使用lstat()判断文件类型

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>

#define _GNU_SOURCE

int main (int argc, char *argv[]){

    struct stat buf;

    for(int i = 1; i < argc; i++){
        if(lstat(argv[i], &buf) < 0){
            printf("lstat invoke error.");
            continue;
        }
        printf("%s is a ", argv[i]);
        if (S_ISREG(buf.st_mode)){
            printf("regular file.");
        }else if(S_ISDIR(buf.st_mode)){
            printf("direcotry.");
        }else if(S_ISBLK(buf.st_mode)){
            printf("block device.");
        }else if(S_ISCHR(buf.st_mode)){
            printf("character device.");
        }else if(S_ISFIFO(buf.st_mode)){
            printf("FIFO.");
        }else if(S_ISLNK(buf.st_mode)){
            printf("soft link.");
        }else if(S_ISSOCK(buf.st_mode)){
            printf("socket.");
        }else
            printf("Unkown type.");
     }

        printf("\n");

        exit(0);
}

编译时出现以下错误：

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？