文件读取操作

读取文件手册

 

函数定义：ssize_t read(int fd, void * buf, size_t count);

函数说明：read()会把参数fd所指的文件传送count 个字节到buf 指针所指的内存中。

返回值：返回值为实际读取到的字节数, 如果返回0, 表示已到达文件尾或是无可读取的数据。若参数count 为0, 则read()不会有作用并返回0。另外，以下情况返回值小于count。

代码实现 

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
int main()
{
	int fd;
	char *buf = "chenglong henshuai?";

	fd = open("./file1",O_RDWR);
	
	if(fd == -1){
		printf("open file1 failed\n");
		fd = open("./file1",O_RDWR|O_CREAT,0600);
		if(fd > 0){
			printf("create file1\n");
		}
	}
	printf("fd = %d\n",fd);
	
	int n_write = write(fd,buf,strlen(buf));
//	write(fd,buf,sizeof(buf));
	
	if(n_write != -1){    //判断是否写入成功，失败返回-1
		printf("write %d byte to file1\n",n_write);
	}	

	char *readBuf;    //野指针
	readBuf = (char *)malloc(sizeof(char )*n_write + 1);	//读取开辟写入+1的大小，强转成char型指针

	int n_read = read(fd,readBuf,n_write);
	
	printf("read %d,context:%s\n",n_read,readBuf);
	
	close(fd);

	return 0;
}

fd = 3
write 19 byte to file1
read 0,context:

问题发现：无法读取文件（光标的问题）

光标位置在chenglong henshuai?问号后面，读取的是光标后面的空内容。

解决方法：1.重新打开文件（老土法）   2.光标移动到头头（好方法）

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
int main()
{
	int fd;
	char *buf = "chenglong henshuai?";

	fd = open("./file1",O_RDWR);
	
	if(fd == -1){
		printf("open file1 failed\n");
		fd = open("./file1",O_RDWR|O_CREAT,0600);
		if(fd > 0){
			printf("create file1\n");
		}
	}
	printf("fd = %d\n",fd);
	
	int n_write = write(fd,buf,strlen(buf));
//	write(fd,buf,sizeof(buf));
	
	if(n_write != -1){
		printf("write %d byte to file1\n",n_write);
	}	

	close(fd);
	fd = open("./file1",O_RDWR);//写完后关闭文件，重新打开

	char *readBuf;
	readBuf = (char *)malloc(sizeof(char )*n_write + 1);	

	int n_read = read(fd,readBuf,n_write);
	
	printf("read %d,context:%s\n",n_read,readBuf);
	
	close(fd);

	return 0;
}

结果：

fd = 3
write 19 byte to file1
read 19,context:chenglong henshuai?