Linux系统调用与文件I/O(一)(2)

read函数:
  #include <unistd.h>
  ssize_t read(int feledes,void *buff,size_t nbytes);
 返回: 读到的字节数，若已到文件末尾则为0，若出错为-1

 write函数:
  #include <unistd.h>
  ssize_t write(int filedes,const void *buff,size_t nbytes);
 返回: 若成功为已写的字节数，若出错为-1
 对于普通文件，写操作从文件的当前位移量处开始。如果打开时制定了O_APPEND选择项，则在每次写操作之 

        前将文件位移量设置在文件的当前结尾处。在一次成功写之后，该文件位移量增加实际写的字节数
 Example: write.c

/*write.c*/
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define MAXSIZE

int main(void)
{
    int i,fd,size,len;
    char *buf="Hello! I'm writing to this file!";
    char buf_r[10];
    
    len = strlen(buf);
    buf_r[10] = '\0';
    if((fd = open("/tmp/hello.c", O_CREAT | O_TRUNC | O_RDWR,0666 ))<0)
    {
        perror("open:");
        exit(1);
    }
    else
    {
        printf("open file:hello.c %d\n",fd);
    }

    if((size = write( fd, buf, len)) < 0)
    {
        perror("write:");
        exit(1);
    }
    else
    {
        printf("Write:%s\n",buf);
    }

    lseek( fd, 0, SEEK_SET ); 
    if((size = read( fd, buf_r, 10))<0)
    { 
        perror("read:");
        exit(1);
    }
    else
    {
        printf("read form file:%s\n",buf_r);
    }

    if( close(fd) < 0 )
    {
        perror("close:");
        exit(1);
    }
    else
    {
        printf("Close hello.c\n");
    }

    exit(0);
}