Manpage of MSGCTL


Section: Linux Programmer's Manual (2)
Updated: 2016-10-08


msgctl - System V message control operations  


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


msgctl() performs the control operation specified by cmdon the System V message queue with identifier msqid.

The msqid_dsdata 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 last change */
    unsigned long   __msg_cbytes; /* Current number of bytes in
                                     queue (nonstandard) */
    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(2) */
    pid_t           msg_lrpid;    /* PID of last msgrcv(2) */

The ipc_permstructure 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 */

Valid values for cmdare:

Copy information from the kernel data structure associated with msqidinto the msqid_dsstructure pointed to by buf. The caller must have read permission on the message queue.
Write the values of some members of the msqid_dsstructure pointed to by bufto the kernel data structure associated with this message queue, updating also its msg_ctimemember. 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_RESOURCEcapability) is required to raise the msg_qbytesvalue beyond the system parameter MSGMNB.
Immediately remove the message queue, awakening all waiting reader and writer processes (with an error return and errnoset 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_SOURCEfeature 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 int msgseg;
                 /* Maximum number of segments;
                    unused within kernel */

The msgmni, msgmax, and msgmnbsettings can be changed via /procfiles of the same name; see proc(5) for details.
MSG_INFO (Linux-specific)
Return a msginfostructure 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 msgpoolfield returns the number of message queues that currently exist on the system; the msgmapfield returns the total number of messages in all queues on the system; and the msgtqlfield returns the total number of bytes in all messages in all queues on the system.
MSG_STAT (Linux-specific)
Return a msqid_dsstructure as for IPC_STAT. However, the msqidargument 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.


On success, IPC_STAT, IPC_SET, and IPC_RMIDreturn 0. A successful IPC_INFOor MSG_INFOoperation 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_STAToperations to obtain information about all queues on the system.) A successful MSG_STAToperation returns the identifier of the queue whose index was given in msqid.

On error, -1 is returned with errnoindicating the error.  


On failure, errnois set to one of the following:
The argument cmdis equal to IPC_STATor MSG_STAT, but the calling process does not have read permission on the message queue msqid, and does not have the CAP_IPC_OWNERcapability in the user namespace that governs its IPC namespace.
The argument cmdhas the value IPC_SETor IPC_STAT, but the address pointed to by bufisn't accessible.
The message queue was removed.
Invalid value for cmdor msqid. Or: for a MSG_STAToperation, the index value specified in msqidreferred to an array slot that is currently unused.
The argument cmdhas the value IPC_SETor 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 caller is not privileged (Linux: does not have the CAP_SYS_ADMINcapability).
An attempt (IPC_SET) was made to increase msg_qbytesbeyond the system parameter MSGMNB, but the caller is not privileged (Linux: does not have the CAP_SYS_RESOURCEcapability).


POSIX.1-2001, POSIX.1-2008, SVr4.  


The inclusion of <sys/types.h>and <sys/ipc.h>isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files.

The IPC_INFO, MSG_STATand MSG_INFOoperations are used by the ipcs(1) program to provide information on allocated resources. In the future these may modified or moved to a /procfilesystem interface.

Various fields in the struct msqid_ds were typed as shortunder Linux 2.2 and have become longunder 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_64flag in cmd.)  


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




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