ex)delNo==1
fseek(delNo+1)  1)AB[C]DE
while(fread(1))   2)ABC[D]E rec==C
fseek(-2)      3)A[B]CDE
fwrite(1)      4)AC[C]DE
while(fread(1))   5)ACC[D]E rec==C
fseek(-2)      6)A[C]CDE
fwrite(1)      →4へ戻る