用gcc -g 編譯結果: Simple_list.c:2: warning: "NULL" redefined /usr/lib/gcc/i686-linux-gnu/4.4.5/include/stddef.h:400: note: this is the location of the previous definition Simple_list.c: In function 『main』: Simple_list.c:31: warning: incompatible implicit declaration of built-in function 『printf』 Simple_list.c:31: warning: format 『%S』 expects type 『wchar_t *』, but argument 2 has type 『char *』 Simple_list.c:32: error: 『P』 undeclared (first use in this function) Simple_list.c:32: error: (Each undeclared identifier is reported only once Simple_list.c:32: error: for each function it appears in.)