msgop(2)msgop(2)Name
msgsnd, msgrcv - message operations
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd (msqid, msgp, msgsz, msgflg)
int msqid;
void *msgp;
size_t msgsz;
int msgflg;
int msgrcv (msqid, msgp, msgsz, msgtyp, msgflg)
int msqid;
void *msgp;
size_t msgsz;
long msgtyp;
int msgflg;
Description
There are two message operations system calls, and
msgsnd
The system call is used to send a message to the queue associated with
the message queue identifier specified by msqid. The msgp parameter
points to a structure containing the message. This structure is com‐
posed of the following members:
long mtype; /∗ message type ∗/
char mtext[]; /∗ message text ∗/
The mtype parameter is a positive integer that can be used by the
receiving process for message selection. For more information, see the
section of this reference page. The mtext parameter is any text of
length msgsz bytes. The msgsz parameter can range from 0 to a system-
imposed maximum.
The msgflg parameter specifies the action to be taken if the number of
bytes already on the queue is equal to msg_qbytes. (For further infor‐
mation, see ) The parameter also specifies what happens when the total
number of messages on all queues system-wide is equal to the system-
imposed limit.
If either of these conditions is true, and if (msgflg & IPC_NOWAIT) is
true, the message is not sent and the calling process returns immedi‐
ately. However, if either of the conditions is true and (msgflg &
IPC_NOWAIT) 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.
· The msqid parameter is removed from the system. For further
information, see When this occurs, errno is set equal to EIDRM,
and a value of -1 is returned.
· The calling process receives a signal that is to be caught. In
this case, the message is not sent and the calling process resumes
execution in the manner prescribed in
The system call fails and no message is sent under the following condi‐
tions:
[EINVAL] The msqid parameter is not a valid message queue identi‐
fier.
[EACCES] Operation permission is denied to the calling process.
For more information, see
[EINVAL] The mtype parameter is less than 1.
[EAGAIN] The message cannot be sent for one of the reasons cited
above and (msgflg & IPC_NOWAIT) is true.
[EINVAL] The msgsz parameter is less than zero or greater than
the system-imposed limit.
[EFAULT] The msgp parameter points to an illegal address.
Upon successful completion, the following actions are taken with
respect to the data structure associated with msqid (for more informa‐
tion, see ):
· The msg_qnum is incremented by 1.
· The msg_lspid is set equal to the process ID of the calling
process.
· The msg_stime is set equal to the current time.
msgrcv
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:
long mtype; /∗ message type ∗/
char mtext[]; /∗ message text ∗/
The mtype parameter is the received message's type, as specified by the
sending process. The mtext parameter is the text of the message. The
msgsz parameter specifies the size, in bytes, of mtext. The received
message is truncated to msgsz bytes if it is larger than msgsz and
(msgflg & MSG_NOERROR) is true. The truncated part of the message is
lost and no indication of the truncation is given to the calling
process.
The msgtyp parameter specifies the type of message requested, as fol‐
lows:
· If msgtyp is equal to 0, the first message on the queue is
received.
· If msgtyp is greater than 0, the first message of type msgtyp is
received.
· If msgtyp is less than 0, the first message of the lowest type
that is less than or equal to the absolute value of msgtyp is
received.
The msgflg parameter specifies the action to be taken if a message of
the desired type is not on the queue. These specified actions are as
follows:
· If (msgflg & IPC_NOWAIT) is true, the calling process returns
immediately with a value of -1 and errno set to ENOMSG.
· If (msgflg & IPC_NOWAIT) is false, the calling process suspends
execution until one of the following occurs:
- A message of the desired type is placed on the queue.
- The msqid parameter is removed from the system. When this
occurs, errno is set equal 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
The system call fails and no message is received under the following
conditions:
[EINVAL] The msqid parameter is not a valid message queue identi‐
fier.
[EACCES] Operation permission is denied to the calling process.
[EINVAL] The msgsz parameter is less than 0.
[E2BIG] The mtext parameter is greater than msgsz and (msgflg &
MSG_NOERROR) is false.
[ENOMSG] The queue does not contain a message of the desired type
and (msgtyp & IPC_NOWAIT) is true.
[EFAULT] The msgp parameter points to an illegal address.
Upon successful completion, the following actions are taken with
respect to the data structure associated with msqid:
· The msg_qnum is decremented by 1.
· The msg_lrpid is set equal to the process ID of the calling
process.
· The msg_rtime is set equal to the current time.
Return Values
If the or system calls return due to the receipt of a signal, a value
of -1 is returned to the calling process, and errno is set to EINTR.
If they return due to removal of msqid from the system, a value of -1
is returned, and errno is set to EIDRM.
Upon successful completion, the return value is as follows:
· The system call returns a value of 0.
· The system call returns a value equal to the number of bytes actu‐
ally placed into mtext.
Otherwise, a value of -1 is returned, and errno is set to indicate the
error.
See Alsoerrno(2), intro(2), msgctl(2), msgget(2), signal(3)msgop(2)