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MSGCTL(2) | System Calls Manual | MSGCTL(2) |
NAME¶
msgctl
—
message control operations
LIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include
<sys/types.h>
#include
<sys/ipc.h>
#include
<sys/msg.h>
int
msgctl
(int
msqid, int
cmd, struct
msqid_ds *buf);
DESCRIPTION¶
Themsgctl
() system call performs some
control operations on the message queue specified by
msqid.
Each message queue has a data structure associated with it, parts of which may
be altered by msgctl
() and parts of which
determine the actions of msgctl
(). The data
structure is defined in
<sys/msg.h>
and contains (amongst others) the following members:
struct msqid_ds { struct ipc_perm msg_perm; /* msg queue permission bits */ struct msg *msg_first; /* first message in the queue */ struct msg *msg_last; /* last message in the queue */ msglen_t msg_cbytes; /* number of bytes in use on the queue */ msgqnum_t msg_qnum; /* number of msgs in the queue */ msglen_t msg_qbytes; /* max # of bytes on the queue */ pid_t msg_lspid; /* pid of last msgsnd() */ pid_t msg_lrpid; /* pid of last msgrcv() */ time_t msg_stime; /* time of last msgsnd() */ time_t msg_rtime; /* time of last msgrcv() */ time_t msg_ctime; /* time of last msgctl() */ };
<sys/ipc.h>
and looks like this:
struct ipc_perm { uid_t cuid; /* creator user id */ gid_t cgid; /* creator group id */ uid_t uid; /* user id */ gid_t gid; /* group id */ mode_t mode; /* r/w permission */ unsigned short seq; /* sequence # (to generate unique ipcid) */ key_t key; /* user specified msg/sem/shm key */ };
msgctl
() is
specified in cmd and is one of:
IPC_STAT
- Gather information about the message queue and place it in the structure pointed to by buf.
IPC_SET
- Set the value of the msg_perm.uid,
msg_perm.gid,
msg_perm.mode and
msg_qbytes fields in the structure
associated with msqid. The values are
taken from the corresponding fields in the structure pointed to by
buf. This operation can only be executed
by the super-user, or a process that has an effective user id equal to
either msg_perm.cuid or
msg_perm.uid in the data structure
associated with the message queue. The value of
msg_qbytes can only be increased by the
super-user. Values for msg_qbytes that
exceed the system limit (MSGMNB from
<sys/msg.h>
) are silently truncated to that limit. IPC_RMID
- Remove the message queue specified by msqid and destroy the data associated with it. Only the super-user or a process with an effective uid equal to the msg_perm.cuid or msg_perm.uid values in the data structure associated with the queue can do this.
RETURN VALUES¶
Themsgctl
() function returns the
value 0 if successful; otherwise the value -1 is returned and
the global variable errno is set to indicate
the error.
ERRORS¶
Themsgctl
() function will fail if:
- [
EPERM
] - The cmd argument is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does the effective uid match either the msg_perm.uid or msg_perm.cuid fields of the data structure associated with the message queue. An attempt is made to increase the value of msg_qbytes through IPC_SET but the caller is not the super-user.
- [
EACCES
] - The command is IPC_STAT and the caller has no read permission for this message queue.
- [
EINVAL
] - The msqid argument is not a valid message queue identifier. cmd is not a valid command.
- [
EFAULT
] - The buf argument specifies an invalid address.
SEE ALSO¶
msgget(2), msgrcv(2), msgsnd(2)HISTORY¶
Message queues appeared in the first release of AT&T System V UNIX.July 9, 2009 | Debian |