Tribblix: manual page: msgsnap.2

MSGSNAP(2) System Calls 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 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