gcc中-pthread与-lpthread的区别
用gcc编译使用了POSIX thread的程序时通常需要加额外的选项，以便使用thread-safe的库及头文件，一些老的书里说直接增加链接选项 -lpthread 就可以了，像这样：
- gcc -c x.c
- gcc x.o -ox -lpthread
gcc -c x.c
gcc x.o -ox -lpthread
而gcc手册里则指出应该在编译和链接时都增加 -pthread 选项，像这样：
- gcc -pthread -c x.c
- gcc x.o -ox -pthread
gcc -pthread -c x.c
gcc x.o -ox -pthread
那么 -pthread 相比于 -lpthread 链接选项究竟多做了什么工作呢？我们可以在verbose模式下执行一下对应的gcc命令行看出来。下面是老式的直接加 -lpthread 链接选项的输出结果：
- $ gcc -v -c x.c
- ...
- /usr/lib/gcc/i486-linux-gnu/4.2.4/cc1 -quiet -v x.c -quiet -dumpbase x.c
- -mtune=generic -auxbase x -version -fstack-protector -fstack-protector -o /tmp/cch4ASTF.s
- ...
- as --traditional-format -V -Qy -o x.o /tmp/cch4ASTF.s
- ...
- $ gcc -v x.o -ox -lpthread
- ...
- /usr/lib/gcc/i486-linux-gnu/4.2.4/collect2 --eh-frame-hdr -m elf_i386 --hash-style=both
- -dynamic-linker /lib/ld-linux.so.2 -ox
- /usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crt1.o
- /usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crti.o
- /usr/lib/gcc/i486-linux-gnu/4.2.4/crtbegin.o
- -L/opt/intel/Compiler/11.1/046/tbb/ia32/cc4.1.0_libc2.4_kernel2.6.16.21/lib/../lib
- -L/usr/lib/gcc/i486-linux-gnu/4.2.4
- -L/usr/lib/gcc/i486-linux-gnu/4.2.4
- -L/usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib
- -L/lib/../lib
- -L/usr/lib/../lib
- -L/opt/intel/Compiler/11.1/046/lib/ia32
- -L/opt/intel/Compiler/11.1/046/tbb/ia32/cc4.1.0_libc2.4_kernel2.6.16.21/lib
- -L/usr/lib/gcc/i486-linux-gnu/4.2.4/../../..
- x.o -lpthread -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc
- --as-needed -lgcc_s --no-as-needed
- /usr/lib/gcc/i486-linux-gnu/4.2.4/crtend.o /usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crtn.o
$ gcc -v -c x.c
...
/usr/lib/gcc/i486-linux-gnu/4.2.4/cc1 -quiet -v x.c -quiet -dumpbase x.c
-mtune=generic -auxbase x -version -fstack-protector -fstack-protector -o /tmp/cch4ASTF.s
...
as --traditional-format -V -Qy -o x.o /tmp/cch4ASTF.s
...
$ gcc -v x.o -ox -lpthread
...
/usr/lib/gcc/i486-linux-gnu/4.2.4/collect2 --eh-frame-hdr -m elf_i386 --hash-style=both
-dynamic-linker /lib/ld-linux.so.2 -ox
/usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crt1.o
/usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crti.o
/usr/lib/gcc/i486-linux-gnu/4.2.4/crtbegin.o
-L/opt/intel/Compiler/11.1/046/tbb/ia32/cc4.1.0_libc2.4_kernel2.6.16.21/lib/../lib
-L/usr/lib/gcc/i486-linux-gnu/4.2.4
-L/usr/lib/gcc/i486-linux-gnu/4.2.4
-L/usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib
-L/lib/../lib
-L/usr/lib/../lib
-L/opt/intel/Compiler/11.1/046/lib/ia32
-L/opt/intel/Compiler/11.1/046/tbb/ia32/cc4.1.0_libc2.4_kernel2.6.16.21/lib
-L/usr/lib/gcc/i486-linux-gnu/4.2.4/../../..
x.o -lpthread -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc
--as-needed -lgcc_s --no-as-needed
/usr/lib/gcc/i486-linux-gnu/4.2.4/crtend.o /usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crtn.o
下面是在编译和链接时分别指定 -pthread 选项的输出结果：
- $ gcc -v -pthread -c x.c
- ...
- /usr/lib/gcc/i486-linux-gnu/4.2.4/cc1 -quiet -v -D_REENTRANT
- x.c -quiet -dumpbase x.c
- -mtune=generic -auxbase x -version -fstack-protector -fstack-protector -o /tmp/cc205IQf.s
- ...
- as --traditional-format -V -Qy -o x.o /tmp/cc205IQf.s
- ...
- $ gcc -v x.o -ox -pthread
- /usr/lib/gcc/i486-linux-gnu/4.2.4/collect2 --eh-frame-hdr -m elf_i386 --hash-style=both
- -dynamic-linker /lib/ld-linux.so.2 -ox
- /usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crt1.o
- /usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crti.o
- /usr/lib/gcc/i486-linux-gnu/4.2.4/crtbegin.o
- -L/opt/intel/Compiler/11.1/046/tbb/ia32/cc4.1.0_libc2.4_kernel2.6.16.21/lib/../lib
- -L/usr/lib/gcc/i486-linux-gnu/4.2.4
- -L/usr/lib/gcc/i486-linux-gnu/4.2.4
- -L/usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib
- -L/lib/../lib
- -L/usr/lib/../lib
- -L/opt/intel/Compiler/11.1/046/lib/ia32
- -L/opt/intel/Compiler/11.1/046/tbb/ia32/cc4.1.0_libc2.4_kernel2.6.16.21/lib
- -L/usr/lib/gcc/i486-linux-gnu/4.2.4/../../..
- x.o -lgcc --as-needed -lgcc_s --no-as-needed -lpthread
- -lc -lgcc
- --as-needed -lgcc_s --no-as-needed
- /usr/lib/gcc/i486-linux-gnu/4.2.4/crtend.o /usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crtn.o
$ gcc -v -pthread -c x.c
...
/usr/lib/gcc/i486-linux-gnu/4.2.4/cc1 -quiet -v -D_REENTRANT
x.c -quiet -dumpbase x.c
-mtune=generic -auxbase x -version -fstack-protector -fstack-protector -o /tmp/cc205IQf.s
...
as --traditional-format -V -Qy -o x.o /tmp/cc205IQf.s
...
$ gcc -v x.o -ox -pthread
/usr/lib/gcc/i486-linux-gnu/4.2.4/collect2 --eh-frame-hdr -m elf_i386 --hash-style=both
-dynamic-linker /lib/ld-linux.so.2 -ox
/usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crt1.o
/usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crti.o
/usr/lib/gcc/i486-linux-gnu/4.2.4/crtbegin.o
-L/opt/intel/Compiler/11.1/046/tbb/ia32/cc4.1.0_libc2.4_kernel2.6.16.21/lib/../lib
-L/usr/lib/gcc/i486-linux-gnu/4.2.4
-L/usr/lib/gcc/i486-linux-gnu/4.2.4
-L/usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib
-L/lib/../lib
-L/usr/lib/../lib
-L/opt/intel/Compiler/11.1/046/lib/ia32
-L/opt/intel/Compiler/11.1/046/tbb/ia32/cc4.1.0_libc2.4_kernel2.6.16.21/lib
-L/usr/lib/gcc/i486-linux-gnu/4.2.4/../../..
x.o -lgcc --as-needed -lgcc_s --no-as-needed -lpthread
-lc -lgcc
--as-needed -lgcc_s --no-as-needed
/usr/lib/gcc/i486-linux-gnu/4.2.4/crtend.o /usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crtn.o
可见编译选项中指定 -pthread 会附加一个宏定义 -D_REENTRANT
，该宏会导致 libc 头文件选择那些thread-safe的实现；链接选项中指定 -pthread 则同 -lpthread 一样，只表示链接 POSIX thread 库。由于 libc 用于适应 thread-safe 的宏定义可能变化，因此在编译和链接时都使用 -pthread 选项而不是传统的 -lpthread 能够保持向后兼容，并提高命令行的一致性。
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