msgrcv, msgsnd - System V message queue operations

#include <sys/msg.h>
int msgsnd(int msqid, const void msgp[.msgsz], size_t msgsz,
 int msgflg);
ssize_t msgrcv(int msqid, void msgp[.msgsz], size_t msgsz, long msgtyp,
 int msgflg);

The msgsnd() and msgrcv() system calls are used to send messages to, and receive messages from, a System V message queue. The calling process must have write permission on the message queue in order to send a message, and read permission to receive a message.

The msgp argument is a pointer to a caller-defined structure of the following general form:

struct msgbuf {
    long mtype;       /* message type, must be > 0 */
    char mtext[1];    /* message data */
};

The mtext field is an array (or other structure) whose size is specified by msgsz, a nonnegative integer value. Messages of zero length (i.e., no mtext field) are permitted. The mtype field must have a strictly positive integer value. This value can be used by the receiving process for message selection (see the description of msgrcv() below).

msgsnd()

The msgsnd() system call appends a copy of the message pointed to by msgp to the message queue whose identifier is specified by msqid.

If sufficient space is available in the queue, msgsnd() succeeds immediately. The queue capacity is governed by the msg_qbytes field in the associated data structure for the message queue. During queue creation this field is initialized to MSGMNB bytes, but this limit can be modified using msgctl(2). A message queue is considered to be full if either of the following conditions is true:

• Adding a new message to the queue would cause the total number of bytes in the queue to exceed the queue's maximum size (the msg_qbytes field).

• Adding another message to the queue would cause the total number of messages in the queue to exceed the queue's maximum size (the msg_qbytes field). This check is necessary to prevent an unlimited number of zero-length messages being placed on the queue. Although such messages contain no data, they nevertheless consume (locked) kernel memory.

If insufficient space is available in the queue, then the default behavior of msgsnd() is to block until space becomes available. If IPC_NOWAIT is specified in msgflg, then the call instead fails with the error EAGAIN.

A blocked msgsnd() call may also fail if:

• the queue is removed, in which case the system call fails with errno set to EIDRM; or

• a signal is caught, in which case the system call fails with errno set to EINTR;see signal(7). (msgsnd() is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTART flag when establishing a signal handler.)

Upon successful completion the message queue data structure is updated as follows:

• msg_lspid is set to the process ID of the calling process.

• msg_qnum is incremented by 1.

• msg_stime is set to the current time.

msgrcv()

The msgrcv() system call removes a message from the queue specified by msqid and places it in the buffer pointed to by msgp.

The argument msgsz specifies the maximum size in bytes for the member mtext of the structure pointed to by the msgp argument. If the message text has length greater than msgsz, then the behavior depends on whether MSG_NOERROR is specified in msgflg. If MSG_NOERROR is specified, then the message text will be truncated (and the truncated part will be lost); if MSG_NOERROR is not specified, then the message isn't removed from the queue and the system call fails returning -1 with errno set to E2BIG.

Unless MSG_COPY is specified in msgflg (see below), the msgtyp argument specifies the type of message requested, as follows:

• If msgtyp is 0, then the first message in the queue is read.

• If msgtyp is greater than 0, then the first message in the queue of type msgtyp is read, unless MSG_EXCEPT was specified in msgflg, in which case the first message in the queue of type not equal to msgtyp will be read.

• If msgtyp is less than 0, then the first message in the queue with the lowest type less than or equal to the absolute value of msgtyp will be read.

The msgflg argument is a bit mask constructed by ORing together zero or more of the following flags:

IPC_NOWAIT

Return immediately if no message of the requested type is in the queue. The system call fails with errno set to ENOMSG.

MSG_COPY (since Linux 3.8)

Nondestructively fetch a copy of the message at the ordinal position in the queue specified by msgtyp (messages are considered to be numbered starting at 0).

This flag must be specified in conjunction with IPC_NOWAIT, with the result that, if there is no message available at the given position, the call fails immediately with the error ENOMSG. Because they alter the meaning of msgtyp in orthogonal ways, MSG_COPY and MSG_EXCEPT may not both be specified in msgflg.

The MSG_COPY flag was added for the implementation of the kernel checkpoint-restore facility and is available only if the kernel was built with the CONFIG_CHECKPOINT_RESTORE option.

MSG_EXCEPT

Used with msgtyp greater than 0 to read the first message in the queue with message type that differs from msgtyp.

MSG_NOERROR

To truncate the message text if longer than msgsz bytes.

If no message of the requested type is available and IPC_NOWAIT isn't specified in msgflg, the calling process is blocked until one of the following conditions occurs:

• A message of the desired type is placed in the queue.

• The message queue is removed from the system. In this case, the system call fails with errno set to EIDRM.

• The calling process catches a signal. In this case, the system call fails with errno set to EINTR. (msgrcv() is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTART flag when establishing a signal handler.)

Upon successful completion the message queue data structure is updated as follows:

msg_lrpid is set to the process ID of the calling process.

msg_qnum is decremented by 1.

msg_rtime is set to the current time.

On success, msgsnd() returns 0 and msgrcv() returns the number of bytes actually copied into the mtext array. On failure, both functions return -1, and set errno to indicate the error.

The program below demonstrates the use of msgsnd() and msgrcv().

The example program is first run with the -s option to send a message and then run again with the -r option to receive a message.

The following shell session shows a sample run of the program:

$ ./a.out -s
sent: a message at Wed Mar  4 16:25:45 2015
$ ./a.out -r
message received: a message at Wed Mar  4 16:25:45 2015

Program source

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <time.h>
#include <unistd.h>
struct msgbuf {
    long mtype;
    char mtext[80];
};
static void
usage(char *prog_name, char *msg)
{
    if (msg != NULL)
        fputs(msg, stderr);
    fprintf(stderr, "Usage: %s [options]\n", prog_name);
    fprintf(stderr, "Options are:\n");
    fprintf(stderr, "-s        send message using msgsnd()\n");
    fprintf(stderr, "-r        read message using msgrcv()\n");
    fprintf(stderr, "-t        message type (default is 1)\n");
    fprintf(stderr, "-k        message queue key (default is 1234)\n");
    exit(EXIT_FAILURE);
}
static void
send_msg(int qid, int msgtype)
{
    time_t         t;
    struct msgbuf  msg;
    msg.mtype = msgtype;
    time(&t);
    snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
             ctime(&t));
    if (msgsnd(qid, &msg, sizeof(msg.mtext),
               IPC_NOWAIT) == -1)
    {
        perror("msgsnd error");
        exit(EXIT_FAILURE);
    }
    printf("sent: %s\n", msg.mtext);
}
static void
get_msg(int qid, int msgtype)
{
    struct msgbuf msg;
    if (msgrcv(qid, &msg, sizeof(msg.mtext), msgtype,
               MSG_NOERROR | IPC_NOWAIT) == -1) {
        if (errno != ENOMSG) {
            perror("msgrcv");
            exit(EXIT_FAILURE);
        }
        printf("No message available for msgrcv()\n");
    } else {
        printf("message received: %s\n", msg.mtext);
    }
}
int
main(int argc, char *argv[])
{
    int  qid, opt;
    int  mode = 0;               /* 1 = send, 2 = receive */
    int  msgtype = 1;
    int  msgkey = 1234;
    while ((opt = getopt(argc, argv, "srt:k:")) != -1) {
        switch (opt) {
        case 's':
            mode = 1;
            break;
        case 'r':
            mode = 2;
            break;
        case 't':
            msgtype = atoi(optarg);
            if (msgtype <= 0)
                usage(argv[0], "-t option must be greater than 0\n");
            break;
        case 'k':
            msgkey = atoi(optarg);
            break;
        default:
            usage(argv[0], "Unrecognized option\n");
        }
    }
    if (mode == 0)
        usage(argv[0], "must use either -s or -r option\n");
    qid = msgget(msgkey, IPC_CREAT | 0666);
    if (qid == -1) {
        perror("msgget");
        exit(EXIT_FAILURE);
    }
    if (mode == 2)
        get_msg(qid, msgtype);
    else
        send_msg(qid, msgtype);
    exit(EXIT_SUCCESS);
}

msgctl(2), msgget(2), capabilities(7), mq_overview(7), sysvipc(7)