文件操作--系统调用： dup. lseek

dup

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

int main1()
{
	int fd = open("test", O_WRONLY|O_CREAT, 0766);
	if (-1 == fd)
	{
		perror("打开tmp.txt文件失败");
		return -1;
	}
	
	int newfd = open("test", O_WRONLY|O_CREAT, 0766);
	if (-1 == newfd)
	{
		perror("打开tmp.txt文件失败");
		return -1;
	}
	
	newfd = dup(fd);  //同步偏移量，此时newfd的偏移量等同fd
	
	printf ("fd    = %d\n", fd);
	printf ("newfd = %d\n", newfd);
	
	write(fd, "hello", 5);
	write(newfd, "world", 5);    //此时test显示为helloworld
	

	close(fd);
	return 0;
}

int main()
{
	int fd = open("test", O_WRONLY|O_CREAT, 0766);
	if (-1 == fd)
	{
		perror("打开tmp.txt文件失败");
		return -1;
	}
	
	int newfd = open("test1", O_WRONLY|O_CREAT, 0766);
	if (-1 == newfd)
	{
		perror("打开tmp.txt文件失败");
		return -1;
	}
	
	// 将文件描述符表中指针部分 复制一份给 newfd
	// 如果复制成功，则会将 原来 newfd 打开的文件关闭
	dup2(fd, newfd);
	
	printf ("fd    = %d\n", fd);        //fd 和 newfd 的值时不同的
	printf ("newfd = %d\n", newfd);
	
	write(fd, "hello", 5);      
	write(newfd, "world", 5);           //test显示为helloworld
	

	close(fd);
	return 0;
}

lseek

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

#define SIZE 1024*1024*40

int main1()
{
	int fd = open("1.ppt", O_RDONLY);
	if (-1 == fd)
	{
		perror("打开tmp.txt文件失败");
		return -1;
	}

	// 获取文件大小
	off_t ret = lseek(fd, 0, SEEK_END);
	
	printf ("文件大小：%ld\n", ret);
		
	close(fd);
	return 0;
}

int main2()
{
	int fd = open("tmp.txt", O_WRONLY|O_CREAT |O_TRUNC, 0766);
	if (-1 == fd)
	{
		perror("打开tmp.txt文件失败");
		return -1;
	}
	
	write(fd, "hello", 5);
	
	off_t ret = lseek(fd, 3, SEEK_SET);
	
	write(fd, "world", 5);
		
	close(fd);
	return 0;
}

// 创建大文件
int main3()
{
	int fd = open("big", O_WRONLY|O_CREAT, 0766);
	if (-1 == fd)
	{
		perror("打开tmp.txt文件失败");
		return -1;
	}
	
	off_t ret = lseek(fd, SIZE-1, SEEK_SET);
	write(fd, "a", 1);
	
	printf ("ret：%ld\n", ret);

	close(fd);
	return 0;
}



// 创建大文件
int main()
{
	truncate("aa.txt", SIZE);
	return 0;
}