Manual browser: msgctl(2)

MSGCTL(2) System Calls Manual MSGCTL(2)


msgctlmessage control operations


Standard C Library (libc, -lc)


#include <sys/msg.h>

msgctl(int msqid, int cmd, struct msqid_ds *buf);


The msgctl() system call performs control operations on the message queue specified by msqid.

Each message queue has a msqid_ds structure associated with it which contains the following members:

    struct ipc_perm msg_perm; /* msg queue permission bits */ 
    msgqnum_t msg_qnum;       /* # 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 msgid_ds structure is defined in <sys/ipc.h> and contains the following members:

    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; /* permission (lower 9 bits) */

The operation to be performed by msgctl() is specified in cmd and is one of:

Gather information about the message queue and place it in the structure pointed to by buf.
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.
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 effective gid can match either msg_perm.cgid or msg_perm.gid.


Upon successful completion, a value of 0 is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.


msgctl() will fail if:
cmd 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 was made to increase the value of msg_qbytes through IPC_SET, but the caller is not the super-user.

cmd is IPC_STAT and the caller has no read permission for this message queue.
msqid is not a valid message queue identifier.

cmd is not a valid command.

buf specifies an invalid address.


The msgctl system call conforms to X/Open System Interfaces and Headers Issue 5 (“XSH5”).


Message queues appeared in the first release of AT&T System V UNIX.
August 25, 1999 NetBSD 7.0