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msgctl.2freebsd
Langue: en
Version: 338370 (ubuntu - 24/10/10)
Section: 2 (Appels système)
BSD mandoc
NAME
msgctl - message control operationsLIBRARY
Lb libcSYNOPSIS
In sys/types.h In sys/ipc.h In sys/msg.h Ft int Fn msgctl int msqid int cmd struct msqid_ds *bufDESCRIPTION
The Fn msgctl system call performs some control operations on the message queue specified by Fa msqid .Each message queue has a data structure associated with it, parts of which may be altered by Fn msgctl and parts of which determine the actions of Fn msgctl . The data structure is defined in 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() */ };
The Vt ipc_perm structure used inside the Vt msqid_ds structure is defined in In 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 */ };
The operation to be performed by Fn msgctl is specified in Fa cmd and is one of:
- IPC_STAT
- Gather information about the message queue and place it in the structure pointed to by Fa 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 Fa msqid . The values are taken from the corresponding fields in the structure pointed to by Fa 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 In sys/msg.h ) are silently truncated to that limit.
- IPC_RMID
- Remove the message queue specified by Fa 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.
The permission to read from or write to a message queue (see msgsnd(2) and msgrcv(2)) is determined by the msg_perm.mode field in the same way as is done with files (see chmod(2)), but the effective uid can match either the msg_perm.cuid field or the msg_perm.uid field, and the effective gid can match either msg_perm.cgid or msg_perm.gid
RETURN VALUES
Rv -std msgctlERRORS
The Fn msgctl function will fail if:- Bq Er EPERM
- The Fa 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.
- Bq Er EACCES
- The command is IPC_STAT and the caller has no read permission for this message queue.
- Bq Er EINVAL
- The Fa msqid argument is not a valid message queue identifier.
cmd is not a valid command.
- Bq Er EFAULT
- The Fa 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 .Contenus ©2006-2024 Benjamin Poulain
Design ©2006-2024 Maxime Vantorre