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# msgctl() - Unix, Linux System Call

# NAME

msgctl - message control operations

# SYNOPSIS

    #include <sys/types.h>
    #include <sys/ipc.h>
    #include <sys/msg.h>

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

# DESCRIPTION

msgctl() performs the control operation specified by cmd on the message queue with identifier msqid.

The msqid_ds data structure is defined in <sys/msg.h> as follows:

    struct msqid_ds {
        struct ipc_perm msg_perm;     /* Ownership and permissions
        time_t         msg_stime;    /* Time of last msgsnd() */
        time_t         msg_rtime;    /* Time of last msgrcv() */
        time_t         msg_ctime;    /* Time of last change */
        unsigned long  __msg_cbytes; /* Current number of bytes in
                                        queue (non-standard) */
        msgqnum_t      msg_qnum;     /* Current number of messages
                                        in queue */
        msglen_t       msg_qbytes;   /* Maximum number of bytes
                                        allowed in queue */
        pid_t          msg_lspid;    /* PID of last msgsnd() */
        pid_t          msg_lrpid;    /* PID of last msgrcv() */
    };

The ipc_perm structure is defined in <sys/ipc.h> as follows (the highlighted fields are settable using IPC_SET):

    struct ipc_perm {
        key_t key;            /* Key supplied to msgget() */
        uid_t uid;            /* Effective UID of owner */
        gid_t gid;            /* Effective GID of owner */
        uid_t cuid;           /* Effective UID of creator */
        gid_t cgid;           /* Effective GID of creator */
        unsigned short mode;  /* Permissions */
        unsigned short seq;   /* Sequence number */
    };

Valid values for cmd are:

Tag | Description
--- | ---
IPC_STAT
| Copy information from the kernel data structure associated with msqid into the msqid_ds structure pointed to by buf. The caller must have read permission on the message queue.
IPC_SET
| Write the values of some members of the msqid_ds structure pointed to by buf to the kernel data structure associated with this message queue, updating also its msg_ctime member. The following members of the structure are updated: msg_qbytes, msg_perm.uid, msg_perm.gid, and (the least significant 9 bits of) msg_perm.mode. The effective UID of the calling process must match the owner (msg_perm.uid) or creator (msg_perm.cuid) of the message queue, or the caller must be privileged. Appropriate privilege (Linux: the CAP_IPC_RESOURCE capability) is required to raise the msg_qbytes value beyond the system parameter MSGMNB.
IPC_RMID
| Immediately remove the message queue, awakening all waiting reader and writer processes (with an error return and errno set to EIDRM). The calling process must have appropriate privileges or its effective user ID must be either that of the creator or owner of the message queue.
IPC_INFO (Linux specific)
| Returns information about system-wide message queue limits and parameters in the structure pointed to by buf. This structure is of type msginfo (thus, a cast is required), defined in <sys/msg.h> if the _GNU_SOURCE feature test macro is defined:

    struct msginfo {
        int msgpool; /* Size in bytes of buffer pool used
                        to hold message data; unused */
        int msgmap;  /* Max. # of entries in message
                        map; unused */
        int msgmax;  /* Max. # of bytes that can be
                        written in a single message */
        int msgmnb;  /* Max. # of bytes that can be written to
                        queue; used to initialize msg_qbytes
                        during queue creation (msgget()) */
        int msgmni;  /* Max. # of message queues */
        int msgssz;  /* Message segment size; unused */
        int msgtql;  /* Max. # of messages on all queues
                        in system; unused */
        unsigned short int msgseg;
                     /* Max. # of segments; unused */
    };

The msgmni, msgmax, and msgmnb settings can be changed via /proc files of the same name; see proc(5) for details.
MSG_INFO (Linux specific)
| Returns a msginfo structure containing the same information as for IPC_INFO, except that the following fields are returned with information about system resources consumed by message queues: the msgpool field returns the number of message queues that currently exist on the system; the msgmap field returns the total number of messages in all queues on the system; and the msgtql field returns the total number of bytes in all messages in all queues on the system.
MSG_STAT (Linux specific)
| Returns a msqid_ds structure as for IPC_STAT. However, the msqid argument is not a queue identifier, but instead an index into the kernel’s internal array that maintains information about all message queues on the system.

# RETURN VALUE

On success, IPC_STAT, IPC_SET, and IPC_RMID return 0. A successful IPC_INFO or MSG_INFO operation returns the index of the highest used entry in the kernel’s internal array recording information about all message queues. (This information can be used with repeated MSG_STAT operations to obtain information about all queues on the system.) A successful MSG_STAT operation returns the identifier of the queue whose index was given in msqid.

On error, -1 is returned with errno indicating the error.

# ERRORS

On failure, errno is set to one of the following:

Tag | Description
--- | ---
EACCES | The argument cmd is equal to IPC_STAT or MSG_STAT, but the calling process does not have read permission on the message queue msqid, and does not have the CAP_IPC_OWNER capability.
EFAULT | The argument cmd has the value IPC_SET or IPC_STAT, but the address pointed to by buf isn’t accessible.
EIDRM | The message queue was removed.
EINVAL | Invalid value for cmd or msqid. Or: for a MSG_STAT operation, the index value specified in msqid referred to an array slot that is currently unused.
EPERM | The argument cmd has the value IPC_SET or IPC_RMID, but the effective user ID of the calling process is not the creator (as found in msg_perm.cuid) or the owner (as found in msg_perm.uid) of the message queue, and the process is not privileged (Linux: it does not have the CAP_SYS_ADMIN capability).

# NOTES

The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the ipcs(8) program to provide information on allocated resources. In the future these may modified or moved to a /proc file system interface.

Various fields in the struct msqid_ds were shorts under Linux 2.2 and have become longs under Linux 2.4. To take advantage of this, a recompilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by an IPC_64 flag in cmd.)

# CONFORMING TO

SVr4, POSIX.1-2001.

# SEE ALSO

- msgget (2)

- msgrcv (2)

- msgsnd (2)

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