msgop

       #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 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 speci-
       fied 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 receiv-
       ing  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 imme-
       diately.  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  modi-
       fied  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
         dler.)

       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  speci-
       fied,  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 consid-
              ered 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,

       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 sys-
         tem 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 han-
         dler.)

       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
       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
       When  msgsnd() fails, errno will be set to one among the following val-
       ues:

       EACCES The calling process does not have write permission on  the  mes-
              sage 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  MSG-
              MAX).

       ENOMEM The  system  does  not  have enough memory to make a copy of the
              message pointed to by msgp.

              queue was removed.

       EINTR  While the process was sleeping to receive a message, the process
              caught a signal; see signal(7).

       EINVAL msgqid 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
       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
       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.

       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
       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)
       {
               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);

       }

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

COLOPHON
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Linux                             2015-08-08                          MSGOP(2)
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