在LINUX，就是IO的输出与输入的问题

先把代码放在这里了，我是看视频学习的，有个问题不是很明白

#include "stdio.h"

#include "stdlib.h"

#include "sys/types.h"

#include "sys/stat.h"

#include "fcntl.h"

#include "string.h"

#include "unistd.h"

int main(int argc,char *argv[])

{

        if(argv<2)

        {

                printf("please input filename!!!\n");

                exit(1);

        }

        else

        {

                    int fd;

                    umask(0000);

                    fd=open(argv[1],O_RDWR|O_CREAT,0666);

                    if(fd<0)

                    {

                            printf("error\n");

                            exit(1);

                    }else

                    {

                        printf("success =%d\n",fd);

                        char     buf[1024];

                        memset(buf,0,1024);

                        int returnum=read(fd,buf,1024);

                        if(returnum!=-1)

                        {

                                printf("buf=%s\n",buf);

                        }else

                        {

                                printf("error\n");

                        }

                        close(fd);

                        printf("closed\n");

                    }

        }

        return 0;

}

//上面是源代码，我就有一个问题搞不明白，就是我手动新建了一个文件比如example.txt文件，我想要读取里面的字符，但是错误，在运行时创建又是可以的，创建好后自己往文件里写字符就能读出来