linux和android端的pthread学习

本文起初主要想写个演示样例实測下pthread_mutex_lock和pthread_mutex_trylock差别。在linux机器上非常快就over了，可是想了一下。pthread是unix系的，在windows没办法直接执行代码非常不方便。于是想到了android，windows上安装ndk，手机root就能够跑pthread代码咯。。。

demo

lock和trylock的差别也非常好理解：，前者是堵塞的。死等知道相互排斥锁被释放；而后者则更加灵活。浅尝辄止。做个尝试不行则干其它事情去。。測试代码pt_lock.c例如以下：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <unistd.h>
#include <sys/types.h>

typedef pthread_t pt_t;
typedef unsigned int uint_t;

pthread_mutex_t mt = PTHREAD_MUTEX_INITIALIZER;

void lock_func(void* arg)
{
	pid_t pid;	//process
	pt_t tid;	// thread

	pid = getpid();
	tid = pthread_self();

	printf("want to lock mutex, msg=%s, tid=%u\n", (char*)arg, (uint_t)tid);

	pthread_mutex_lock( &mt );
	printf("I[tid=%u] am using, (*|^_^|*)\n", (uint_t)tid);
	sleep(10);
	pthread_mutex_unlock( &mt );
}

void try_lock_func(void* arg)
{
	uint_t tid = (uint_t)pthread_self();
	int counter = 0;

	while ( pthread_mutex_trylock( &mt ) )
	{
		sleep(1);
		++counter;

		printf("after sleep 1s, i [tid=%u] want to try again, iter=%d.\n", tid, counter);
	}
	printf("It is my[tid=%u] turn, so long i waited...msg=%s\n", tid, (char*)arg);
	pthread_mutex_unlock( &mt );
}

#define XX_CREATE_FAILED(err)	\
	printf("create thread error : %s\n", strerror(err));\
	return 1;	

int main()
{
	int rc;
	pt_t pt1, pt2, pt3;

	const char* msg1 = "block";
	const char* msg2 = "unblock";

	rc = pthread_create(&pt1, NULL, (void*)&lock_func, (void*)msg1);	if (rc != 0)
	{
		XX_CREATE_FAILED(rc);
	}

	rc = pthread_create(&pt2, NULL, (void*)&lock_func, (void*)msg1);	if (rc != 0)
	{
		XX_CREATE_FAILED(rc);
	}
	sleep(1);

	rc = pthread_create(&pt3, NULL, (void*)&try_lock_func, (void*)msg2);	if (rc != 0)
	{
		XX_CREATE_FAILED(rc);
	}

	pthread_join(pt1, NULL);
	pthread_join(pt2, NULL);
	pthread_join(pt3, NULL);
	return 0;
}

代码思路也非常好理解：创建三个线程。1和2通过lock方式去争抢mt相互排斥锁，3线程则灵活，每隔1秒去检測下mt相互排斥锁能否够用，不会堵塞。

线程1或者2占有mt的时间为10秒。

linux run

xx@h10-1-152-72:~/peteryfren/cpp/pthread> gcc -Wall -o pt_lock pt_lock.c -lpthread
xx@h10-1-152-72:~/peteryfren/cpp/pthread> ./pt_lock 

输出结果与预期一致，id=1082132800线程先占有mt。10s内线程tid=1090525504堵塞，而线程tid=1098918208每隔1s測试下mt可用性。

want to lock mutex, msg=block, tid=1082132800
I[tid=1082132800] am using, (*|^_^|*)
want to lock mutex, msg=block, tid=1090525504
after sleep 1s, i [tid=1098918208] want to try again, iter=1.
after sleep 1s, i [tid=1098918208] want to try again, iter=2.
after sleep 1s, i [tid=1098918208] want to try again, iter=3.
after sleep 1s, i [tid=1098918208] want to try again, iter=4.
after sleep 1s, i [tid=1098918208] want to try again, iter=5.
after sleep 1s, i [tid=1098918208] want to try again, iter=6.
after sleep 1s, i [tid=1098918208] want to try again, iter=7.
after sleep 1s, i [tid=1098918208] want to try again, iter=8.
I[tid=1090525504] am using, (*|^_^|*)
after sleep 1s, i [tid=1098918208] want to try again, iter=9.
after sleep 1s, i [tid=1098918208] want to try again, iter=10.
after sleep 1s, i [tid=1098918208] want to try again, iter=11.
after sleep 1s, i [tid=1098918208] want to try again, iter=12.
after sleep 1s, i [tid=1098918208] want to try again, iter=13.
after sleep 1s, i [tid=1098918208] want to try again, iter=14.
after sleep 1s, i [tid=1098918208] want to try again, iter=15.
after sleep 1s, i [tid=1098918208] want to try again, iter=16.
after sleep 1s, i [tid=1098918208] want to try again, iter=17.
after sleep 1s, i [tid=1098918208] want to try again, iter=18.
after sleep 1s, i [tid=1098918208] want to try again, iter=19.
It is my[tid=1098918208] turn, so long i waited...msg=unblock

android run

android上编译和执行pt_lock參考前一篇blog：http://blog.csdn.net/ryfdizuo/article/details/28891649   详细批处理run.cmd例如以下：

@echo "1. build .o file"

@Rem fPIE flag is used in compiling stage.
D:\android-ndk-r9b-windows-x86\toolchains\arm-linux-androideabi-4.6\prebuilt\windows\bin\arm-linux-androideabi-gcc.exe --sysroot=D:\android-ndk-r9b-windows-x86\platforms\android-13\arch-arm -fPIE -c pt_lock.c

@echo "2. build exec file"

@Rem pie flag is used in linking stage.
D:\android-ndk-r9b-windows-x86\toolchains\arm-linux-androideabi-4.6\prebuilt\windows\bin\arm-linux-androideabi-gcc.exe --sysroot=D:\android-ndk-r9b-windows-x86\platforms\android-13\arch-arm -pie -o pt_lock pt_lock.o

@pause

里面的gcc和android系统库路径须要依据机器上实际位置改动。

注意android上gcc编译与linux上有点点区别：ndk中gcc不须要须要显式指定-lpthread。可能pthread默认就会链接。

android-ndk-r9b-windows-x86\platforms\android-13\arch-arm\usr\lib文件夹下的这些so载入须要指定。。在Android.mk中类似。

命令行下执行run.cmd，编译pt_lock，然后推到手机上，注意我的nexus4必须是/data/local/tmp以下才有权限，执行后有错误，详细步骤：

E:\GitHub\ndk_tutorial\pthread_playground>adb shell
root@mako:/ # cd /data/local/tmp
cd /data/local/tmp
root@mako:/data/local/tmp # mv /sdcard/pt_lock ./
mv /sdcard/pt_lock ./
root@mako:/data/local/tmp # chmod 751 pt_lock
chmod 751 pt_lock
root@mako:/data/local/tmp # ./pt_lock
error: only position independent executables (PIE) are supported.

报了PIE不支持的错误，n4上安装的是android L系统。gg后知道编译的链接的须要添加pie等设置，參见[ref2]，fPIE是编译时候的选项，pie是链接时候的选项。再次又一次编译，推到手机上执行OK。。。pt_lock.c不须要做不论什么改动。

pthread学习

1.线程相关

1）回调函数类型 void* (*pfunc) (void* arg) 。參数通过void*传入，多个參数是通过结构体/数组打包。

2）线程属性，能够设置线程类型，通常是joinable。设置线程栈的大小。假设线程内部要定义大数组一定小心栈越界。

3）线程终止的几种情况：

• 回调函数执行完毕。正常退出
• 线程内调用pthread_exit函数。无论是否完毕都退出。该函数后面的全部代码都不会被执行（尤其printf等语句）。
• 其它线程显式调用pthread_cancel 结束当前线程。
• 整个进程被终止，由于调用了exit等函数。它以下的全部线程都被杀死。资源被回收。

• main函数创建N个线程。两种做法等待其它线程结束：显式pthread_join等待其它线程结束；main函数中调用pthread_exit，等待其它线程结束。

2. 相互排斥锁相关

pthread_mutex_init

pthread_mutex_lock。堵塞等待

pthread_mutex_trylock，非堵塞等待

pthread_mutex_destroy

3. 信号量相关

pthread_mutex_init

pthread_mutex_wait 堵塞当前线程，直到满足某些条件时。伪代码例如以下：

begin

pthread_mutex_unlock( mutex )  堵塞之前释放相互排斥锁

block_on_cond( condition ) 条件堵塞

pthread_mutex_lock(mutex) 条件满足后 占有相互排斥锁，完毕工作后须要释放相互排斥锁

end

该语句必须在循环中运行，而不是if中，由于线程之间可能有虚假的唤醒行为，spurious wakeup。当唤醒时须要再次确定预期的条件是否满足。假设不满足继续等待。

相关的讨论參见：http://a-shi.org/blog/2012/07/27/pthread_cond_wait_mutex_while/

pthread_mutex_signal，仅仅有一个线程等待时发送信号量

pthread_mutex_broadcast 多个线程同一时候等待时使用

pthread_mutex_destroy

4. 生存者/消费者，读写。哲学家进餐模型

http://www.cs.nmsu.edu/~jcook/Tools/pthreads/examples.html

refer

1. gcc以下的pie和fPIE 选项标志解说，http://richardustc.github.io/blog/2013/05/pie/

2. android L系统下PIE错误，http://blog.csdn.net/hxdanya/article/details/39371759

3. pthread http://man7.org/linux/man-pages/man3/pthread_create.3.html