FreeBSD Manual Pages
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 The msgctl() system call performs some control operations on the mes- sage 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 ac- tions 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 */ 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 ipc_perm structure used inside the msqid_ds structure is defined 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 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 associ- ated with msqid. The values are taken from the correspond- ing fields in the structure pointed to by buf. This opera- tion 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 in- creased by the super-user. Values for msg_qbytes that ex- ceed 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. 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 ef- fective gid can match either msg_perm.cgid or msg_perm.gid. RETURN VALUES The msgctl() 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 The msgctl() 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 ef- fective uid match either the msg_perm.uid or msg_perm.cuid fields of the data structure associ- ated 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. FreeBSD 13.2 July 9, 2020 MSGCTL(2)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY
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