NAME
msgop - message operations
SYNOPSIS
# include <sys/types.h>
# include <sys/ipc.h>
# include <sys/msg.h>
int msgsnd ( int msqid, struct msgbuf *msgp, int msgsz, int
msgflg )
int msgrcv ( int msqid, struct msgbuf *msgp, int msgsz, long
msgtyp, int msgflg )
DESCRIPTION
To send or receive a message, the calling process allocates
a structure that looks like the following
struct msgbuf {
long mtype; /* message type, must be > 0 */
char mtext[1]; /* message data */
};
but with an array mtext of size msgsz, a non-negative
integer value. The structure member mtype must have a
strictly positive integer value that can be used by the
receiving process for message selection (see the section
about msgrcv).
The calling process must have write access permissions to
send and read access permissions to receive a message on the
queue.
The msgsnd system call enqueue a copy of the message pointed
to by the msgp argument on the message queue whose identif-
ier is specified by the value of the msqid argument.
The argument msgflg specifies the system call behaviour if
enqueuing the new message will require more than msg_qbytes
in the queue. Asserting IPC_NOWAIT the message will not be
sent and the system call fails returning with errno set to
EAGAIN. Otherwise the process is suspended until the condi-
tion for the suspension no longer exists (in which case the
message is sent and the system call succeeds), or the queue
is removed (in which case the system call fails with errno
set to EIDRM), or the process receives a signal that has to
be caught (in which case the system call fails with errno
set to EINTR).
Upon successful completion the message queue data structure
is updated as follows:
msg_lspid is set to the process-ID of the calling pro-
cess.
msg_qnum is incremented by 1.
msg_stime is set to the current time.
The system call msgrcv reads a message from the message
queue specified by msqid into the msgbuf pointed to by the
msgp argument removing from the queue, on success, the read
message.
The argument msgsz specifies the maximum size in bytes for
the member mtext of the structure pointed to by the msgp
argument. If the message text has length greater than
msgsz, then if the msgflg argument asserts MSG_NOERROR, the
message text will be truncated (and the truncated part will
be lost), otherwise the message isn't removed from the queue
and the system call fails returning with errno set to E2BIG.
The argument msgtyp specifies the type of message requested
as follows:
If msgtyp is 0, then the message on the queue's front
is read.
If msgtyp is greater than 0, then the first message on
the queue of type msgtyp is read if MSG_EXCEPT isn't
asserted by the msgflg argument, otherwise the first
message on the queue of type not equal to msgtyp will
be read.
If msgtyp is less than 0, then the first message on the
queue with the lowest type less than or equal to the
absolute value of msgtyp will be read.
The msgflg argument asserts none, one or more (or-ing them)
among the following flags:
IPC_NOWAIT For immediate return if no message of the
requested type is on the queue. The system call fails
with errno set to ENOMSG.
MSG_EXCEPT Used with msgtyp greater than 0 to read the
first message on the queue with message type that
differs from msgtyp.
MSG_NOERROR To truncate the message text if longer than
msgsz bytes.
If no message of the requested type is available and
IPC_NOWAIT isn't asserted in msgflg, the calling process is
blocked until one of the following conditions occurs:
A message of the desired type is placed on the queue.
The message queue is removed from the system. In such
a case the system call fails with errno set to EIDRM.
The calling process receives a signal that has to be
caught. In such a case the system call fails with
errno set to EINTR.
Upon successful completion the message queue data structure
is updated as follows:
msg_lrpid is set to the process-ID of the calling pro-
cess.
msg_qnum is decremented by 1.
msg_rtime is set to the current time.
RETURN VALUE
On a failure both functions return -1 with errno indicating
the error, otherwise msgsnd returns 0 and msgrvc returns the
number of bytes actually copied into the mtext array.
ERRORS
When msgsnd fails, at return errno will be set to one among
the following values:
EAGAIN The message can't be sent due to the msg_qbytes
limit for the queue and IPC_NOWAIT was asserted
in mgsflg.
EACCES The calling process has no write access permis-
sions on the message queue.
EFAULT The address pointed to by msgp isn't accessible.
EIDRM The message queue was removed.
EINTR Sleeping on a full message queue condition, the
process received a signal that had to be caught.
EINVAL Invalid msqid value, or nonpositive mtype value,
or invalid msgsz value (less than 0 or greater
than the system value MSGMAX).
ENOMEM The system has not enough memory to make a copy
of the supplied msgbuf.
When msgrcv fails, at return errno will be set to one among
the following values:
E2BIG The message text length is greater than msgsz and
MSG_NOERROR isn't asserted in msgflg.
EACCES The calling process has no read access permis-
sions on the message queue.
EFAULT The address pointed to by msgp isn't accessible.
EIDRM While the process was sleeping to receive a mes-
sage, the message queue was removed.
EINTR While the process was sleeping to receive a mes-
sage, the process received a signal that had to
be caught.
EINVAL Illegal msgqid value, or msgsz less than 0.
ENOMSG IPC_NOWAIT was asserted in msgflg and no message
of the requested type existed on the message
queue.
NOTES
The followings are system limits affecting a msgsnd system
call:
MSGMAX Maximum size for a message text: the implementa-
tion set this value to 4080 bytes.
MSGMNB Default maximum size in bytes of a message queue:
policy dependent. The super-user can increase
the size of a message queue beyond MSGMNB by a
msgctl system call.
The implementation has no intrinsic limits for the system
wide maximum number of message headers (MSGTQL) and for the
system wide maximum size in bytes of the message pool
(MSGPOOL).
CONFORMING TO
SVr4, SVID.
SEE ALSO
ipc(5), msgctl(2), msgget(2), msgrcv(2), msgsnd(2).