1、使用两个线程完成两个文件的拷贝，分支线程1拷贝前一半，分支线程2拷贝后一半，主线程回收两个分支线程的资源

#include <myhead.h>

int copy_file(const char *srcfile,const char *destfile,int start,int len);

struct Buff
{
	const char *srcfile;
	const char *destfile;
	int len;
	int flag;
};


void *task(void *arg)
{
	struct Buff *buff = (struct Buff *)arg;
	if(buff->flag == 1)
	{
		copy_file(buff->srcfile,buff->destfile,0,buff->len/2);
	}else if (buff->flag == 2)
	{
		copy_file(buff->srcfile,buff->destfile,buff->len/2,buff->len-buff->len/2);
	}
}

int get_file_len(const char *srcfile,const char *destfile)
{
	//以只读打开源文件
	int sfd = open(srcfile,O_RDONLY);
	if(sfd == -1)
	{
		perror("open srcfile error");
		return -1;
	}

	//以创建的形式打开目标文件	
	int dfd = open(destfile,O_WRONLY|O_CREAT|O_TRUNC,0664);
	if(dfd == -1)
	{
		perror("open destfile error");
		return -1;
	}

	int size = lseek(sfd,0,SEEK_END);

	//关闭文件
	close(sfd);
	close(dfd);

	return size;
}

int copy_file(const char *srcfile,const char *destfile,int start,int len)
{
	//以只读的形式打开源文件
	int sfd = open(srcfile,O_RDONLY);
	if(sfd == -1)
	{
		perror("open srcfile error");
		return -1;
	}

	int dfd = open(destfile,O_WRONLY);
	if(dfd == -1)
	{
		perror("open destfile error");
		return -1;
	}

	lseek(sfd,start,SEEK_SET);
	lseek(dfd,start,SEEK_SET);

	char buf[128]="";
	int sum =0;

	while(1)
	{
		//从源文件中读取数据
		int res = read(sfd,buf,sizeof(buf));
		sum += res;

		if(res==0||sum>len)
		{
			write(dfd,buf,res-(sum-len));
			break;
		}

		write(dfd,buf,res);

	}
	close(sfd);
	close(dfd);
	
	printf("拷贝成功\n");
}

int main(int argc, const char *argv[])
{
	if(argc != 3)     //判断是否传入两个文件
	{
		printf("input file error\n");
		return -1;
	}

	const char *srcfile = argv[1];
	const char *destfile = argv[2];
	
	int len = get_file_len(srcfile,destfile);   //求出源文件的长度
	struct Buff buff1={srcfile,destfile,len,1};
	struct Buff buff2={srcfile,destfile,len,2};
	//定义两个线程变量
	pthread_t tid1,tid2;
	//创建两个线程
	if((pthread_create(&tid1,NULL,task,&buff1)) != 0)
	{
		printf("tid1 创建失败\n");
		return -1;
	}
	if((pthread_create(&tid2,NULL,task,&buff2)) != 0)
	{
		printf("tid2 创建失败\n");
		return -1;
	}
	

	//阻塞等两个线程的回收
	pthread_join(tid1,NULL);
	pthread_join(tid2,NULL);
	return 0;	
}