msgsnap(2) (man pages section 2: System Calls)

man pages section 2: System Calls

msgsnap(2)

NAME

msgsnap– message queue snapshot operation

SYNOPSIS

#include <sys/msg.h>

msqid

buf

bufsz

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:

If msgtyp is 0, all of the messages on the queue are read.
If msgtyp is greater than 0, all messages of type msgtyp are read.
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.

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 identifier 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 attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
MT-Level	Async-Signal-Safe