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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 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 mes‐
	      sage 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 MSG‐
	      MAX).

       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_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	speci‐
	      fied 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 3.15 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)
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