【原创】多线程应用中pthread库使用问题

在 linux 下开发多线程应用，大多情况下我们都会使用 pthread （即 POSIX thread）这个库。该库遵循 POSIX.1-2001 标准。

在使用该库的过程中，肯定有人见过各种 gcc 选项配置方式：

• -pthread
• -pthreads
• -lpthread

但应该有很多朋友没有搞清楚，这几种方式的区别和联系。本文的目的就在于此。

在 GCC man 手册中，我们可以找到如下内容

...
Machine Dependent Options
...
HPPA Options
...
       -threads
           Add support for multithreading with the dce thread library under HP-UX.  This option sets flags for both the preprocessor and linker.
...
IA-64 Options
...
       -pthread
           Add support for multithreading using the POSIX threads library.  This option sets flags for both the preprocessor and linker.  It does 
           not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.  These are HP-UX specific flags.
...
IBM RS/6000 and PowerPC Options
...
       -pthread
           Adds support for multithreading with the pthreads library.  This option sets flags for both the preprocessor and linker.
...
SPARC Options
...
       -threads
           Add support for multithreading using the Solaris threads library.  This option sets flags for both the preprocessor and linker.  
           This option does not affect the thread safety of object code produced by the compiler or that of libraries supplied with it.

       -pthreads
           Add support for multithreading using the POSIX threads library.  This option sets flags for both the preprocessor and linker.  
           This option does not affect the thread safety of object code produced  by the compiler or that of libraries supplied with it.

       -pthread
           This is a synonym for -pthreads.
...

可以看到，这几个选项都是 Machine Dependent 的。而什么是 Machine Dependent ，则可以参考 这里 。

还有以下内容

...
Options for Linking
... 
       -llibrary
       -l library
           Search the library named library when linking.  (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.)

           It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified.  Thus, foo.o
           -lz bar.o searches library z after file foo.o but before bar.o.  If bar.o refers to functions in z, those functions may not be loaded.

           The linker searches a standard list of directories for the library, which is actually a file named liblibrary.a.  The linker then uses this file as if it had been specified
           precisely by name.

           The directories searched include several standard system directories plus any that you specify with -L.

           Normally the files found this way are library files---archive files whose members are object files.  The linker handles an archive file by scanning through it for members
           which define symbols that have so far been referenced but not defined.  But if the file that is found is an ordinary object file, it is linked in the usual fashion.  The only
           difference between using an -l option and specifying a file name is that -l surrounds library with lib and .a and searches several directories.

从 GCC 选项分类上，上面两种选项分别属于
GCC Command Options -> Hardware Models and Configurations -> xxx Options
GCC Command Options -> Options for Linking


而 -pthread 和 -pthreads 的关系可以简单认为，是在某一种 Hardware Model 上支持的同义选项。

至于为何推荐使用 -pthread 选项，可以参考下图中的说明



下面实验验证一下，几种方式的效果

[root@Betty thread_test]# uname -a
Linux Betty 2.6.32-358.el6.x86_64 #1 SMP Fri Feb 22 00:31:26 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
[root@Betty thread_test]# gcc -V
gcc: ‘-V’选项必须有一个参数
[root@Betty thread_test]# gcc -v
使用内建 specs。
目标：x86_64-redhat-linux
配置为：../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk --disable-dssi --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre --enable-libgcj-multifile --enable-java-maintainer-mode --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib --with-ppl --with-cloog --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
线程模型：posix
gcc 版本 4.4.7 20120313 (Red Hat 4.4.7-11) (GCC) 
[root@Betty thread_test]#
[root@Betty thread_test]#
[root@Betty thread_test]# ll
总用量 5320
-rw-r--r-- 1 root root   45125 8月  20 09:24 event.h
lrwxrwxrwx 1 root root      21 8月  19 15:02 libevent-2.0.so.5 -> libevent-2.0.so.5.1.9
-rwxr-xr-x 1 root root  968698 8月  19 15:02 libevent-2.0.so.5.1.9
-rw-r--r-- 1 root root 1571978 8月  19 15:02 libevent.a
lrwxrwxrwx 1 root root      26 8月  19 15:02 libevent_core-2.0.so.5 -> libevent_core-2.0.so.5.1.9
-rwxr-xr-x 1 root root  585233 8月  19 15:02 libevent_core-2.0.so.5.1.9
-rw-r--r-- 1 root root  978626 8月  19 15:02 libevent_core.a
lrwxrwxrwx 1 root root      26 8月  19 15:02 libevent_core.so -> libevent_core-2.0.so.5.1.9
lrwxrwxrwx 1 root root      27 8月  19 15:02 libevent_extra-2.0.so.5 -> libevent_extra-2.0.so.5.1.9
-rwxr-xr-x 1 root root  404860 8月  19 15:02 libevent_extra-2.0.so.5.1.9
-rw-r--r-- 1 root root  593424 8月  19 15:02 libevent_extra.a
lrwxrwxrwx 1 root root      27 8月  19 15:02 libevent_extra.so -> libevent_extra-2.0.so.5.1.9
lrwxrwxrwx 1 root root      29 8月  19 15:02 libevent_openssl-2.0.so.5 -> libevent_openssl-2.0.so.5.1.9
-rwxr-xr-x 1 root root   94281 8月  19 15:02 libevent_openssl-2.0.so.5.1.9
-rw-r--r-- 1 root root  131932 8月  19 15:02 libevent_openssl.a
lrwxrwxrwx 1 root root      29 8月  19 15:02 libevent_openssl.so -> libevent_openssl-2.0.so.5.1.9
lrwxrwxrwx 1 root root      30 8月  19 15:02 libevent_pthreads-2.0.so.5 -> libevent_pthreads-2.0.so.5.1.9
-rwxr-xr-x 1 root root   18438 8月  19 15:02 libevent_pthreads-2.0.so.5.1.9
-rw-r--r-- 1 root root   18678 8月  19 15:02 libevent_pthreads.a
lrwxrwxrwx 1 root root      30 8月  19 15:02 libevent_pthreads.so -> libevent_pthreads-2.0.so.5.1.9
lrwxrwxrwx 1 root root      21 8月  19 15:02 libevent.so -> libevent-2.0.so.5.1.9
-rw-r--r-- 1 root root    9458 8月  20 09:24 thread.h
-rw-r--r-- 1 root root     757 8月  20 10:22 thread_test.c
[root@Betty thread_test]# 
[root@Betty thread_test]# 
[root@Betty thread_test]# gcc thread_test.c -o thread_test_1 -levent_core -levent_pthreads -lpthread
[root@Betty thread_test]# gcc thread_test.c -o thread_test_2 -levent_core -levent_pthreads -pthread  
[root@Betty thread_test]# gcc thread_test.c -o thread_test_3 -levent_core -levent_pthreads -pthreads 
gcc: 无法识别的选项‘-pthreads’
[root@Betty thread_test]# 
[root@Betty thread_test]# ll
总用量 5344
-rw-r--r-- 1 root root   45125 8月  20 09:24 event.h
lrwxrwxrwx 1 root root      21 8月  19 15:02 libevent-2.0.so.5 -> libevent-2.0.so.5.1.9
-rwxr-xr-x 1 root root  968698 8月  19 15:02 libevent-2.0.so.5.1.9
-rw-r--r-- 1 root root 1571978 8月  19 15:02 libevent.a
lrwxrwxrwx 1 root root      26 8月  19 15:02 libevent_core-2.0.so.5 -> libevent_core-2.0.so.5.1.9
-rwxr-xr-x 1 root root  585233 8月  19 15:02 libevent_core-2.0.so.5.1.9
-rw-r--r-- 1 root root  978626 8月  19 15:02 libevent_core.a
lrwxrwxrwx 1 root root      26 8月  19 15:02 libevent_core.so -> libevent_core-2.0.so.5.1.9
lrwxrwxrwx 1 root root      27 8月  19 15:02 libevent_extra-2.0.so.5 -> libevent_extra-2.0.so.5.1.9
-rwxr-xr-x 1 root root  404860 8月  19 15:02 libevent_extra-2.0.so.5.1.9
-rw-r--r-- 1 root root  593424 8月  19 15:02 libevent_extra.a
lrwxrwxrwx 1 root root      27 8月  19 15:02 libevent_extra.so -> libevent_extra-2.0.so.5.1.9
lrwxrwxrwx 1 root root      29 8月  19 15:02 libevent_openssl-2.0.so.5 -> libevent_openssl-2.0.so.5.1.9
-rwxr-xr-x 1 root root   94281 8月  19 15:02 libevent_openssl-2.0.so.5.1.9
-rw-r--r-- 1 root root  131932 8月  19 15:02 libevent_openssl.a
lrwxrwxrwx 1 root root      29 8月  19 15:02 libevent_openssl.so -> libevent_openssl-2.0.so.5.1.9
lrwxrwxrwx 1 root root      30 8月  19 15:02 libevent_pthreads-2.0.so.5 -> libevent_pthreads-2.0.so.5.1.9
-rwxr-xr-x 1 root root   18438 8月  19 15:02 libevent_pthreads-2.0.so.5.1.9
-rw-r--r-- 1 root root   18678 8月  19 15:02 libevent_pthreads.a
lrwxrwxrwx 1 root root      30 8月  19 15:02 libevent_pthreads.so -> libevent_pthreads-2.0.so.5.1.9
lrwxrwxrwx 1 root root      21 8月  19 15:02 libevent.so -> libevent-2.0.so.5.1.9
-rw-r--r-- 1 root root    9458 8月  20 09:24 thread.h
-rwxr-xr-x 1 root root    8008 8月  20 15:12 thread_test_1
-rwxr-xr-x 1 root root    8008 8月  20 15:12 thread_test_2
-rwxr-xr-x 1 root root    7976 8月  20 15:12 thread_test_3
-rw-r--r-- 1 root root     757 8月  20 10:22 thread_test.c
[root@Betty thread_test]# 
[root@Betty thread_test]# ldd thread_test_*
thread_test_1:
        linux-vdso.so.1 =>  (0x00007fff4a7ff000)
        libevent_core-2.0.so.5 => not found
        libevent_pthreads-2.0.so.5 => not found
        libpthread.so.0 => /lib64/libpthread.so.0 (0x000000388cc00000)
        libc.so.6 => /lib64/libc.so.6 (0x000000388c800000)
        /lib64/ld-linux-x86-64.so.2 (0x000000388c400000)
thread_test_2:
        linux-vdso.so.1 =>  (0x00007fffbc3ff000)
        libevent_core-2.0.so.5 => not found
        libevent_pthreads-2.0.so.5 => not found
        libpthread.so.0 => /lib64/libpthread.so.0 (0x000000388cc00000)
        libc.so.6 => /lib64/libc.so.6 (0x000000388c800000)
        /lib64/ld-linux-x86-64.so.2 (0x000000388c400000)
thread_test_3:
        linux-vdso.so.1 =>  (0x00007fffef754000)
        libevent_core-2.0.so.5 => not found
        libevent_pthreads-2.0.so.5 => not found
        libc.so.6 => /lib64/libc.so.6 (0x000000388c800000)
        /lib64/ld-linux-x86-64.so.2 (0x000000388c400000)
[root@Betty thread_test]#

可以看到，从结果上讲，使用 -lpthread 和 -pthread 效果相同，而 -ptheads 不被支持。

最后，可以通过如下命令确认 libpthread 库属于哪个 rpm 包

[root@Betty thread_test]# rpm -qf /lib64/libpthread.so.0 
glibc-2.12-1.149.el6_6.9.x86_64

PS：上面的实验只是用于对于几种选项配置方式，实际上 libevent_pthreads.so 内部已经包含了 libpthread.so 。