svmsg/svmsg_receive.c

/* svmsg_receive.c

   Usage: svmsg_receive [-nex] [-t msg-type] msqid [max-bytes]

   Experiment with the msgrcv() system call to receive messages from a
   System V message queue.

   See also svmsg_send.c.
*/
#define _GNU_SOURCE             /* Get definition of MSG_EXCEPT */
#include <sys/types.h>
#include <sys/msg.h>
#include "tlpi_hdr.h"

#define MAX_MTEXT 1024

struct mbuf {
    long mtype;                 /* Message type */
    char mtext[MAX_MTEXT];      /* Message body */
};

static void
usageError(const char *progName, const char *msg)
{
    if (msg != NULL)
        fprintf(stderr, "%s", msg);
    fprintf(stderr, "Usage: %s [options] msqid [max-bytes]\n", progName);
    fprintf(stderr, "Permitted options are:\n");
    fprintf(stderr, "    -e       Use MSG_NOERROR flag\n");
    fprintf(stderr, "    -t type  Select message of given type\n");
    fprintf(stderr, "    -n       Use IPC_NOWAIT flag\n");
#ifdef MSG_EXCEPT
    fprintf(stderr, "    -x       Use MSG_EXCEPT flag\n");
#endif
    exit(EXIT_FAILURE);
}

int
main(int argc, char *argv[])
{
    int msqid, flags, type;
    ssize_t msgLen;
    size_t maxBytes;
    struct mbuf msg;            /* Message buffer for msgrcv() */
    int opt;                    /* Option character from getopt() */

    /* Parse command-line options and arguments */

    flags = 0;
    type = 0;
    while ((opt = getopt(argc, argv, "ent:x")) != -1) {
        switch (opt) {
        case 'e':       flags |= MSG_NOERROR;   break;
        case 'n':       flags |= IPC_NOWAIT;    break;
        case 't':       type = atoi(optarg);    break;
#ifdef MSG_EXCEPT
        case 'x':       flags |= MSG_EXCEPT;    break;
#endif
        default:        usageError(argv[0], NULL);
        }
    }

    if (argc < optind + 1 || argc > optind + 2)
        usageError(argv[0], "Wrong number of arguments\n");

    msqid = getInt(argv[optind], 0, "msqid");
    maxBytes = (argc > optind + 1) ?
                getInt(argv[optind + 1], 0, "max-bytes") : MAX_MTEXT;

    /* Get message and display on stdout */

    msgLen = msgrcv(msqid, &msg, maxBytes, type, flags);
    if (msgLen == -1)
        errExit("msgrcv");

    printf("Received: type=%ld; length=%ld", msg.mtype, (long) msgLen);
    if (msgLen > 0)
        printf("; body=%s", msg.mtext);
    printf("\n");

    exit(EXIT_SUCCESS);
}