为什么在linux下编写代码，可以直接包含系统头文件
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <assert.h>
#include <stdio.h>
#include <string.h>
static bool stop = false;
static void handle_term(int sig)
{
stop = true;
}
int main(int argc, char *argv[])
{
signal(SIGTERM, handle_term);
if (argc <= 3)
{
printf("usage: %s ip_address port_number backlog\n", basename(argv[0]));
return 1;
}
const char *ip = argv[1];
int port = atoi(argv[2]);
int backlog = atoi(argv[3]);
int sock = socket(PF_INET, SOCK_STREAM, 0);
assert(sock >= 0);
struct sockaddr_in address;
bzero(&address, sizeof(address));
address.sin_family = AF_INET;
inet_pton(AF_INET, ip, &address.sin_addr);
address.sin_port = htons(port);
int ret = bind(sock, (struct sockaddr*)&address, sizeof(address));
assert(ret != -1);
ret = listen(sock, backlog);
assert(ret != -1);
while (!stop)
{
sleep(1);
}
close(sock);
return 0;
}直接用g++可以编译成功，但是我用env查看环境变量发现没有包含/usr/include这个路径呀？一些头文件不是在这个文件夹下的吗？请问为什么可以编译成功呢，怎么找到这些头文件的呢？
