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MSGSNAP(2)							    MSGSNAP(2)

       msgsnap - message queue snapshot operation

       #include <sys/msg.h>

       msgsnap(int msqid, void *buf, size_t bufsz, long msgtyp);

       The  msgsnap()  function	 reads all of the messages of type msgtyp from
       the queue associated with the message  queue  identifier	 specified  by
       msqid and places them in the user-defined buffer pointed to by buf.

       The  buf	 argument  points to a user-defined buffer that on return will
       contain first a buffer header structure:

	 struct msgsnap_head {
	      size_t  msgsnap_size;   /* bytes used/required in the buffer */
	      size_t  msgsnap_nmsg;   /* number of messages in the buffer */

       followed by msgsnap_nmsg messages, each of which starts with a  message

	 struct msgsnap_mhead {
	      size_t  msgsnap_mlen;   /* number of bytes in the message */
	      long    msgsnap_mtype;  /* message type */

       and followed by msgsnap_mlen bytes containing the message contents.

       Each  subsequent	 message header is located at the first byte following
       the previous message contents, rounded up to a sizeof(size_t) boundary.

       The bufsz argument specifies the size  of buf in bytes.	 If  bufsz  is
       less  than sizeof(msgsnap_head), msgsnap() fails with EINVAL.  If bufsz
       is insufficient to contain all of  the  requested  messages,  msgsnap()
       succeeds	 but  returns with msgsnap_nmsg set to 0 and with msgsnap_size
       set to the required size of the buffer in bytes.

       The msgtyp argument specifies the types of messages requested  as  fol‐

	   o	  If msgtyp is 0, all of the messages on the queue are read.

	   o	  If msgtyp is greater than 0, all messages of type msgtyp are

	   o	  If msgtyp is less than 0, all messages with type  less  than
		  or equal to the absolute value of msgtyp are read.

       The msgsnap() function is a non-destructive operation. Upon completion,
       no changes are made to the data structures associated with msqid.

       Upon successful completion, msgsnap()  returns  0.   Otherwise,	−1  is
       returned and errno is set to indicate the error.

       The msgsnap() function will fail if:

		 Operation  permission	is denied to the calling process.  See

		 The msqid argument is not a valid message queue identifier or
		 the value of bufsz is less than sizeof(struct msgsnap_head).

		 The buf argument points to an illegal address.

       The  msgsnap()  function	 returns  a  snapshot of messages on a message
       queue at one point in time.  The queue contents can change  immediately
       following return from msgsnap().

       Example 1 msgsnap() example

       This  is	 sample C code indicating how to use the msgsnap function (see

	 process_msgid(int msqid)
	      size_t bufsize;
	      struct msgsnap_head *buf;
	      struct msgsnap_mhead *mhead;
	      int i;

	      /* allocate a minimum-size buffer */
	      buf = malloc(bufsize = sizeof(struct msgsnap_head));

	      /* read all of the messages from the queue */
	      for (;;) {
		   if (msgsnap(msqid, buf, bufsize, 0) != 0) {
		   if (bufsize >= buf->msgsnap_size)  /* we got them all */
		   /* we need a bigger buffer */
		   buf = realloc(buf, bufsize = buf->msgsnap_size);

	      /* process each message in the queue (there may be none) */
	      mhead = (struct msgsnap_mhead *)(buf + 1);  /* first message */
	      for (i = 0; i < buf->msgsnap_nmsg; i++) {
		   size_t mlen = mhead->msgsnap_mlen;

		   /* process the message contents */
		   process_message(mhead->msgsnap_mtype, (char *)(mhead+1), mlen);

		   /* advance to the next message header */
		   mhead = (struct msgsnap_mhead *)
			((char *)mhead + sizeof(struct msgsnap_mhead) +
			((mlen + sizeof(size_t) - 1) & ~(sizeof(size_t) - 1)));


       See attributes(5) for descriptions of the following attributes:

       │MT-Level       │ Async-Signal-Safe │

       ipcrm(1),   ipcs(1),   Intro(2),	  msgctl(2),   msgget(2),   msgids(2),
       msgrcv(2), msgsnd(2), attributes(5)

				  Mar 8, 2000			    MSGSNAP(2)

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