/app/oracle

另外，如果有多个rpath，路径最好用绝对路径，不然有些时候configure 那一步过不了， 如ffmpeg。

bash 脚本里的到绝对路径的方法：

# bash脚本里 = 前后不能有空格，等号前后不能有空格，赋值
VPU_WRAPPER=../../vpu-helper/vpu_wrapper
pushd $VPU_WRAPPER
VPU_WRAPPER_DIR=`pwd`
popd

EXTRA_CFLAGS="$EXTRA_CFLAGS -I$VPU_WRAPPER_DIR"
EXTRA_LDFLAGS="$EXTRA_LDFLAGS -L$VPU_WRAPPER_DIR/lib"
echo $EXTRA_LDFLAGS
# pushd 和 popd 会改变当前文件专属的目录路径，只对当前文件有效，退出当前文件的执行后立即失效

makefile里

CROSS_DIR    = ../arm-2012.09/bin
CROSS_PREFIX = arm-none-linux-gnueabi-
CC    = $(shell cd $(CROSS_DIR) && pwd)/$(CROSS_PREFIX)gcc
AR    = $(shell cd $(CROSS_DIR) && pwd)/$(CROSS_PREFIX)ar
STRIP = $(shell cd $(CROSS_DIR) && pwd)/$(CROSS_PREFIX)strip
RANLIB= $(shell cd $(CROSS_DIR) && pwd)/$(CROSS_PREFIX)ranlib
@echo "CC is: '$(CC)'"
$(warning "CC is: $(CC)")

# $(shell cd $(CROSS_DIR))，改变目录只针对当前语句有效，退出当前语句后目录改变结果立即失效，即这一句不影响文件里的目录路径，只影响自己这一句。