root@VM-16-14-ubuntu:~/note# gcc symlinkat.c -o symlinkat
symlinkat.c: In function ‘main’:
symlinkat.c:8:19: warning: implicit declaration of function ‘open’; did you mean ‘popen’? [-Wimplicit-function-declaration]
8 | if ((fd = open("/root/test", 0)) < 0)
| ^~~~
| popen#加入头文件 fcntl.h
root@VM-16-14-ubuntu:~/note# cat symlinkat.c
#include <stdio.h>
#include "apue.h"
#include "fcntl.h"
root@VM-16-14-ubuntu:~/note# gcc symlinkat.c -o symlinkat
root@VM-16-14-ubuntu:~/note#