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



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  mes-
       sage, 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 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 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_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 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 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 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 mod-
              ified 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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-04-23                          MSGOP(2)