msgop(2)msgop(2)NAME
msgsnd, msgrcv - message operations
SYNOPSISDESCRIPTION
The system call sends a message to the queue associated with the mes‐
sage queue identifier specified by msqid.
msgp points to a user-defined buffer that must contain first a field of
type that specifies the type of the message, followed by a data portion
that will hold the data bytes of the message. The structure below is
an example of what this user-defined buffer might look like:
mtype is a positive integer that can be used by the receiving process
for message selection (see below). mtext is any text of length msgsz
bytes. msgsz can range from 0 to a system-imposed maximum.
msgflg specifies the action to be taken if one or more of the following
is true:
· The number of bytes already on the queue is equal to (see in glos‐
sary(9)).
· The total number of messages on all queues system-wide is equal to
the system-imposed limit.
These actions are as follows:
If (msgflg is true, the message is not sent and the calling
process returns immediately.
If (msgflg is false, the calling process suspends execution
until one of the following occurs:
· The condition responsible for the suspension no longer
exists, in which case the message is sent.
· msqid is removed from the system (see msgctl(2)). When this
occurs, is set to [EIDRM] and a value of is returned.
· The calling process receives a signal to be caught. In this
case, the message is not sent and the calling process resumes
execution in the manner prescribed in signal(5).
Upon successful completion, the following actions are taken with
respect to the data structure associated with msqid:
is incremented by 1.
is set to the process ID of the calling process.
is set to the current time.
The system call reads a message from the queue associated with the mes‐
sage queue identifier specified by msqid and places it in the structure
pointed to by msgp. This structure is composed of the following mem‐
bers:
mtype is the received message's type as specified by the sending
process. mtext is the text of the message. msgsz specifies the size
in bytes of mtext. The received message is truncated to msgsz bytes if
it is larger than msgsz and (msgflg is true. The truncated part of the
message is lost and no indication of the truncation is given to the
calling process.
msgtyp specifies the type of message requested as follows:
msgtyp = 0 First message on the queue is received.
msgtyp > 0 First message of type msgtyp is received.
msgtyp < 0 First message of the lowest type that is less than
or equal to the absolute value of msgtyp is
received.
msgflg specifies the action to be taken if a message of the desired
type is not on the queue. These are as follows:
If (msgflg is true, the calling process returns immediately with
a value of and set to [ENOMSG].
If (msgflg is false, the calling process suspends execution
until one of the following occurs:
· A message of the desired type is placed on the queue.
· msqid is removed from the system. When this occurs, is set
to [EIDRM] and a value of −1 is returned.
· The calling process receives a signal that is to be caught.
In this case, a message is not received and the calling
process resumes execution in the manner prescribed in sig‐
nal(5)).
Upon successful completion, the following actions are taken with
respect to the data structure associated with msqid.
is decremented by 1.
is set to the process ID of the calling process.
is set to the current time.
RETURN VALUE
Upon successful completion, the return value is as follows:
returns a value of
returns a value equal to the number of bytes actually placed
into mtext.
Otherwise, a value of is returned and is set to indicate the error.
ERRORS
If fails, is set to one of the following values.
[E2BIG] mtext is greater than msgsz and (msgflg is false.
[EACCES] Operation permission is denied to the calling
process.
[EFAULT] msgp points to an illegal address. The reliable
detection of this error is implementation depen‐
dent.
[EIDRM] The message queue identifier msqid has been
removed from the system.
[EINTR] The function was interrupted by a signal.
[EINVAL] msqid is not a valid message queue identifier.
[EINVAL] msgsz is less than 0.
[ENOMSG] The queue does not contain a message of the
desired type and (msgflg is true.
If fails, is set to one of the following values.
[EACCES] Operation permission is denied to the calling
process.
[EAGAIN] The message cannot be sent for one of the reasons
cited above and (msgflg is true.
[EFAULT] msgp points to an illegal address. The reliable
detection of this error is implementation depen‐
dent.
[EIDRM] The message queue identifier msqid has been
removed from the system.
[EINTR] was interrupted by a signal.
[EINVAL] msqid is not a valid message queue identifier.
[EINVAL] mtype is less than 1.
[EINVAL] msgsz is less than zero or greater than the sys‐
tem-imposed limit.
SEE ALSOipcs(1), msgctl(2), msgget(2), stdipc(3C), privileges(5), signal(5).
STANDARDS CONFORMANCEmsgop(2)
