msgop - message operations
# include <sys/types.h>
# include <sys/ipc.h>
# include <sys/msg.h>
int msgsnd ( int msqid, struct msgbuf *msgp, size_t msgsz, int msgflg )
ssize_t msgrcv ( int msqid, struct msgbuf *msgp, size_t msgsz, long
msgtyp, int msgflg )
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 enqueues a copy of the message pointed to by the
msgp argument on the message queue whose identifier 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 condition 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 process.
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 process.
msg_qnum is decremented by 1.
msg_rtime is set to the current time.
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.
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 permissions 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_NOER-
ROR isn't asserted in msgflg.
EACCES The calling process has no read access permissions on the
message queue.
EFAULT The address pointed to by msgp isn't accessible.
EIDRM While the process was sleeping to receive a message, the
message queue was removed.
EINTR While the process was sleeping to receive a message, 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.
The followings are system limits affecting a msgsnd system call:
MSGMAX Maximum size for a message text: the implementation 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).
SVr4, SVID.
The pointer argument is declared as struct msgbuf * with libc4, libc5,
glibc 2.0, glibc 2.1. It is declared as void * (const void * for
msgsnd()) with glibc 2.2, following the SUSv2.
ipc(5), msgctl(2), msgget(2), msgrcv(2), msgsnd(2)
Linux 0.99.13 2000-07-10 MSGOP(2)
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