msg.h File

Purpose

Describes the structures that are used by the subroutines that perform message queueing operations.

Syntax

#include <sys/msg.h>

Description

The msg.h file defines the following symbolic constants, types, and structures:

Types:

unsigned int  msgqnum_t;
unsigned int  msglen_t;

Symbolic Constants:

Item	Description
MSG_NOERROR	no error if big message */
MSG_R	read permission */
MSG_W	write permission */
MSG_RWAIT	a reader is waiting for a message */
MSG_WWAIT	a writer is waiting to send */
MSG_STAT	Number of bytes to copy for IPC_STAT command
MSGXBUFSIZE	the length of everything but mtext[1] and padding
MSG_SYSSPACE	for rmsgsnd() flags
XMSG	for rmsgrcv() flags

There is one msg queue id data structure for each q in the system. The msqid_ds structure contains the following members:

struct ipc_perm    msg_perm;           operation permission 
struct
void               *__msg_first;       ptr to first message on q
void               *__msg_last;        ptr to last message on q
unsigned int        __msg_cbytes;      current # bytes on q
msgqnum_t          msg_qnum;           # of messages on q
msglen_t           msg_qbytes;         max # of bytes on q
pid_t              msg_lspid;          pid of last msgsnd
pid_t              msg_lrpid;          pid of last msgrcv
time_t             msg_stime;          last msgsnd time
time_t             msg_rtime;          last msgrcv time
time_t             msg_ctime;          last change time
int                __msg_rwait;        wait list for message
receive
int                __msg_wwait;        wait list for message send
unsigned short     __msg_reqevents;    select/poll requested
events

The msg_hdr struct contains the following members:

time_t             mtime;              time message was sent
uid_t              muid;               author's effective uid
gid_t              mgid;               author's effective gid
pid_t              mpid;               author's process id
mtyp_t             mtype;              message type

There is one msg structure for each message that may be in the system. The msg structure contains the following members:

struct msg         *msg_next;          ptr to next message on q
struct msg_hdr     msg_attr;           message attributes
unsigned int       msg_ts;             message text size
char               *msg_spot;          pointer to message text

The structure msgbuf is the user message buffer template for msgsnd and msgrcv system calls and contains the following members:

mtyp_t             mtype;              message type
char               mtext[1];           message text

The msgxbuf structure is the user message buffer template for the msgxrcv system call and contains the following members:

time_t              mtime;             time message was sent
uid_t               muid;              author's effective uid
gid_t               mgid;              author's effective gid
pid_t               mpid;              author's process id
mtyp_t              mtype;             Message type
char                mtext[1];          Message text

The msginfo structure contains the following members:

int              msgmax,        max message size
int              msgmnb,        max # bytes on queue
int              msgmni,        # of message queue identifiers
int              msgmnm;        max # messages per queue identifier

The time_t, size_t, off_t, mtyp_t, pid_t, and gid_t types are as defined in <sys/types.h>.

The following are declared as functions:

int msgget(key_t, int);
int msgrcv(int, void *, size_t, long, int);
int msgsnd(int, const void *, size_t, int);
int msgctl(int, int, struct msqid_ds *);
int msgxrcv(int, struct msgxbuf*, int, long, int);

In addition, all of the symbols from <sys/ipc.h> will be defined when this header is included.