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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 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

       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 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

       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.

       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_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

       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

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

COLOPHON

       This  page  is  part of release 4.16 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/.