#include <arpa/inet.h>
uint64_t htonll(uint64_t hostlonglong);
uint32_t htonl(uint32_t hostlong);
uint16_t htons(uint16_t hostshort);
uint64_t ntohll(uint64_t netlonglong);
uint32_t ntohl(uint32_t netlong);
uint16_t ntohs(uint16_t netshort);

#include <arpa/inet.h>
#include <stdio.h>
int main()
{
 uint32_t host = 0x01020304; // high->low: 01 02 03 04
 uint32_t network = htonl(host); // high->low: 04 03 02 01
 printf("%p\n", network); // 0x4030201
}

#include <sys/socket.h>
int socket(int domain, int type, int protocol);

Name  Purpose    Man page
AF_UNIX, AF_LOCAL Local communication  unix(7)
AF_INET  IPv4 Internet protocols  ip(7)
AF_INET6  IPv6 Internet protocols  ipv6(7)
AF_IPX  IPX - Novell protocols
AF_NETLINK  Kernel user interface device netlink(7)
AF_X25  ITU-T X.25 / ISO-8208 protocol x25(7)
AF_AX25  Amateur radio AX.25 protocol
AF_ATMPVC  Access to raw ATM PVCs
AF_APPLETALK AppleTalk   ddp(7)
AF_PACKET  Low level packet interface packet(7)
AF_ALG  Interface to kernel crypto API

SOCK_STREAM Provides sequenced, reliable, two-way, connection-based byte streams. An out-of-band data transmission mechanism may be supported.

SOCK_DGRAM Supports datagrams (connectionless, unreliable messages of a fixed maximum length).

SOCK_SEQPACKET Provides a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length; a consumer is required to read an entire packet with each input system call.

SOCK_RAW Provides raw network protocol access.

SOCK_RDM Provides a reliable datagram layer that does not guarantee ordering.

SOCK_PACKET Obsolete and should not be used in new programs; see packet(7).

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

struct sockaddr {
 sa_family_t sa_family;
 char sa_data[14];
};
// sa_familt_t 是無(wú)符號(hào)整型，Ubuntu 下是 unsigned short int

struct sockaddr_in {
 short int sin_family; /* Address family */
 unsigned short int sin_port; /* Port number */
 struct in_addr sin_addr; /* Internet address */
 unsigned char sin_zero[8]; /* Same size as struct sockaddr */
};

typedef uint32_t in_addr_t;
struct in_addr{
 in_addr_t s_addr;
};

int listen(int sockfd, int backlog);

int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

in_addr_t inet_addr(const char *cp);

int main()
{
 const char *ip = "127.0.0.1"; // 7f.00.00.01
 printf("%p\n", inet_addr(ip)); // 0x0100007f
}

#include <sys/types.h>
#include <sys/socket.h>
ssize_t send(int sockfd, const void *buf, size_t len, int flags);
ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,
  const struct sockaddr *dest_addr, socklen_t addrlen);
ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);

#include <sys/types.h>
#include <sys/socket.h>
ssize_t recv(int sockfd, void *buf, size_t len, int flags);
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
   struct sockaddr *src_addr, socklen_t *addrlen);
ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);

int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

#include <stdlib.h>
#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <pthread.h>
#include <string.h>
#include <unistd.h>
#define PORT 8887
#define QUEUE 10
const char *pattern = "Hello, I am the server. Your msg is received, which is: %s";

typedef struct
{
 struct sockaddr_in addr;
 socklen_t addr_len;
 int connectfd;
} thread_args;

void *handle_thread(void *arg)
{
 thread_args *targs = (thread_args *)arg;
 pthread_t tid = pthread_self();
 printf("tid = %u and socket = %d\n", tid, targs->connectfd);
 char send_buf[BUFSIZ] = {0}, recv_buf[BUFSIZ] = {0};
 while (1)
 {
 int len = recv(targs->connectfd, recv_buf, BUFSIZ, 0);
 printf("[Client %d] %s", targs->connectfd, recv_buf);
 
 if (strcmp("q\n", recv_buf) == 0)
  break;
 
 sprintf(send_buf, pattern, recv_buf);
 send(targs->connectfd, send_buf, strlen(send_buf), 0);

 memset(send_buf, 0, BUFSIZ), memset(recv_buf, 0, BUFSIZ);
 }
 close(targs->connectfd);
 free(targs);
 pthread_exit(NULL);
}

int main()
{
 int listenfd = socket(AF_INET, SOCK_STREAM, 0);
 printf("server is listening at socket fd = %d\n", listenfd);
 struct sockaddr_in addr;
 addr.sin_family = AF_INET;
 addr.sin_port = htons(PORT);
 addr.sin_addr.s_addr = htonl(INADDR_ANY);

 if (bind(listenfd, (struct sockaddr *)&addr, sizeof(addr)) == -1)
 {
 perror("bind error\n");
 exit(-1);
 }

 if (listen(listenfd, QUEUE) == -1)
 {
 perror("listen error\n");
 exit(-1);
 }

 while (1)
 {
 thread_args *targs = malloc(sizeof(thread_args));
 targs->connectfd = accept(listenfd, (struct sockaddr *)&targs->addr, &targs->addr_len);
 // int newfd = accept(sockfd, NULL, NULL);
 pthread_t tid;
 pthread_create(&tid, NULL, handle_thread, (void *)targs);
 pthread_detach(tid);
 }
 close(listenfd);
}

#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define PORT 8887
const char *target_ip = "127.0.0.1";

int main()
{
 int sockfd = socket(AF_INET, SOCK_STREAM, 0);
 printf("client socket = %d\n", sockfd);
 struct sockaddr_in addr;
 addr.sin_family = AF_INET;
 addr.sin_port = htons(PORT);
 addr.sin_addr.s_addr = inet_addr(target_ip);

 if (connect(sockfd, (struct sockaddr *)&addr, sizeof(struct sockaddr_in)) < 0)
 {
 perror("connect error\n");
 exit(-1);
 }

 char send_buf[BUFSIZ], recv_buf[BUFSIZ];
 while (fgets(send_buf, BUFSIZ, stdin) != NULL)
 {
 if (strcmp(send_buf, "q\n") == 0)
  break;

 send(sockfd, send_buf, strlen(send_buf), 0);
 printf("[Client] %s\n", send_buf);

 recv(sockfd, recv_buf, BUFSIZ, 0);
 printf("[Server] %s\n", recv_buf);

 memset(send_buf, 0, BUFSIZ), memset(recv_buf, 0, BUFSIZ);
 }
 close(sockfd);
 exit(0);
}