Two processes comunicating via shared memory in UNIX

We develop two programs here that illustrate the passing of a simple piece of memery (a string) between the processes if running simulatenously:

server.c

-- simply creates the string and shared memory portion.

client.c

-- attaches itself to the created shared memory portion and uses the string (printf).

 

 server.c

 #include <sys/types.h>  
 #include <sys/ipc.h>  
 #include <sys/shm.h>  
 #include <stdio.h>  
 #define SHMSZ   27  
 main()  
 {  
   char c;  
   int shmid;  
   key_t key;  
   char *shm, *s;  
   /*  
    * We'll name our shared memory segment  
    * "5678".  
    */  
   key = 5678;  
   /*  
    * Create the segment.  
    */  
   if ((shmid = shmget(key, SHMSZ, IPC_CREAT | 0666)) < 0) {  
     perror("shmget");  
     exit(1);  
   }  
   /*  
    * Attach the segment to our data space.  
    */  
   if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {  
     perror("shmat");  
     exit(1);  
   }  
   /*  
    * Now put some things into the memory for the  
    * other process to read.  
    */  
   s = shm;  
   for (c = 'a'; c <= 'z'; c++)  
     *s++ = c;  
   *s = NULL;  
   /*  
    * Finally, we wait until the other process   
    * changes the first character of our memory  
    * to '*', indicating that it has read what   
    * we put there.  
    */  
   while (*shm != '*')  
     sleep(1);  
   exit(0);  
 }  

 /*  
  * client - client program to demonstrate shared memory.  
  */  
 #include <sys/types.h>  
 #include <sys/ipc.h>  
 #include <sys/shm.h>  
 #include <stdio.h>  
 #define SHMSZ   27  
 main()  
 {  
   int shmid;  
   key_t key;  
   char *shm, *s;  
   /*  
    * We need to get the segment named  
    * "5678", created by the server.  
    */  
   key = 5678;  
   /*  
    * Locate the segment.  
    */  
   if ((shmid = shmget(key, SHMSZ, 0666)) < 0) {  
     perror("shmget");  
     exit(1);  
   }  
   /*  
    * Now we attach the segment to our data space.  
    */  
   if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {  
     perror("shmat");  
     exit(1);  
   }  
   /*  
    * Now read what the server put in the memory.  
    */  
   for (s = shm; *s != NULL; s++)  
     putchar(*s);  
   putchar('\n');  
   /*  
    * Finally, change the first character of the   
    * segment to '*', indicating we have read   
    * the segment.  
    */  
   *shm = '*';  
   exit(0);  
 }