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 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(7) for descriptions of the following attributes:

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