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msgctl
Provides message control operations.
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgctl (int msqid, int cmd, struct msqid_ds *buffer);
Parameters
msqid Passes a message queue identifier returned by a call to
msgget().
cmd Passes a command defining the control operation to perform.
Valid commands are defined in the "Description" section below.
buffer Passes a pointer to a buffer of type struct msqid_ds (defined
in the <sys/msg.h> header). Operations on the buffer are
defined by cmd. Refer to the "Description" section below.
Return Values
0 Success.
-1 An error occurred, and errno is set to indicate the error
condition.
Description
The msgctl() function provides message control operations on the message
queue and data structure associated with msqid. Control operations are
defined by the cmd parameter. Following are valid commands to be passed
in cmd:
IPC_RMID Deallocate the message queue identifier specified by msqid and
purge the message queue and data structure associated with it.
The calling process must have either [REV BEG]MPE/iX SM
capability[REV END], or be the owner or creator of msqid (have
an effective user ID equal to the value of either the
msg_perm.uid (owner) or msg_perm.cuid (creator) fields in the
data structure associated with msqid).
IPC_SET Copy data from the following fields of the msqid_ds structure
pointed to by buffer to the corresponding fields in the data
structure associated with msqid:
* msg_perm.uid (owner user ID)
* msg_perm.gid (owner group ID)
* Low order 9 bits of msg_perm.mode
* msg_qbytes
The calling process must have either [REV BEG]MPE/iX SM
capability[REV END] or an effective user ID equal to the value
of either the msg_perm.uid or msg_perm.cuid fields in the data
structure associated with msqid. The caller must have [REV
BEG]MPE/iX SM capability[REV END] to increase the value of the
msg_qbytes field of the data structure associated with msqid.
IPC_STAT Copy all data from the structure associated with msqid to the
data structure pointed to by buffer.
Implementation Considerations
None.
Errors
If an error occurs, errno is set to one of the following values.
EACCES CAUSE cmd specifies IPC_STAT and the calling process does
not have read permission.
ACTION Ensure that the calling process has read permission
for the msqid.
EFAULT CAUSE The system detected a NULL or bad address in
attempting to use the buffer argument.
ACTION Check to see if the pointer is correctly initialized.
EINVAL CAUSE msqid is not a valid message queue identifier, or cmd
is not a valid command.
ACTION Check that the msqid parameter is valid and the
associated message queue has not been removed, or
that cmd is valid.
EPERM CAUSE One of the following:
* cmd specifies IPC_RMID or IPC_SET and the
calling process does not have either [REV
BEG]MPE/iX SM capability[REV END] or an
effective user ID equal to the value of either
the msg_perm.uid or msg_perm.cuid fields in
the data structure associated with msqid.
* cmd specifies IPC_SET and the calling process
tried to increase the value of msg_qbytes
without having [REV BEG]MPE/iX SM
capability[REV END].
ACTION Ensure that the calling process has the appropriate
effective user ID or capability required to perform
the requested command.
ESYSERR CAUSE An operating system error occurred that does not map
directly to any of the above errors.
ACTION Examine the MPE/iX process error stack for the type
of system error.
See Also
msgget(), msgrcv(), msgsnd(), SVID2 (Section 12)