Manpage of MSGOP

MSGOP

Section: Linux Programmer's Manual (2)
Updated: 2015-08-08
Index
 

NAME

msgrcv, msgsnd - System V message queue operations  

SYNOPSIS

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

DESCRIPTION

The msgsnd() and msgrcv() system calls are used, respectively, 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 msgpargument 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 mtextfield is an array (or other structure) whose size is specified by msgsz, a nonnegative integer value. Messages of zero length (i.e., no mtextfield) are permitted. The mtypefield 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 msgpto 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_qbytesfield in the associated data structure for the message queue. During queue creation this field is initialized to MSGMNBbytes, 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_qbytesfield).
*
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_qbytesfield). 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_NOWAITis 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 errnoset to EIDRM; or
*
a signal is caught, in which case the system call fails with errnoset to EINTR;seesignal(7). (msgsnd() is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTARTflag when establishing a signal handler.)

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

msg_lspidis set to the process ID of the calling process.
msg_qnumis incremented by 1.
msg_stimeis set to the current time.
 

msgrcv()

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

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

Unless MSG_COPYis specified in msgflg(see below), the msgtypargument specifies the type of message requested, as follows:

*
If msgtypis 0, then the first message in the queue is read.
*
If msgtypis greater than 0, then the first message in the queue of type msgtypis read, unless MSG_EXCEPTwas specified in msgflg, in which case the first message in the queue of type not equal to msgtypwill be read.
*
If msgtypis less than 0, then the first message in the queue with the lowest type less than or equal to the absolute value of msgtypwill be read.

The msgflgargument 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 errnoset 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 msgtypin orthogonal ways, MSG_COPYand MSG_EXCEPTmay not both be specified in msgflg.

The MSG_COPYflag was added for the implementation of the kernel checkpoint-restore facility and is available only if the kernel was built with the CONFIG_CHECKPOINT_RESTOREoption.

MSG_EXCEPT
Used with msgtypgreater 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 msgszbytes.

If no message of the requested type is available and IPC_NOWAITisn'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 errnoset to EIDRM.
*
The calling process catches a signal. In this case, the system call fails with errnoset to EINTR. (msgrcv() is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTARTflag when establishing a signal handler.)

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

msg_lrpidis set to the process ID of the calling process.
msg_qnumis decremented by 1.
msg_rtimeis set to the current time.
 

RETURN VALUE

On failure both functions return -1 with errnoindicating the error, otherwise msgsnd() returns 0 and msgrcv() returns the number of bytes actually copied into the mtextarray.  

ERRORS

When msgsnd() fails, errnowill be set to one among the following values:
EACCES
The calling process does not have write permission on the message queue, and does not have the CAP_IPC_OWNERcapability.
EAGAIN
The message can't be sent due to the msg_qbyteslimit for the queue and IPC_NOWAITwas specified in msgflg.
EFAULT
The address pointed to by msgpisn't accessible.
EIDRM
The message queue was removed.
EINTR
Sleeping on a full message queue condition, the process caught a signal.
EINVAL
Invalid msqidvalue, or nonpositive mtypevalue, or invalid msgszvalue (less than 0 or greater than the system value MSGMAX).
ENOMEM
The system does not have enough memory to make a copy of the message pointed to by msgp.

When msgrcv() fails, errnowill be set to one among the following values:

E2BIG
The message text length is greater than msgszand MSG_NOERRORisn't specified in msgflg.
EACCES
The calling process does not have read permission on the message queue, and does not have the CAP_IPC_OWNERcapability.
EFAULT
The address pointed to by msgpisn't accessible.
EIDRM
While the process was sleeping to receive a message, the message queue was removed.
EINTR
While the process was sleeping to receive a message, the process caught a signal; see signal(7).
EINVAL
msqidwas invalid, or msgszwas less than 0.
EINVAL (since Linux 3.14)
msgflgspecified MSG_COPY, but not IPC_NOWAIT.
EINVAL (since Linux 3.14)
msgflgspecified both MSG_COPYand MSG_EXCEPT.
ENOMSG
IPC_NOWAITwas specified in msgflgand no message of the requested type existed on the message queue.
ENOMSG
IPC_NOWAITand MSG_COPYwere specified in msgflgand the queue contains less than msgtypmessages.
ENOSYS (since Linux 3.8)
MSG_COPYwas specified in msgflg, and this kernel was configured without CONFIG_CHECKPOINT_RESTORE.
 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4.

The MSG_EXCEPTand MSG_COPYflags are Linux-specific; their definitions can be obtained by defining the _GNU_SOURCEfeature test macro.  

NOTES

The inclusion of <sys/types.h>and <sys/ipc.h>isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files.

The msgpargument is declared as struct msgbuf * in glibc 2.0 and 2.1. It is declared as void *in glibc 2.2 and later, as required by SUSv2 and SUSv3.

The following limits on message queue resources affect the msgsnd() call:

MSGMAX
Maximum size of a message text, in bytes (default value: 8192 bytes). On Linux, this limit can be read and modified via /proc/sys/kernel/msgmax.
MSGMNB
Maximum number of bytes that can be held in a message queue (default value: 16384 bytes). On Linux, this limit can be read and modified via /proc/sys/kernel/msgmnb. A privileged process (Linux: a process with the CAP_SYS_RESOURCEcapability) can increase the size of a message queue beyond MSGMNBusing the msgctl(2) IPC_SEToperation.

The implementation has no intrinsic system-wide limits on the number of message headers (MSGTQL) and the number of bytes in the message pool (MSGPOOL).  

BUGS

In Linux 3.13 and earlier, if msgrcv() was called with the MSG_COPYflag, but without IPC_NOWAIT, and the message queue contained less than msgtypmessages, then the call would block until the next message is written to the queue. At that point, the call would return a copy of the message, regardlessof whether that message was at the ordinal position msgtyp. This bug is fixed in Linux 3.14.

Specifying both MSG_COPYand MSC_EXCEPTin msgflgis a logical error (since these flags impose different interpretations on msgtyp). In Linux 3.13 and earlier, this error was not diagnosed by msgrcv(). This bug is fixed in Linux 3.14.  

EXAMPLE

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 -ssent: a message at Wed Mar  4 16:25:45 2015

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

Program source

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.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)
{
    struct msgbuf msg;
    time_t t;

    msg.mtype = msgtype;

    time(&t);
    snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s",
            ctime(&t));

    if (msgsnd(qid, (void *) &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, (void *) &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 aqsaq:
            mode = 1;
            break;
        case aqraq:
            mode = 2;
            break;
        case aqtaq:
            msgtype = atoi(optarg);
            if (msgtype <= 0)
                usage(argv[0], "-t option must be greater than 0\n");
            break;
        case aqkaq:
            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);
}
 

SEE ALSO

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


 

Index

NAME
SYNOPSIS
DESCRIPTION
msgsnd()
msgrcv()
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
BUGS
EXAMPLE
Program source
SEE ALSO

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Time: 22:27:42 GMT, June 20, 2016