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Linux 2.6 - man page for msgrcv (linux section 2)

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 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 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_qbytes field in the associated data structure for the  mes-
       sage  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 sig-
	 nal(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  struc-
       ture  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  fol-
       lowing 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  sig-
	 nal 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  (MSG-
       POOL).

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

COLOPHON
       This  page  is  part of release 3.27 of the Linux man-pages project.  A description of the
       project,  and  information  about  reporting  bugs,  can  be  found   at   http://www.ker-
       nel.org/doc/man-pages/.

Linux					    2008-04-23					 MSGOP(2)


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