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.\" Copyright (c) 2000, 2020, Oracle and/or its affiliates.
.TH msgsnap 2 "25 Sep 2020" "Oracle Solaris 11.4" "System Calls"
.SH NAME
msgsnap \- message queue snapshot operation
.SH SYNOPSIS

.LP
.nf
#include <sys/msg.h>

\fBint\fR \fBmsgsnap\fR(\fBint\fR \fImsqid\fR, \fBvoid *\fR\fIbuf\fR, \fBsize_t\fR \fIbufsz\fR, \fBlong\fR \fImsgtyp\fR);
.fi
.SH DESCRIPTION
.sp
.LP
The \fBmsgsnap()\fR function reads all of the messages of type \fImsgtyp\fR from the queue associated with the message queue identifier specified by \fImsqid\fR and places them in the user-defined buffer pointed to by \fIbuf\fR. 
.sp
.LP
The \fIbuf\fR argument points to a user-defined buffer that on return will contain first a buffer header structure:
.sp
.in +2
.nf

struct msgsnap_head {
     size_t  msgsnap_size;   /* bytes used/required in the buffer */
     size_t  msgsnap_nmsg;   /* number of messages in the buffer */
};
.fi
.in -2
.sp
.sp
.LP
followed by \fBmsgsnap_nmsg\fR messages, each of which starts with a message header:
.sp
.in +2
.nf
struct msgsnap_mhead {
     size_t  msgsnap_mlen;   /* number of bytes in the message */
     long    msgsnap_mtype;  /* message type */
};
.fi
.in -2
.sp
.sp
.LP
and followed by \fBmsgsnap_mlen\fR bytes containing the message contents.
.sp
.LP
Each subsequent message header is located at the first byte following the previous message contents, rounded up to a \fBsizeof(size_t)\fR boundary.
.sp
.LP
The \fIbufsz\fR argument specifies the size of \fIbuf\fR in bytes. If \fIbufsz\fR is less than \fBsizeof(msgsnap_head)\fR, \fBmsgsnap()\fR fails with \fBEINVAL\fR. If \fIbufsz\fR is insufficient to contain all of the requested messages, \fBmsgsnap()\fR succeeds but returns with \fBmsgsnap_nmsg\fR set to 0 and with \fBmsgsnap_size\fR set to the required size of the buffer in bytes. 
.sp
.LP
The \fImsgtyp\fR argument specifies the types of messages requested as follows:
.RS +4
.TP
.ie t \(bu
.el o
If \fImsgtyp\fR is 0, all of the messages on the queue are read.

.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fImsgtyp\fR is greater than 0, all messages of type \fImsgtyp\fR are read.

.RE
.RS +4
.TP
.ie t \(bu
.el o
If \fImsgtyp\fR is less than 0, all messages with type less than or equal to the absolute value of \fImsgtyp\fR are read.

.RE
.sp
.LP
The \fBmsgsnap()\fR function is a non-destructive operation. Upon completion, no changes are made to the data structures associated with \fImsqid\fR.
.SH RETURN VALUES
.sp
.LP
Upon successful completion, \fBmsgsnap()\fR returns \fB0\fR. Otherwise, \fB\(mi1\fR is returned and \fBerrno\fR is set to indicate the error.
.SH ERRORS
.sp
.LP
The \fBmsgsnap()\fR function will fail if:
.sp
.ne 2
.mk
.na
\fB\fBEACCES\fR\fR
.ad
.RS 10n
.rt
Operation permission is denied to the calling process. See \fBIntro\fR(2).
.RE

.sp
.ne 2
.mk
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
.rt
The \fImsqid\fR argument is not a valid message queue identifier or the value of \fIbufsz\fR is less than \fBsizeof(struct msgsnap_head)\fR.
.RE

.sp
.ne 2
.mk
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 10n
.rt
The \fIbuf\fR argument points to an illegal address.
.RE

.SH USAGE
.sp
.LP
The \fBmsgsnap()\fR function returns a snapshot of messages on a message queue at one point in time. The queue contents can change immediately following return from \fBmsgsnap()\fR.
.SH EXAMPLES
.LP
\fBExample 1\fR \fBmsgsnap()\fR example

.sp
.LP
This is sample C code indicating how to use the msgsnap function (see \fBmsgids\fR(2)).

.sp
.in +2
.nf
void
process_msgid(int msqid)
{
     size_t bufsize = sizeof(struct msgsnap_head);
     struct msgsnap_head *buf;
     struct msgsnap_mhead *mhead;
     int i;

/* allocate a minimum-size buffer */
     buf = malloc(bufsize);

/* 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 = reallocf(buf, buf->msgsnap_size);
          if (buf == NULL) {
               perror("reallocf");
               return;
          }
          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);
}
.fi
.in -2
.sp
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp
.TS
tab(
) box;
cw(2.75i) |cw(2.75i) 
lw(2.75i) |lw(2.75i) 
.
ATTRIBUTE TYPE
ATTRIBUTE VALUE
_
Interface Stability
Committed
_
MT-Level
Async-Signal-Safe
.TE
.sp
.SH SEE ALSO
.sp
.LP
\fBipcrm\fR(1), \fBipcs\fR(1), \fBIntro\fR(2), \fBmsgctl\fR(2), \fBmsgget\fR(2), \fBmsgids\fR(2), \fBmsgrcv\fR(2), \fBmsgsnd\fR(2), \fBattributes\fR(7)
.SH HISTORY
.sp
.LP
The \fBmsgsnap()\fR function was added to Solaris in the Solaris 8 10/00 (Update 2) release.