关于 c:linux clone() 返回 -1 作为 child_pid

linux clone() returning -1 as child_pid

我有以下程序：

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#define _GNU_SOURCE
#include<sched.h>
#include<stdio.h>
#include<stdlib.h>
#include<sys/wait.h>
#include<unistd.h>

static char child_stack[2048];

static int child_fn() {
printf("Pid: %ld\
", (long) getpid());
return 0;
}

int main() {
pid_t child_pid = clone(child_fn, child_stack+1024, CLONE_NEWPID | CLONE_NEWNS | SIGCHLD, NULL);
printf("clone()= %ld\
", (long) child_pid);

waitpid(child_pid, NULL, 0);
return 0;
}

我得到以下输出

1 2	# ./clone clone()= -1

谁能帮我理解为什么会这样？

Note: I'm running this under a docker image with ubuntu:latest. Also notice that the prompt is # instead of $, so does it have anything to do with the user who's running this process, probably root in this case?