以Append方式打开文件，设置偏移量无效

 1 #include<stdio.h>
 2 
 3 int main()
 4 {
 5     FILE * fd = fopen("btoo1.c", "ab+");
 6     fpos_t p ; 
 7     int fp = fgetpos(fd, &p);
 8     printf("bef seek: fgetpos = %ld, ftell = %d\n", p, ftell(fd));
 9     fseek(fd, 12, SEEK_SET);
10     fgetpos(fd, &p);
11     printf("bef seek: fgetpos = %ld, ftell = %d\n", p, ftell(fd));
12     fwrite("*****", 1, 5, fd);
13     fclose(fd);
14     return 0;
15 }