代码来自《POSIX多线程程序设计》
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/*
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* atfork.c
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* Demonstrate the use of "fork handlers" to protect data invariants
across a fork.
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*/
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#include <sys/types.h>
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#include <pthread.h>
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#include <sys/wait.h>
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#include "errors.h"
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pid_t self_pid; /* pid of current process */
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pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
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void fork_prepare(void)
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{
- pthread_mutex_lock(&mutex);
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}
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void fork_parent(void)
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{
- pthread_mutex_unlock(&mutex);
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}
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void fork_child(void)
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{
- self_pid = getpid();
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pthread_mutex_unlock(&mutex);
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}
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void *thread_routine(void *arg)
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{
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pid_t child_pid;
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child_pid = fork();
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if(child_pid == (pid_t)-1)
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errno_abort("Fork");
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pthread_mutex_lock(&mutex);
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pthread_mutex_unlock(&mutex);
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printf("After fork: %d (%d)\n", child_pid, self_pid);
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if(child_pid != 0){ // parent
process
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if ((pid_t)-1 == waitpid(child_pid, (int*)0, 0))
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errno_abort("Wait for child");
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}
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return NULL;
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}
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int main(int argc, char *argv[])
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{
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pthread_t fork_thread;
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int atfork_flag = 1;
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if(argc > 1)
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atfork_flag = atoi (argv[1]);
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if(atfork_flag){
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pthread_atfork(fork_prepare, fork_parent, fork_child);
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}
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self_pid = getpid();
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pthread_mutex_lock(&mutex);
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pthread_create(&fork_thread, NULL, thread_routine, NULL);
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sleep(5);
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pthread_mutex_unlock (&mutex);
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pthread_join(fork_thread, NULL);
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return 0;
- }
编译运行结果:
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digdeep@ubuntu:~/pthread/learnthread$
gcc -Wall -lpthread -o atfork atfork.c /tmp/cckSrItT.o: In function `main':
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atfork.c:(.text+0x195): undefined
reference to `pthread_atfork'
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collect2: ld returned 1 exit status
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digdeep@ubuntu:~/pthread/learnthread$
gcc -Wall -pthread -o atfork atfork.c
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digdeep@ubuntu:~/pthread/learnthread$ ./atfork
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After fork: 2637 (2635)
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After fork: 0 (2637)
- digdeep@ubuntu:~/pthread/learnthread$
于是想搞清楚 -lpthread和-pthread的区别:
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digdeep@ubuntu:~$ gcc
--version
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gcc (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2
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Copyright (C) 2010 Free Software Foundation, Inc.
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This is free software; see the source for copying conditions. There is NO
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warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
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digdeep@ubuntu:~$ man
gcc | grep pthread
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-mvxworks -G num -pthread
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-mno-unaligned-doubles -mv8plus -mno-v8plus -mvis -mno-vis -threads -pthreads
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-pthread
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implies -pthread, and thus is only
supported on targets that have support for
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-pthread.
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implies -pthread, and thus is only
supported on targets that have support for
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-pthread.
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option and the -pthread option are
incompatible.
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-pthread
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Adds support for multithreading with the pthreads library. This option sets
flags for
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-pthreads
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-pthread
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This is a synonym for -pthreads.
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digdeep@ubuntu:~$
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digdeep@ubuntu:~$ man
gcc | grep lpthread
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digdeep@ubuntu:~$
- digdeep@ubuntu:~$
从上面的输出可以看到,目前gcc 4.5.2中已经没有了关于 -lpthread的介绍了。所以以后的多线程编译应该用
-pthread,而不是-lpthread。
仔细的阅读man gcc中的关于pthread的介绍:
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-pthread
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Adds support for multithreading with the pthreads library. This option sets
flags for
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both the preprocessor and linker.
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-pthreads
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Add support for multithreading using the POSIX threads library. This option sets
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flags for both the preprocessor and linker. This option does
not affect the thread
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safety of object code produced by the compiler or that of libraries supplied with it.
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-pthread
- This is a synonym for -pthreads.
从上面的 man gcc中的详细说明可以看出:
1)-pthread和-pthreads的含义是相同的。
2)-pthread或者-pthreads的编译选项是用于在编译时增加多线程的支持。该选项同时对“预处理器”和“链接器”产
生作用。
3)-pthread或者-pthreads的编译选项,即不影响编译器产生的目标代码的线程安全性,也不影响对提供的支持
多线程的函数库libraries(的选择).(gcc
会自动链接系统当前版本推荐的 thread lib 以及对应的 thread
safe 的 c func。 http://kasicass.blog.163.com/blog/static/395619200992410313759/)
但是,下面的输出却又说明,我们不能使用 -pthreads:
- digdeep@ubuntu:~/pthread/learnthread$ gcc -Wall -pthreads -o atfork atfork.c
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gcc: unrecognized option '-pthreads'
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/tmp/ccZZmWbD.o: In function `main':
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atfork.c:(.text+0x195): undefined
reference to `pthread_atfork'
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atfork.c:(.text+0x1cf): undefined
reference to `pthread_create'
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atfork.c:(.text+0x1fb): undefined
reference to `pthread_join'
- collect2: ld returned 1 exit status
所以:一句话,我们现在应该使用 -pthread 而不是 -lpthread。
另:下面的英文文档详细说明了相关的情况。
该文档的摘要:
下面是摘录自命令 gcc -dumpspecs的输出:
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*cpp:
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%{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}
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*lib:
- %{pthread:-lpthread} %{shared:-lc} %{!shared:%{mieee-fp:-lieee} %{profile:-lc_p}%{!profile:-lc}}
%{pthread:-lpthread} 与上面的说明有点矛盾!!有点糊涂了。NND!