msgsnap(2)

NAME

     msgsnap - message queue snapshot operation

SYNOPSIS

     #include <sys/msg.h>

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

DESCRIPTION

     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 header:

     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 follows:

        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 read.

        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 asso-
     ciated with msqid.

RETURN VALUES

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

ERRORS

     The msgsnap() function will fail if:

     EACCES
           Operation permission is denied to the calling process.
           See intro(2).

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

     EFAULT
           The buf argument points to an illegal address.

USAGE

     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().

EXAMPLES

     Example 1: msgsnap() example

     This is sample C code indicating  how  to  use  the  msgsnap
     function (see msgids(2)).

     void
     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) {
                    perror("msgsnap");
                         free(buf);
                         return;
               }
               if (bufsize >= buf->msgsnap_size)  /* we got them all */
                    break;
               /* 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)));
          }

          free(buf);
     }

ATTRIBUTES

     See attributes(5) for descriptions of the  following  attri-
     butes:

     ____________________________________________________________
    |       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
    |_____________________________|_____________________________|
    | MT-Level                    | Async-Signal-Safe           |
    |_____________________________|_____________________________|

SEE ALSO

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