1，思维导图
2，创建子父

#include<myhead.h>
int main(int argc, const char *argv[])
{
	pid_t pid;
	pid=fork();
	if(pid>0)
	{
		int fp3=open("./3.txt",O_RDONLY);
		int fp4=open("./4.txt",O_CREAT|O_TRUNC|O_WRONLY,0664);
		char str1[5];
		int count1=0;
		while(count1=read(fp3,str1,sizeof(str1)))
		{
			if(write(fp4,str1,count1)!=count1)
			{
				break;
			}
		}
		close(fp3);
		close(fp4);
	}
	else if(pid==0)
	{
		int fp1=open("./1.txt",O_RDONLY);
		int fp2=open("./2.txt",O_CREAT|O_TRUNC|O_WRONLY,0664);
		char str2[5];
		int count2=0;
		while(count2=read(fp1,str2,sizeof(str2)))
		{
			if(write(fp2,str2,count2)!=count2)
			{
				break;
			}
		}
		close(fp1);
		close(fp2);
	}
	return 0;
}

进程，子进程实现1.txt复制2.txt，父进程实现3.txt复制4.txt