使用lstat()判断文件类型

filetype.c: In function ‘main’:filetype.c:14:9: warning: implicit declaration of function ‘lstat’ [-Wimplicit-function-declaration]filetype.c:31:9: warning: implicit declaration of function ‘S_ISSOCK’ [-Wimplicit-function-declaration]/tmp/ccLbzony.o: In function `main':filetype.c:(.text+0x182): undefined reference to `S_ISSOCK'collect2: error: ld returned 1 exit status

第一个不warning不影响运行，第二个又是怎么回事？