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NAME
msgctl - System V 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 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 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_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 */
};
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_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 int 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).
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 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 error, -1 is returned 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 in the user
namespace that governs its IPC namespace.
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 caller is not privileged (Linux: does not have the CAP_SYS_ADMIN
capability).
EPERM An attempt (IPC_SET) was made to increase msg_qbytes beyond the system parameter MSGMNB, but the
caller is not privileged (Linux: does not have the CAP_SYS_RESOURCE capability).
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
NOTES
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_STAT and MSG_INFO operations are used by the ipcs(1) program to provide information on
allocated resources. In the future these may modified or moved to a /proc filesystem interface.
Various fields in the struct msqid_ds were typed as short under Linux 2.2 and have become long 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.)
SEE ALSO
msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7), sysvipc(7)
COLOPHON
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