threads 2-局部变量和全局变量

本文主要是介绍threads 2-局部变量和全局变量，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

main里面定义的变量都是属于主线程的
主线程创建的子线程不能访问之
需用全局变量进行线程间通信

[root@localhost ch12]# cat test4.c
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>void* thread_function(void *arg);
void *thread_result;char *str="hello";
int flag=0;
int main()
{
puts(str);
int res;
pthread_t a_thread;
res=pthread_create(&a_thread,NULL,&thread_function,NULL);
if(res!=0){perror("pthread_create error");exit(EXIT_FAILURE);		}
printf("already create a thread\n");res = pthread_join(a_thread, &thread_result);
puts(str);
}void* thread_function(void *arg)
{
str="hi";
puts(str);
pthread_exit("thanks");
}[root@localhost ch12]# 

[root@localhost ch12]# make test4
cc -D_REENTRANT  -lpthread  test4.c   -o test4
[root@localhost ch12]# ./test4
hello
already create a thread
hi
hi

如果将line 11 ,char *str="hello";放到main里面
则编译不会通过

[root@localhost ch12]# make test4
cc -D_REENTRANT  -lpthread  test4.c   -o test4
test4.c: In function ‘thread_function’:
test4.c:31:1: error: ‘str’ undeclared (first use in this function)
test4.c:31:1: note: each undeclared identifier is reported only once for each function it appears in
make: *** [test4] Error 1

局部变量虽然不可以如此明目张胆的通信---被编译器揪出来le
但可冒充pthread_create的参数来进行父子线程的数据交换,如下

[root@localhost ch12]# cat test4.c 
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>void* thread_function(void *arg);
void *thread_result;int flag=0;
int main()
{
char str[]="hello";
puts(str);
int res;
pthread_t a_thread;
res=pthread_create(&a_thread,NULL,&thread_function,str);
if(res!=0){perror("pthread_create error");exit(EXIT_FAILURE);		}
printf("already create a thread\n");res = pthread_join(a_thread, &thread_result);
puts(str);
}void* thread_function(void *arg)
{
strcpy(arg, "hi");
puts(arg);
pthread_exit("thanks");
}[root@localhost ch12]# make test4
cc -D_REENTRANT  -lpthread  test4.c   -o test4
[root@localhost ch12]# ./test4
hello
already create a thread
hi
hi

定义主线程的局部变量str，充当pthread_create的参数传递给子线程，传的是地址
，以至于在子线程中修改了str，主线程中也可得知
也可单向传递，主线程-->子线程，即传值,如下

[root@localhost ch12]# cat test4.c
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>void* thread_function(void *arg);
void *thread_result;int flag=0;
int main()
{
char *str="hello";
puts(str);
int res;
pthread_t a_thread;
res=pthread_create(&a_thread,NULL,&thread_function,str);
if(res!=0){perror("pthread_create error");exit(EXIT_FAILURE);		}
printf("already create a thread\n");res = pthread_join(a_thread, &thread_result);
puts(str);
}void* thread_function(void *arg)
{
arg="hi";
puts(arg);
pthread_exit("thanks");
}[root@localhost ch12]# make test4
cc -D_REENTRANT  -lpthread  test4.c   -o test4
[root@localhost ch12]# ./test4
hello
already create a thread
hi
hello

可见在主线程中第二次打印的str仍然是hello
下面是pthread_create的原型

[root@localhost ch12]# man 3 pthread_create

       #include <pthread.h>int pthread_create(pthread_t *thread, const pthread_attr_t *attr,void *(*start_routine) (void *), void *arg);

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