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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 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:

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: EACCES

EINVAL EFAULT

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 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);
}

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE ATTRIBUTE VALUE

MT-Level Async-Signal-Safe

ipcrm (1), ipcs (1), Intro (2), msgctl (2), msgget (2), msgids (2), msgrcv (2), msgsnd (2), attributes (7)