The Source

#include <stdio.h>
 #include <sys/types.h>
 #include <sys/ipc.h>
 #include <sys/shm.h>
 
 #define SEGSIZE 100
 
 main(int argc, char *argv[])
 {
         key_t key;
         int   shmid, cntr;
         char  *segptr;
 
         if(argc == 1)
                 usage();
 
         /* Create unique key via call to ftok() */
         key = ftok(".", 'S');
 
         /* Open the shared memory segment - create if necessary */
         if((shmid = shmget(key, SEGSIZE, IPC_CREAT|IPC_EXCL|0666)) == -1) 
         {
                 printf("Shared memory segment exists - opening as client\n");
 
                 /* Segment probably already exists - try as a client */
                 if((shmid = shmget(key, SEGSIZE, 0)) == -1) 
                 {
                         perror("shmget");
                         exit(1);
                 }
         }
         else
         {
                 printf("Creating new shared memory segment\n");
         }
 
         /* Attach (map) the shared memory segment into the current process */
         if((segptr = shmat(shmid, 0, 0)) == -1)
         {
                 perror("shmat");
                 exit(1);
         }
         
         switch(tolower(argv[1][0]))
         {
                 case 'w': writeshm(shmid, segptr, argv[2]);
                           break;
                 case 'r': readshm(shmid, segptr);
                           break;
                 case 'd': removeshm(shmid);
                           break;
                 case 'm': changemode(shmid, argv[2]);
                           break;
                  default: usage();
 
         }
 }
 
 writeshm(int shmid, char *segptr, char *text)
 {
         strcpy(segptr, text);
         printf("Done...\n");
 }
 
 readshm(int shmid, char *segptr)
 {
         printf("segptr: %s\n", segptr);
 }
 
 removeshm(int shmid)
 {
         shmctl(shmid, IPC_RMID, 0);
         printf("Shared memory segment marked for deletion\n");
 }
 
 changemode(int shmid, char *mode) 
 {
         struct shmid_ds myshmds;
 
         /* Get current values for internal data structure */
         shmctl(shmid, IPC_STAT, &myshmds);
 
         /* Display old permissions */
         printf("Old permissions were: %o\n", myshmds.shm_perm.mode);
 
         /* Convert and load the mode */ 
         sscanf(mode, "%o", &myshmds.shm_perm.mode);
 
         /* Update the mode */
         shmctl(shmid, IPC_SET, &myshmds);
 
         printf("New permissions are : %o\n", myshmds.shm_perm.mode);
 }
 
 usage()
 {
         fprintf(stderr, "shmtool - A utility for tinkering with shared memory\n");
         fprintf(stderr, "\nUSAGE:  shmtool (w)rite <text>\n");
         fprintf(stderr, "                (r)ead\n");
         fprintf(stderr, "                (d)elete\n");
         fprintf(stderr, "                (m)ode change <octal mode>\n");
         exit(1);
 }