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The Source

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <ctype.h>
#include <string.h>

#define SEGSIZE 100

void usage();
void writeshm(int shmid, char *segptr, char *text);
void changemode(int shmid, char *mode); 
void removeshm(int shmid);
void readshm(int shmid, char *segptr);

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 = (char *)shmat(shmid, 0, 0)) == (char *)-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);
}