Basic shared memory program in C - c

Question

I want to make a basic chat application in C using Shared memory. I am working in Linux. The application consist in writing the client and the server can read, and if the server write the client can read the message.
I tried to do this, but I can't achieve the communication between client and server. The code is the following:
Server.c
int
main(int argc, char **argv)
{
char *msg;
static char buf[SIZE];
int n;
msg = getmem();
memset(msg, 0, SIZE);
initmutex();
while ( true )
{
if( (n = read(0, buf, sizeof buf)) > 0 )
{
enter();
sprintf(msg, "%.*s", n, buf);
printf("Servidor escribe: %s", msg);
leave();
}else{
enter();
if ( strcmp(buf, msg) )
{
printf("Servidor lee: %s", msg);
strcpy(buf, msg);
}
leave();
sleep(1);
}
}
return 0;
}
Client.c
int
main(int argc, char **argv)
{
char *msg;
static char buf[SIZE-1];
int n;
msg = getmem();
initmutex();
while(true)
{
if ( (n = read(0, buf, sizeof buf)) > 0 )
{
enter();
sprintf(msg, "%.*s", n, buf);
printf("Cliente escribe: %s", msg);
leave();
}else{
enter();
if ( strcmp(buf, msg) )
{
printf("Cliente lee: %s", msg);
strcpy(buf, msg);
}
leave();
sleep(1);
}
}
printf("Cliente termina\n");
return 0;
}
The shared memory module is the folowing:
#include "common.h"
void
fatal(char *s)
{
perror(s);
exit(1);
}
char *
getmem(void)
{
int fd;
char *mem;
if ( (fd = shm_open("/message", O_RDWR|O_CREAT, 0666)) == -1 )
fatal("sh_open");
ftruncate(fd, SIZE);
if ( !(mem = mmap(NULL, SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)) )
fatal("mmap");
close(fd);
return mem;
}
static sem_t *sd;
void
initmutex(void)
{
if ( !(sd = sem_open("/mutex", O_RDWR|O_CREAT, 0666, 1)) )
fatal("sem_open");
}
void
enter(void)
{
sem_wait(sd);
}
void
leave(void)
{
sem_post(sd);
}

Answer 1

By the time you'll have this working right, you'll have re-invented a pipe. A socket is the better solution, that will also cover the more common scenario where the server and the client are not running on the same machine.

Answer 2

/* client */
#include <stdio.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#define SIZEOFSHMSEG 50 /* Size of the shared mem segment */
#include<stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include<fcntl.h>
#define PORT 6002
int main()
{ char buffer[512];
int rc, shmid, i,l,choice,n;
void *shm_address;
struct shmid_ds shmid_struct;
char c,ch;
char HOST[30];
char hostname[100];
int sd;
struct sockaddr_in sin;
struct sockaddr_in pin;
struct hostent *hp;
printf("enter the server's ip address:");
scanf("%s",HOST);
n=strlen(HOST);
HOST[n]='\0';
if ((sd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
perror("socket");
exit(1);
}
/* fill in the socket structure with host information */
memset(&pin, 0, sizeof(pin));
pin.sin_family = AF_INET;
pin.sin_addr.s_addr = inet_addr(HOST);
pin.sin_port = htons(PORT);
/* connect to PORT on HOST */
if (connect(sd,(struct sockaddr *) &pin, sizeof(pin)) == -1) {
perror("connect");
exit(1);
}
/* shared memory setup */
shmid = shmget(2222, SIZEOFSHMSEG, 0666 | IPC_CREAT | IPC_EXCL);
if (shmid == -1)
{
printf("main: shmget() failed\n");
return -1;
}
/* Attach the shared memory segment to the server process. */
shm_address = shmat(shmid, NULL, 0);
if ( shm_address==NULL )
{
printf("main: shmat() failed\n");
return -1;
}
fcntl(sd, F_SETFL, O_NONBLOCK);
buffer[0]='\0';
strcpy((char *)shm_address,buffer);
for(;;)
{
printf("----------------------------------------------------------\n");
printf("1.read from shared mem\n2.write to shared area\n3.exit\n");
printf("ur choice:");
scanf("%d",&choice);
n = read(sd, buffer, 512);
buffer[n] = '\0';
strcpy((char *) shm_address, buffer);
if(choice==1)
{ strcpy(buffer,(char *)shm_address);
printf("display:%s\n",buffer);
}
if(choice==2)
{ printf("enter the data:");
scanf(" %[^\n]",buffer);
n=strlen(buffer);
buffer[n]='\0';
strcpy((char *) shm_address, buffer);
write(sd, buffer, strlen(buffer));
}
if(choice==3)
{ printf("terminated\n");
break;
}
printf("----------------------------------------------------------\n");
}
close(sd);
rc = shmdt(shm_address);
if (rc==-1)
{
printf("main: shmdt() failed\n");
return -1;
}
rc = shmctl(shmid, IPC_RMID, &shmid_struct);
if (rc==-1)
{
printf("main: shmctl() failed\n");
return -1;
}
return 0;
}

Answer 3

/* server */
#include <stdio.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#define SIZEOFSHMSEG 50 /* Size of the shared mem segment */
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include<fcntl.h>
#define PORT 6002
int main()
{ char buffer[512];
int rc, shmid, i,l,choice,n;
void * shm_address;
struct shmid_ds shmid_struct;
char c;
int sd, sd_current, cc;
int addrlen;
struct sockaddr_in sin;
struct sockaddr_in pin;
/* get an internet domain socket */
if ((sd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
perror("socket");
exit(1);
}
/* complete the socket structure */
memset(&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = htonl(INADDR_ANY);
sin.sin_port = htons(PORT);
/* bind the socket to the port number */
if (bind(sd, (struct sockaddr *) &sin, sizeof(sin)) == -1) {
perror("bind");
exit(1);
}
/* show that we are willing to listen */
if (listen(sd, 5) == -1) {
perror("listen");
exit(1);
}
printf("waiting for client....\n");
addrlen = sizeof(pin);
if ((sd_current = accept(sd, (struct sockaddr *) &pin, &addrlen)) == -1) {
perror("accept");
exit(1);
}
printf("connected..\n");
/* shared memory setup */
shmid = shmget(1111, SIZEOFSHMSEG, 0666 | IPC_CREAT | IPC_EXCL);
if (shmid == -1)
{
printf("main: shmget() failed\n");
return -1;
}
/* Attach the shared memory segment to the server process. */
shm_address = shmat(shmid, NULL, 0);
if ( shm_address==NULL )
{
printf("main: shmat() failed\n");
return -1;
}
buffer[0]='\0';
strcpy((char *)shm_address,buffer);
fcntl(sd_current, F_SETFL, O_NONBLOCK);
for(;;)
{
printf("----------------------------------------------------------\n");
printf("1.read from shared mem\n2.write to shared area\n3.exit\n");
printf("ur choice:");
scanf("%d",&choice);
n=read(sd_current, buffer, 512);
buffer[n] = '\0';
strcpy((char *) shm_address, buffer);
if(choice==1)
{ strcpy(buffer,(char *)shm_address);
printf("display:%s\n",buffer);
}
if(choice==2)
{ printf("enter the data:");
scanf(" %[^\n]",buffer);
n=strlen(buffer);
buffer[n]='\0';
strcpy((char *) shm_address, buffer);
write(sd_current, buffer, strlen(buffer));
}
if(choice==3)
{ printf("terminated\n");
break;
}
printf("----------------------------------------------------------\n");
}
close(sd_current);
close(sd);
rc = shmdt(shm_address);
if (rc==-1)
{
printf("main: shmdt() failed\n");
return -1;
}
rc = shmctl(shmid, IPC_RMID, &shmid_struct);
if (rc==-1)
{
printf("main: shmctl() failed\n");
return -1;
}
return 0;
}