5 分钟
Linux 动态链接库详解（三）动态库查找
本篇将介绍在构建时（链接阶段）以及运行时的动态库的查找过程。
构建时查找
在构建过程中，动态库的查找发生在链接阶段，即由 gcc 或 clang 等命令间接调用 ld 命令进行链接时，搜索动态链接库。
使用 gcc 或 clang 构建依赖动态链接库的代码时，可以通过如下，如下方式设置额外的搜索路径：
LIBRARY_PATH环境变量（注意不是LD_LIBRARY_PATH，LD_LIBRARY_PATH不识别），多个以:分割。-L命令行参数，可以配置多，如gcc ... -L /path/to/dir-a -L /path/to/dir-b。
除了以上自定义的搜索路径外，还会搜索 linker script 配置的默认搜索路径，以 debian 11 为例，通过 ld --verbose | grep SEARCH_DIR | tr -s ' ;' \\012 可以查看，其值为：
/usr/local/lib/x86_64-linux-gnu/lib/x86_64-linux-gnu/usr/lib/x86_64-linux-gnu/usr/local/lib64/lib64/usr/lib64/usr/local/lib/lib/usr/lib/usr/x86_64-linux-gnu/lib64/usr/x86_64-linux-gnu/lib
注意 /etc/ld.so.conf 配置的路径不会在构建（链接阶段）搜索（该配置文件影响运行阶段的搜索，详见下文）。
如上配置的搜索优先级顺序为：
- 命令行
-L参数指定的路径（包含用户自定义的以及 gcc 添加的）。 - 环境变量
LIBRARY_PATH指定的路径。 - 链接器 linker script 配置的默认搜索路径。
最后，在编译真实的项目时，一般使用 Makefile 来定义编译过程，Makefile 识别如下环境变量，可以用来配置自定义搜索路径：
验证如下：01-sample/03-build-stage-search.sh
#!/usr/bin/env bash
cd $(dirname $(readlink -f $0))
rm -rf build/bin && mkdir -p build/bin
echo '=== 方式 1: 找不到库情况'
gcc -I ./build/include -o ./build/bin/main ./main.c && echo '构建成功' || echo '构建失败'
echo
echo '=== 方式 2: 使用 LD_LIBRARY_PATH 指定查找路径'
LD_LIBRARY_PATH=./build/lib gcc -I ./build/include -o ./build/bin/main ./main.c -l sample && echo '构建成功' || echo '构建失败'
echo
echo '=== 方式 3: 使用 -L 指定查找路径'
gcc -I ./build/include -o ./build/bin/main ./main.c -L ./build/lib -l sample && echo '构建成功' || echo '构建失败'
echo
echo '=== 方式 4: 使用 LIBRARY_PATH 指定查找路径'
LIBRARY_PATH=./build/lib gcc -I ./build/include -o ./build/bin/main ./main.c -l sample && echo '构建成功' || echo '构建失败'
echo
echo '=== 方式 5: 使用 -L LIBRARY_PATH 指定错误的路径观察查找路径'
mkdir -p /tmp/by_LIBRARY_PATH /tmp/by_-l
LIBRARY_PATH=/tmp/by_LIBRARY_PATH gcc -I ./build/include -o ./build/bin/main ./main.c -L /tmp/by_-l -l sample -Wl,-verbose && echo '构建成功' || echo '构建失败'
echo输出如下：
bash ./01-sample/03-build-stage-search.sh
=== 方式 1: 找不到库情况
/usr/bin/ld: /tmp/ccLFcL27.o: in function `main':
main.c:(.text+0xa): undefined reference to `print_hello'
collect2: error: ld returned 1 exit status
构建失败
=== 方式 2: 使用 LD_LIBRARY_PATH 指定查找路径
/usr/bin/ld: 找不到 -lsample: 没有那个文件或目录
collect2: error: ld returned 1 exit status
构建失败
=== 方式 3: 使用 -L 指定查找路径
构建成功
=== 方式 4: 使用 LIBRARY_PATH 指定查找路径
构建成功
=== 方式 5: 使用 -L LIBRARY_PATH 指定错误的路径观察查找路径
...
试图打开 /tmp/by_-l/libsample.so 失败
试图打开 /tmp/by_-l/libsample.a 失败
试图打开 /usr/lib/gcc/x86_64-linux-gnu/12/libsample.so 失败
试图打开 /usr/lib/gcc/x86_64-linux-gnu/12/libsample.a 失败
试图打开 /usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/libsample.so 失败
试图打开 /usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/libsample.a 失败
试图打开 /usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib/libsample.so 失败
试图打开 /usr/lib/gcc/x86_64-linux-gnu/12/../../../../lib/libsample.a 失败
试图打开 /lib/x86_64-linux-gnu/libsample.so 失败
试图打开 /lib/x86_64-linux-gnu/libsample.a 失败
试图打开 /lib/../lib/libsample.so 失败
试图打开 /lib/../lib/libsample.a 失败
试图打开 /usr/lib/x86_64-linux-gnu/libsample.so 失败
试图打开 /usr/lib/x86_64-linux-gnu/libsample.a 失败
试图打开 /usr/lib/../lib/libsample.so 失败
试图打开 /usr/lib/../lib/libsample.a 失败
试图打开 /tmp/by_LIBRARY_PATH/libsample.so 失败
试图打开 /tmp/by_LIBRARY_PATH/libsample.a 失败
试图打开 /usr/lib/gcc/x86_64-linux-gnu/12/../../../libsample.so 失败
试图打开 /usr/lib/gcc/x86_64-linux-gnu/12/../../../libsample.a 失败
试图打开 /usr/local/lib/x86_64-linux-gnu/libsample.so 失败
试图打开 /usr/local/lib/x86_64-linux-gnu/libsample.a 失败
试图打开 /lib/x86_64-linux-gnu/libsample.so 失败
试图打开 /lib/x86_64-linux-gnu/libsample.a 失败
试图打开 /usr/lib/x86_64-linux-gnu/libsample.so 失败
试图打开 /usr/lib/x86_64-linux-gnu/libsample.a 失败
试图打开 /usr/lib/x86_64-linux-gnu64/libsample.so 失败
试图打开 /usr/lib/x86_64-linux-gnu64/libsample.a 失败
试图打开 /usr/local/lib64/libsample.so 失败
试图打开 /usr/local/lib64/libsample.a 失败
试图打开 /lib64/libsample.so 失败
试图打开 /lib64/libsample.a 失败
试图打开 /usr/lib64/libsample.so 失败
试图打开 /usr/lib64/libsample.a 失败
试图打开 /usr/local/lib/libsample.so 失败
试图打开 /usr/local/lib/libsample.a 失败
试图打开 /lib/libsample.so 失败
试图打开 /lib/libsample.a 失败
试图打开 /usr/lib/libsample.so 失败
试图打开 /usr/lib/libsample.a 失败
试图打开 /usr/x86_64-linux-gnu/lib64/libsample.so 失败
试图打开 /usr/x86_64-linux-gnu/lib64/libsample.a 失败
试图打开 /usr/x86_64-linux-gnu/lib/libsample.so 失败
试图打开 /usr/x86_64-linux-gnu/lib/libsample.a 失败
/usr/bin/ld: 找不到 -lsample: 没有那个文件或目录
...
collect2: error: ld returned 1 exit status
构建失败运行时查找
在可执行文件执行过程中，动态库的查找发生在程序装载阶段，由操作系统内核调用 ld.so （/lib64/ld-linux-x86-64.so.2） 这个 so 内的代码实现。
默认情况下， ld.so 查找 /etc/ld.so.conf （引用了 /etc/ld.so.conf.d/*.conf） 配置的默认搜索路径，以 debian 11 为例，其默认值为：
/usr/local/lib/usr/local/lib/x86_64-linux-gnu/lib/x86_64-linux-gnu/usr/lib/x86_64-linux-gnu
可以通过修改 /etc/ld.so.conf （并执行 sudo ldconfig 来更新缓存 /etc/ld.so.cache，通过 ldconfig -p 可查看发现的所有库文件） 配置文件来修改系统的运行动态库搜索路径。
除了上述方式外，还可以通过如下方式添加动态库搜索路径：
LD_LIBRARY_PATH环境变量。- 在构建（链接阶段）时，通过
-rpath参数指定动态库搜索路径（写入到了可执行文件 ELF.dynamic段的DT_RPATH或DT_RUNPATH，通过readelf -d可查询）。 LD_AUDIT环境变量指定一个xxx.so文件，可以使用 C 语言实现更加智能灵活的动态库搜索。
如上配置的搜索优先级顺序为：
- 环境变量
LD_LIBRARY_PATH指定的路径。 - 可执行文件中 ELF
.dynamic段DT_RPATH或DT_RUNPATH指定的路径。 /etc/ld.so.conf配置的哪些。
验证如下：01-sample/04-runtime-stage-search.sh
#!/usr/bin/env bash
cd $(dirname $(readlink -f $0))
rm -rf build/bin && mkdir -p build/bin
echo '=== 准备: 指定 -rpath'
gcc -I ./build/include -o ./build/bin/main ./main.c -L ./build/lib -l sample -Wl,-rpath,/tmp/by_-rpath
echo
echo '=== 验证: 观察查找路径'
mkdir -p /tmp/by_LD_LIBRARY_PATH /tmp/by_-rpath
LD_DEBUG=libs LD_LIBRARY_PATH=/tmp/by_LD_LIBRARY_PATH ./build/bin/main
echo输出如下：
=== 准备: 指定 -rpath
=== 验证: 观察查找路径
655097: find library=libsample.so [0]; searching
655097: search path=/tmp/by_LD_LIBRARY_PATH/glibc-hwcaps/x86-64-v2:/tmp/by_LD_LIBRARY_PATH/tls/x86_64/x86_64:/tmp/by_LD_LIBRARY_PATH/tls/x86_64:/tmp/by_LD_LIBRARY_PATH/tls/x86_64:/tmp/by_LD_LIBRARY_PATH/tls:/tmp/by_LD_LIBRARY_PATH/x86_64/x86_64:/tmp/by_LD_LIBRARY_PATH/x86_64:/tmp/by_LD_LIBRARY_PATH/x86_64:/tmp/by_LD_LIBRARY_PATH (LD_LIBRARY_PATH)
655097: trying file=/tmp/by_LD_LIBRARY_PATH/glibc-hwcaps/x86-64-v2/libsample.so
655097: trying file=/tmp/by_LD_LIBRARY_PATH/tls/x86_64/x86_64/libsample.so
655097: trying file=/tmp/by_LD_LIBRARY_PATH/tls/x86_64/libsample.so
655097: trying file=/tmp/by_LD_LIBRARY_PATH/tls/x86_64/libsample.so
655097: trying file=/tmp/by_LD_LIBRARY_PATH/tls/libsample.so
655097: trying file=/tmp/by_LD_LIBRARY_PATH/x86_64/x86_64/libsample.so
655097: trying file=/tmp/by_LD_LIBRARY_PATH/x86_64/libsample.so
655097: trying file=/tmp/by_LD_LIBRARY_PATH/x86_64/libsample.so
655097: trying file=/tmp/by_LD_LIBRARY_PATH/libsample.so
655097: search path=/tmp/by_-rpath/glibc-hwcaps/x86-64-v2:/tmp/by_-rpath/tls/x86_64/x86_64:/tmp/by_-rpath/tls/x86_64:/tmp/by_-rpath/tls/x86_64:/tmp/by_-rpath/tls:/tmp/by_-rpath/x86_64/x86_64:/tmp/by_-rpath/x86_64:/tmp/by_-rpath/x86_64:/tmp/by_-rpath (RUNPATH from file ./build/bin/main)
655097: trying file=/tmp/by_-rpath/glibc-hwcaps/x86-64-v2/libsample.so
655097: trying file=/tmp/by_-rpath/tls/x86_64/x86_64/libsample.so
655097: trying file=/tmp/by_-rpath/tls/x86_64/libsample.so
655097: trying file=/tmp/by_-rpath/tls/x86_64/libsample.so
655097: trying file=/tmp/by_-rpath/tls/libsample.so
655097: trying file=/tmp/by_-rpath/x86_64/x86_64/libsample.so
655097: trying file=/tmp/by_-rpath/x86_64/libsample.so
655097: trying file=/tmp/by_-rpath/x86_64/libsample.so
655097: trying file=/tmp/by_-rpath/libsample.so
655097: search cache=/etc/ld.so.cache
655097: search path=/lib/x86_64-linux-gnu/glibc-hwcaps/x86-64-v2:/lib/x86_64-linux-gnu/tls/x86_64/x86_64:/lib/x86_64-linux-gnu/tls/x86_64:/lib/x86_64-linux-gnu/tls/x86_64:/lib/x86_64-linux-gnu/tls:/lib/x86_64-linux-gnu/x86_64/x86_64:/lib/x86_64-linux-gnu/x86_64:/lib/x86_64-linux-gnu/x86_64:/lib/x86_64-linux-gnu:/usr/lib/x86_64-linux-gnu/glibc-hwcaps/x86-64-v2:/usr/lib/x86_64-linux-gnu/tls/x86_64/x86_64:/usr/lib/x86_64-linux-gnu/tls/x86_64:/usr/lib/x86_64-linux-gnu/tls/x86_64:/usr/lib/x86_64-linux-gnu/tls:/usr/lib/x86_64-linux-gnu/x86_64/x86_64:/usr/lib/x86_64-linux-gnu/x86_64:/usr/lib/x86_64-linux-gnu/x86_64:/usr/lib/x86_64-linux-gnu:/lib/glibc-hwcaps/x86-64-v2:/lib/tls/x86_64/x86_64:/lib/tls/x86_64:/lib/tls/x86_64:/lib/tls:/lib/x86_64/x86_64:/lib/x86_64:/lib/x86_64:/lib:/usr/lib/glibc-hwcaps/x86-64-v2:/usr/lib/tls/x86_64/x86_64:/usr/lib/tls/x86_64:/usr/lib/tls/x86_64:/usr/lib/tls:/usr/lib/x86_64/x86_64:/usr/lib/x86_64:/usr/lib/x86_64:/usr/lib (system search path)
655097: trying file=/lib/x86_64-linux-gnu/glibc-hwcaps/x86-64-v2/libsample.so
655097: trying file=/lib/x86_64-linux-gnu/tls/x86_64/x86_64/libsample.so
655097: trying file=/lib/x86_64-linux-gnu/tls/x86_64/libsample.so
655097: trying file=/lib/x86_64-linux-gnu/tls/x86_64/libsample.so
655097: trying file=/lib/x86_64-linux-gnu/tls/libsample.so
655097: trying file=/lib/x86_64-linux-gnu/x86_64/x86_64/libsample.so
655097: trying file=/lib/x86_64-linux-gnu/x86_64/libsample.so
655097: trying file=/lib/x86_64-linux-gnu/x86_64/libsample.so
655097: trying file=/lib/x86_64-linux-gnu/libsample.so
655097: trying file=/usr/lib/x86_64-linux-gnu/glibc-hwcaps/x86-64-v2/libsample.so
655097: trying file=/usr/lib/x86_64-linux-gnu/tls/x86_64/x86_64/libsample.so
655097: trying file=/usr/lib/x86_64-linux-gnu/tls/x86_64/libsample.so
655097: trying file=/usr/lib/x86_64-linux-gnu/tls/x86_64/libsample.so
655097: trying file=/usr/lib/x86_64-linux-gnu/tls/libsample.so
655097: trying file=/usr/lib/x86_64-linux-gnu/x86_64/x86_64/libsample.so
655097: trying file=/usr/lib/x86_64-linux-gnu/x86_64/libsample.so
655097: trying file=/usr/lib/x86_64-linux-gnu/x86_64/libsample.so
655097: trying file=/usr/lib/x86_64-linux-gnu/libsample.so
655097: trying file=/lib/glibc-hwcaps/x86-64-v2/libsample.so
655097: trying file=/lib/tls/x86_64/x86_64/libsample.so
655097: trying file=/lib/tls/x86_64/libsample.so
655097: trying file=/lib/tls/x86_64/libsample.so
655097: trying file=/lib/tls/libsample.so
655097: trying file=/lib/x86_64/x86_64/libsample.so
655097: trying file=/lib/x86_64/libsample.so
655097: trying file=/lib/x86_64/libsample.so
655097: trying file=/lib/libsample.so
655097: trying file=/usr/lib/glibc-hwcaps/x86-64-v2/libsample.so
655097: trying file=/usr/lib/tls/x86_64/x86_64/libsample.so
655097: trying file=/usr/lib/tls/x86_64/libsample.so
655097: trying file=/usr/lib/tls/x86_64/libsample.so
655097: trying file=/usr/lib/tls/libsample.so
655097: trying file=/usr/lib/x86_64/x86_64/libsample.so
655097: trying file=/usr/lib/x86_64/libsample.so
655097: trying file=/usr/lib/x86_64/libsample.so
655097: trying file=/usr/lib/libsample.so
655097:
./build/bin/main: error while loading shared libraries: libsample.so: cannot open shared object file: No such file or directory