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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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msgctl(2) System Calls Manual msgctl(2)
msgctl - System V message control operations
Standard C library (libc, -lc)
#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:
0400 Read by user
0200 Write by user
0040 Read by group
0020 Write by group
0004 Read by others
0002 Write by others
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.
EACCES The argument op 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 op 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 op 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 op 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).
POSIX.1-2008.
POSIX.1-2001, SVr4.
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 op.)
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.
msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7),
sysvipc(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 msgctl(2)
Pages that refer to this page: ipcrm(1), ipcs(1), ipc(2), msgget(2), msgop(2), syscalls(2), capabilities(7), sysvipc(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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