一道关于文件的面试题

/*设有文件a.file, 请写出程序在文件的开头插入 字符串 “ file start: ”；*/#include <stdio.h>#include <stdlib.h>#include <string.h>int main(){int len;char ch;char str[1024] = "file start:";FILE *fp;len = strlen(str);fp = fopen("a.file","r+");if (fp == NULL){printf("打开文件失败!\n");exit(0);}while((ch = getc(fp)) != EOF){str[len] = ch;len++;}if (!fseek(fp,0,SEEK_SET)){for (len = 0; len<(int)strlen(str); len++){fputc(str[len],fp);}}fclose(fp);return 0;}