Resource for Source: C/C++/Resource for Source - C-C++.iso/codelib6/v_08_11/8n11059b

Text File | 1995-11-01 | 4.8 KB | 214 lines

/*    Program:            usrsnd
 *
 *    Filename:            listing1.c
 *
 *    Description:        Demonstration program that uses a message
 *                    queue to enable users to send one line messages
 *                    to each other.
 *
 *    Contents:            main(), send_mssg(), recv_mssg(),
 *                    get_msg_queue()
 *
 *    Author/Programmer:    W. J. Freda
 *                    Automated Concepts Inc.
 */
#include <stdio.h>
#include <signal.h>
#include <pwd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

#define KEY     0x5642412
static struct mssg  {
    long mtype;            /* Message type */
    char mtext[BUFSIZ];        /* Message text */
} Msgbuf;
static int Msqid;            /* Message queue identifier */
static int Firstime=1;

/*    MAIN
 *
 *    DESCRIPTION:    Provides a simple menu that ties the
 *                send_msg() and recv_msg() functions
 *                together.
 *
 *    INPUT:        NONE
 *    OUTPUT:        Program termination
*/
main()
{
    char *mssg, *recv_mssg();
    char selection[80];

    while(1)  {
        printf("\n\n\t1) Send a message to a specific user\n");
        printf("\t2) Receive messages\n");
        printf("\t3) Exit\n");
        printf("\n\t\tEnter selection: ");

        gets(selection);

        switch(*selection)  {
            case '1':
                send_mssg();
                break;
            case '2':
                if ((mssg=recv_mssg()) == NULL)
                    printf("\n\nNo messages on queue\n\n");
                else
                    printf("\n\n%s\n\n",mssg);
                break;
            case '3':
                exit(0);
            default:
                printf("\tInvalid selection.\n\n");
                break;
        }
    }
}


/*    SEND_MSSG FUNCTION
 *
 *    DESCRIPTION:    Prompts the user for a username and message
 *                and sends the message to that user via a message
 *                queue.  The user can read the message using
 *                rcv_mssg().
 *
 *    INPUT:        NONE
 *    OUTPUT:        0    If successful
 *                -1    Otherwise
*/
send_mssg()
{
    extern int Msqid;            /* Message queue identifier  */
    extern struct mssg Msgbuf;    /* Message structure         */
    extern int errno;
    int msglen;                /* Length of message        */
    char message[BUFSIZ];        /* Message text             */
    char username[20];
    struct passwd *pass, *getpwnam();

    if (Firstime)  {
        if ((Msqid=get_msg_queue()) == -1)
            return(-1);

        Firstime=0;
    }

    /* Prompt for user name */
    printf("\n\nSend message to which user? ");
    gets(username);

    /* Validate username */
    setpwent();
    pass=getpwnam(username);
    endpwent();

    if (pass == NULL)  {
        printf("Invalid username.\n\n");
        return(0);
    }

    /* Prompt user for a message to send */
    printf("Enter message: ");
    gets(Msgbuf.mtext);

    if (*(Msgbuf.mtext) == NULL)
        /* Nothing to send */
        return(0);

    /* Set the message length */
    msglen=strlen(Msgbuf.mtext);

    /* Set the message type equal to the userid
     * we want to send the message to.
    */
    Msgbuf.mtype = pass->pw_uid;

    /* Send the message.  The IPC_NOWAIT flag informs msgsnd()
     * not to send the message if the message queue is full.
     * Without the IPC_NOWAIT flag this process would be put
     * to sleep by the kernel until the message can be sent.
    */
    if (msgsnd(Msqid, &Msgbuf, msglen, IPC_NOWAIT) == -1)  {
        /* Error occurred */
        return(-1);
    }

    return(0);
}


/*    RECV_MSSG FUNCTION        char *recv_mssg()
 *
 *    DESCRIPTION:    Attempts to retrieve a message from the
 *                message queue.
 *
 *    INPUT:        NONE
 *    OUTPUT:        text    Returns a pointer to the first
 *                    character of the message text.
 *                NULL    If there are no messages or an
 *                    error occurred.
*/
char *recv_mssg()
{
    extern int Msqid;            /* Message queue identifier  */
    extern struct mssg Msgbuf;    /* Message structure         */
    extern int Firstime;
    int uid;                    /* User id                   */
    int len;

    if (Firstime)  {
        if ((Msqid=get_msg_queue()) == -1)  {
            return(NULL);
        }
        Firstime=0;
    }

    /* Get the user id */
    uid=getuid();

    /* Receive a message.  The IPC_NOWAIT flag informs msgrcv()
      * to return immediately if no message is on queue.  Without
      * the IPC_NOWAIT flag this process would be put to sleep by
      * the kernel until a message of type uid is received.
    */
    if ((len=msgrcv(Msqid, &Msgbuf, BUFSIZ, uid, IPC_NOWAIT)) == -1)
        return(NULL);

    /* Null terminate the message */
    Msgbuf.mtext[len]=NULL;
    return(Msgbuf.mtext);
}

/*    GET_MSG_QUEUE FUNCTION
 *
 *    DESCRIPTION:    Gets and possibly creates a message
 *                queue identifier via the msgget(2)
 *                system call.
 *
 *    INPUT:        NONE
 *    OUTPUT:        qid    Message queue ID
 *                -1    Otherwise
*/
get_msg_queue()
{
    int qid;

    /* Acquire a message queue.
      * The IPC_CREAT flag informs msgget() to create
      * the message queue with the permission 0666.
      * The IPC_CREAT flags inform msgget() to create
      * the message queue if it doesn't already exist.
    */
    if ((qid=msgget((key_t)KEY, 0666 | IPC_CREAT)) == -1)  {
        return(-1);
    }

    return(qid);
}