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 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 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 msgid_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 cmd
are:
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.
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.
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.
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.
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.
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.
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 error, -1 is returned with errno
indicating the error.
On failure, errno
is set to one of the following:
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.
The argument cmd
has the value IPC_SET or IPC_STAT, but the address pointed to by buf
isn't accessible.
The message queue was removed.
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.
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).
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.1-2001, 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_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
.)
msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7), sysvipc(7)
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.