我在/home/bbs/src/lib目錄裡，執行 make 出現下面情形
gcc -O2 -s -pipe -fomit-frame-pointer -Wunused -I../include -c f_map.c
f_map.c: In function `f_map':
f_map.c:31: `NULL' undeclared (first use in this function)
f_map.c:31: (Each undeclared identifier is reported only once
f_map.c:31: for each function it appears in.)
make: *** [f_map.o] Error 1
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^