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NAME

       msgrcv, msgsnd - message 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 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 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 defined 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).)  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.

       The argument msgtyp 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_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.

       EAGAIN No message was available in the queue and IPC_NOWAIT was specified in msgflg.

       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 msgqid was invalid, or msgsz was less than 0.

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

CONFORMING TO

       SVr4, POSIX.1-2001.

NOTES

       The  msgp argument is declared as struct msgbuf * with libc4, libc5, glibc 2.0, glibc 2.1.
       It is declared as void * with 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 for a message text: 8192 bytes (on Linux, this limit can be  read  and
              modified via /proc/sys/kernel/msgmax).

       MSGMNB Default maximum size in bytes of a message queue: 16384 bytes (on Linux, this limit
              can be read and modified via /proc/sys/kernel/msgmnb).  The superuser can  increase
              the size of a message queue beyond MSGMNB by a msgctl(2) system call.

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

SEE ALSO

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

COLOPHON

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       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.