$ pwd
/home/jhl/working/myapp/server
$ ls
main.out  Makefile  server.c  server.d  server.o
$
$ cat server.c
// server.c
// 1628872598@qq.com
// ref: The Linux Programming inTerface, sec 56.5, tlpi-disk/sockets/is_seqnum_sv.c

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <time.h>

#define PORTNO "12345"
#define BACKLOG 5

int main(void)
{
        struct addrinfo hints, *result, *rp;
        struct sockaddr addr;
        socklen_t addrlen;
        int lfd, afd, optval;
        int rt;
        int errsv = 0;
        char host[NI_MAXHOST];
        char service[NI_MAXSERV];
        char buf[128] = {'\0'};
        time_t tm;

        memset(&hints, 0, sizeof hints);
        hints.ai_canonname = NULL;
        hints.ai_addr = NULL;
        hints.ai_next = NULL;
        hints.ai_socktype = SOCK_STREAM;
        hints.ai_family = AF_UNSPEC;
        hints.ai_flags = AI_PASSIVE | AI_NUMERICSERV;

        rt = getaddrinfo(NULL, PORTNO, &hints, &result);
        errsv = errno;
        if (rt){
                printf("%s: %s\n", "getaddrinfo gai_strerror", gai_strerror(rt));
                if (errsv)
                        perror("getaddrinfo");
                return rt;
        }

        optval = 1;
        for (rp = result; rp; rp = rp->ai_next){
                lfd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
                if (lfd == -1)
                        continue;
                if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof optval) == -1){
                        perror("setsockopt");
                        return -1;
                }
                if (!bind(lfd, rp->ai_addr, rp->ai_addrlen))
                        break;
                close(lfd);
        }
        if (!rp){
                printf("Could not bind socket to any address\n");
                return !rp;
        }
        if (listen(lfd, BACKLOG) == -1){
                perror("listen");
                return -1;
        }
        freeaddrinfo(result);

        for (;;){
                addrlen = sizeof addr;
                afd = accept(lfd, &addr, &addrlen);
                if (afd == -1){
                        perror("accept");
                        continue;
                }
                if (!getnameinfo(&addr, addrlen, host, NI_MAXHOST, service, NI_MAXSERV, 0))
                        printf("Connection from %s %s\n", host, service);
                else
                        printf("Unknown %s %s\n", host, service);

                for (;;){
                        // recv
                        memset(buf, 0, sizeof buf);
                        rt = recv(afd, buf, sizeof buf - 1, MSG_DONTWAIT);
                        errsv = errno;
                        if (rt == -1){
                                printf("%s: %s\n", "recv", strerror(errsv));
                                if (errsv == ECONNREFUSED || errsv == ENOTCONN || errsv == ECONNRESET){
                                        close(afd);
                                        break;
                                }
                        }
                        if (rt == 0){
                                printf("%s: %s\n", "recv", "Peer has performed an orderly shutdown");
                                close(afd);
                                break;
                        }
                        if (strlen(buf))
                                printf("recv: client time: [%d] %s", strlen(buf), buf);
                        else
                                printf("[NODATA]\n");
                        fflush(stdout);

                        // send
                        time(&tm);
                        snprintf(buf, sizeof buf, "%s", ctime(&tm));
                        rt = send(afd, buf, strlen(buf), MSG_DONTWAIT);
                        errsv = errno;
                        if (rt == -1){
                                printf("%s: %s\n", "send", strerror(errsv));
                                if (errsv == ECONNRESET || errsv == ENOTCONN || errsv == EPIPE){
                                        close(afd);
                                        break;
                                }
                        }
                        sleep(1);
                }
        }
}
$


$ pwd
/home/jhl/working/myapp/client
$ ls
client.c  client.d  client.o  main.out  Makefile
$
$ cat client.c
// client.c
// 1628872598@qq.com
// ref: The Linux Programming inTerface sec 56.5, tlpi-dist/sockets/is_seqnum_cl.c

#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <time.h>

#define PORTNO "12345"

int main(int argc, char *argv[])
{
        struct addrinfo hints, *result, *rp;
        int fd;
        int rt;
        int errsv = 0;
        char buf[128] = {'\0'};
        time_t tm;

        if (argc < 2){
                printf("%s <host>\n", argv[0]);
                return -1;
        }

        memset(&hints, 0, sizeof hints);
        hints.ai_canonname = NULL;
        hints.ai_addr = NULL;
        hints.ai_next = NULL;
        hints.ai_family = AF_UNSPEC;
        hints.ai_socktype = SOCK_STREAM;
        hints.ai_flags = AI_PASSIVE | AI_NUMERICSERV;
        rt = getaddrinfo(argv[1], PORTNO, &hints, &result);
        errsv = errno;

        if (rt){
                printf("%s: %s\n", "getaddrinfo gai_strerror", gai_strerror(rt));
                if (errsv)
                        perror("getaddrinfo");
                return rt;
        }
        for (rp = result; rp; rp = rp->ai_next){
                fd = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
                if (fd == -1)
                        continue;
                if (connect(fd, rp->ai_addr, rp->ai_addrlen) != -1)
                        break;
                close(fd);
        }
        if (!rp){
                printf("Could not connect socket to any address\n");
                return !rp;
        }
        freeaddrinfo(result);

        for (;;){
                // send
                time(&tm);
                snprintf(buf, sizeof buf, "%s", ctime(&tm));
                rt = send(fd, buf, strlen(buf), MSG_DONTWAIT);
                errsv = errno;
                if (rt == -1){
                        printf("%s: %s\n", "send", strerror(errsv));
                        if (errsv == ECONNRESET || errsv == ENOTCONN || errsv == EPIPE){
                                close(fd);
                                break;
                        }
                }

                // recv
                memset(buf, 0, sizeof buf);
                rt = recv(fd, buf, sizeof buf - 1, MSG_DONTWAIT);
                errsv = errno;
                if (rt == -1){
                        printf("%s: %s\n", "recv", strerror(errsv));
                        if (errsv == ECONNREFUSED || errsv == ENOTCONN || errsv == ECONNRESET){
                                close(fd);
                                break;
                        }
                }
                if (rt == 0){
                        printf("%s: %s\n", "recv", "Peer has performed an orderly shutdown");
                        close(fd);
                        break;
                }
                if (strlen(buf))
                        printf("recv: server time: [%d] %s", strlen(buf), buf);
                else
                        printf("[NODATA]\n");
                fflush(stdout);
                sleep(1);
        }
        close(fd);
        return 0;
}
$
$ cat Makefile
# Visual Stupid Makefile 1628872598@qq.com
# Layout: [include/]  src/
# All .c and Makefile in src, .h can be in include. No other sub-directories.
# Project related: CPPFLAGS
# .cpp, CXX, CXXFLAGS for C++

OUT      = main.out
SRCS     = $(wildcard *.c)
OBJS     = $(patsubst %.c,%.o,$(SRCS))
CC       = gcc
CXX      = g++
CFLAGS   = -std=c99 -Wall -W
CXXFLAGS = -std=c++0x -Wall -W
CPPFLAGS = -I ../include
LDFLAGS  =

CFLAGS   += -g
#CFLAGS  += -DNDEBUG
CFLAGS   += -D_POSIX_SOURCE -D_BSD_SOURCE
LDFLAGS  +=

$(OUT): $(OBJS)
        $(CC) $(LDFLAGS) $^ -o $@

include $(SRCS:.c=.d)

%.d: %.c
        @set -e; rm -f $@; \
        $(CC) -M $(CPPFLAGS) $< > $@.$$$$; \
        sed 's,\($*\)\.o[ :]*,\1.o $@ : ,g' < $@.$$$$ > $@; \
        rm -f $@.$$$$

.PHONY  : clean
clean:
        rm *.d *.d.* $(OBJS) $(OUT)

$


Screenshot:

https://public.sn2.livefilestore.com/y1piEZTEe_epMlD_bItB4B4gOe6kZep4JDwJcGYSQL3GL0Q-CFgTQRIu_3-8ePa35JPCy5XzX2-M9iiFujXNP3RLA/server.png?psid=1

https://public.sn2.livefilestore.com/y1pF1dbaT4I33KHodiCwSUKq8G55C68Av2r7WsoXGEFOXeBmKnr1fHf1hyRvCQiQREDJIUbSrVhzA2gCnC52Xb5WA/client.png?psid=1