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NAME

       msgrcv, msgsnd - System V message queue operations

SYNOPSIS

       #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 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 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.

ERRORS

       msgsnd() can fail with the following errors:

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

       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.

       msgrcv() can fail with the following errors:

       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)
              Both  MSG_COPY  and  IPC_NOWAIT  were  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  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.

EXAMPLES

       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, &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);
       }

SEE ALSO

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

COLOPHON

       This  page  is  part of release 5.13 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/.