msgctl - System V message control operations

#include <sys/msg.h>
int msgctl(int msqid, int op, struct msqid_ds *buf);

msgctl() performs the control operation specified by op on the System V 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(2) */
    time_t          msg_rtime;  /* Time of last msgrcv(2) */
    time_t          msg_ctime;  /* Time of creation or last
                                   modification by msgctl() */
    unsigned long   msg_cbytes; /* # of bytes in queue */
    msgqnum_t       msg_qnum;   /* # number of messages in queue */
    msglen_t        msg_qbytes; /* Maximum # of bytes in queue */
    pid_t           msg_lspid;  /* PID of last msgsnd(2) */
    pid_t           msg_lrpid;  /* PID of last msgrcv(2) */
};

The fields of the msqid_ds structure are as follows:

msg_perm

This is an ipc_perm structure (see below) that specifies the access permissions on the message queue.

msg_stime

Time of the last msgsnd(2) system call.

msg_rtime

Time of the last msgrcv(2) system call.

msg_ctime

Time of creation of queue or time of last msgctl() IPC_SET operation.

msg_cbytes

Number of bytes in all messages currently on the message queue. This is a nonstandard Linux extension that is not specified in POSIX.

msg_qnum

Number of messages currently on the message queue.

msg_qbytes

Maximum number of bytes of message text allowed on the message queue.

msg_lspid

ID of the process that performed the last msgsnd(2) system call.

msg_lrpid

ID of the process that performed the last msgrcv(2) system call.

The ipc_perm structure is defined as follows (the highlighted fields are settable using IPC_SET):

struct ipc_perm {
    key_t          __key;       /* Key supplied to msgget(2) */
    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 */
};

The least significant 9 bits of the mode field of the ipc_perm structure define the access permissions for the message queue. The permission bits are as follows:

Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.

Valid values for op 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_SYS_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. The third argument to msgctl() is ignored in this case.

IPC_INFO (Linux-specific)

Return 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 kibibytes of buffer pool
                    used to hold message data;
                    unused within kernel */
    int msgmap;  /* Maximum number of entries in message
                    map; unused within kernel */
    int msgmax;  /* Maximum number of bytes that can be
                    written in a single message */
    int msgmnb;  /* Maximum number of bytes that can be
                    written to queue; used to initialize
                    msg_qbytes during queue creation
                    (msgget(2)) */
    int msgmni;  /* Maximum number of message queues */
    int msgssz;  /* Message segment size;
                    unused within kernel */
    int msgtql;  /* Maximum number of messages on all queues
                    in system; unused within kernel */
    unsigned short msgseg;
                 /* Maximum number of segments;
                    unused within kernel */
};

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)

Return 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)

Return 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.

MSG_STAT_ANY (Linux-specific, since Linux 4.17)

Return a msqid_ds structure as for MSG_STAT. However, msg_perm.mode is not checked for read access for msqid meaning that any user can employ this operation (just as any user may read /proc/sysvipc/msg to obtain the same information).

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 or MSG_STAT_ANY operations to obtain information about all queues on the system.) A successful MSG_STAT or MSG_STAT_ANY operation returns the identifier of the queue whose index was given in msqid.

On failure, -1 is returned and errno is set to indicate the error.

msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7), sysvipc(7)