#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
int main()
{
    int sockfd;

    if ((sockfd=socket(AF_INET,SOCK_STREAM,0))<0)
        err_sys("can't create socket");

    close(sockfd);
    return 0;
}

我收到链接器错误：

/tmp/cciuUVhz.o: In function `main':
getsockopt.c:(.text+0x38): undefined reference to `err_sys'
collect2: ld returned 1 exit status

函数err_sys()在哪里定义？