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



NAME
       msgrcv, msgsnd - message operations

SYNOPSIS
       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/ipc.h>&gt;
       #include <&lt;sys/msg.h>&gt;

       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  fol-
       lowing 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 non-negative 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 defined by the msg_bytes 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).)   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  han-
         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.

       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  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
                  message queue, and does not have the CAP_IPC_OWNER  capabil-
                  ity.

       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 non-positive 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  val-
       ues:

       E2BIG      The  message text length is greater than msgsz and MSG_NOER-
                  ROR 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 capabil-
                  ity.

       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
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-04-23                          MSGOP(2)