MSGCTL(2)

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:

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.

RETURN VALUE
On success, the return value will be 0 otherwise -1 with errno indicating the error.

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

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.

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
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, SVID. SVID does not document the EIDRM error condition.

SEE ALSO
msgget(2), msgrcv(2), msgsnd(2), ipc(5), capabilities(7)