if(strcmp(data,"this")==0)   《--这里的  0是 什么意思？
fseek(fp,-(long)strlen("this"),SEEK_CUR）《-- 这里的  long是什么意思？

#include<stdio.h>
#include<string.h>

int main()
{
    FILE *fp=fopen("a.txt","r+");
    char data[100];

    if(fp==NULL)
    {
        printf("no data \n");
        return 0;
    }

    while((fscanf(fp,"%s",data))!=EOF)
    {
        if(strcmp(data,"this")==0)
        {
            fseek(fp,-(long)strlen("this"),SEEK_CUR);
            fputs("that",fp);

            fflush(fp);

        }
    }

    fclose(fp);

}