NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

MSGOP(2)                  Linux Programmer's Manual                 MSGOP(2)

NAME         top

       msgrcv, msgsnd - System V message queue operations

SYNOPSIS         top

       #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         top

       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 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.

RETURN VALUE         top

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

ERRORS         top

       When msgsnd() fails, errno will 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_OWNER capability.

       EAGAIN The message can't be sent due to the msg_qbytes limit for the
              queue and IPC_NOWAIT was specified in msgflg.

       EFAULT The address pointed to by msgp isn't accessible.

       EIDRM  The message queue was removed.

       EINTR  Sleeping on a full message queue condition, the process caught
              a signal.

       EINVAL Invalid msqid value, or nonpositive mtype value, or invalid
              msgsz value (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, errno will be set to one among the following
       values:

       E2BIG  The message text length is greater than msgsz and MSG_NOERROR
              isn't specified in msgflg.

       EACCES The calling process does not have read permission on the
              message queue, and does not have the CAP_IPC_OWNER capability
              in the user namespace that governs its IPC namespace.

       EFAULT The address pointed to by msgp isn'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 msqid was invalid, or msgsz was less than 0.

       EINVAL (since Linux 3.14)
              msgflg specified MSG_COPY, but not IPC_NOWAIT.

       EINVAL (since Linux 3.14)
              msgflg specified both MSG_COPY and MSG_EXCEPT.

       ENOMSG IPC_NOWAIT was specified in msgflg and no message of the
              requested type existed on the message queue.

       ENOMSG IPC_NOWAIT and MSG_COPY were specified in msgflg and the queue
              contains less than msgtyp messages.

       ENOSYS (since Linux 3.8)
              MSG_COPY was specified in msgflg, and this kernel was
              configured without CONFIG_CHECKPOINT_RESTORE.

CONFORMING TO         top

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

       The MSG_EXCEPT and MSG_COPY flags are Linux-specific; their
       definitions can be obtained by defining the _GNU_SOURCE feature test
       macro.

NOTES         top

       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 msgp argument 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_RESOURCE
              capability) can increase the size of a message queue beyond
              MSGMNB using the msgctl(2) IPC_SET operation.

       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         top

       In Linux 3.13 and earlier, if msgrcv() was called with the MSG_COPY
       flag, but without IPC_NOWAIT, and the message queue contained less
       than msgtyp messages, 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, regardless of whether that message was
       at the ordinal position msgtyp.  This bug is fixed in Linux 3.14.

       Specifying both MSG_COPY and MSC_EXCEPT in msgflg is 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         top

       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 <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 '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);
       }

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2016-10-08                         MSGOP(2)