1MSGCTL(2)                  Linux Programmer's Manual                 MSGCTL(2)
2
3
4

NAME

6       msgctl - System V message control operations
7

SYNOPSIS

9       #include <sys/types.h>
10       #include <sys/ipc.h>
11       #include <sys/msg.h>
12
13       int msgctl(int msqid, int cmd, struct msqid_ds *buf);
14

DESCRIPTION

16       msgctl()  performs  the  control operation specified by cmd on the Sys‐
17       tem V message queue with identifier msqid.
18
19       The msqid_ds data structure is defined in <sys/msg.h> as follows:
20
21           struct msqid_ds {
22               struct ipc_perm msg_perm;     /* Ownership and permissions */
23               time_t          msg_stime;    /* Time of last msgsnd(2) */
24               time_t          msg_rtime;    /* Time of last msgrcv(2) */
25               time_t          msg_ctime;    /* Time of last change */
26               unsigned long   __msg_cbytes; /* Current number of bytes in
27                                                queue (nonstandard) */
28               msgqnum_t       msg_qnum;     /* Current number of messages
29                                                in queue */
30               msglen_t        msg_qbytes;   /* Maximum number of bytes
31                                                allowed in queue */
32               pid_t           msg_lspid;    /* PID of last msgsnd(2) */
33               pid_t           msg_lrpid;    /* PID of last msgrcv(2) */
34           };
35
36       The ipc_perm structure is defined as follows  (the  highlighted  fields
37       are settable using IPC_SET):
38
39           struct ipc_perm {
40               key_t          __key;       /* Key supplied to msgget(2) */
41               uid_t          uid;         /* Effective UID of owner */
42               gid_t          gid;         /* Effective GID of owner */
43               uid_t          cuid;        /* Effective UID of creator */
44               gid_t          cgid;        /* Effective GID of creator */
45               unsigned short mode;        /* Permissions */
46               unsigned short __seq;       /* Sequence number */
47           };
48
49       Valid values for cmd are:
50
51       IPC_STAT
52              Copy  information from the kernel data structure associated with
53              msqid into the msqid_ds structure pointed to by buf.  The caller
54              must have read permission on the message queue.
55
56       IPC_SET
57              Write  the  values  of  some  members  of the msqid_ds structure
58              pointed to by buf to the kernel data structure  associated  with
59              this  message  queue,  updating  also its msg_ctime member.  The
60              following members of  the  structure  are  updated:  msg_qbytes,
61              msg_perm.uid,  msg_perm.gid,  and  (the least significant 9 bits
62              of) msg_perm.mode.  The effective UID  of  the  calling  process
63              must  match  the owner (msg_perm.uid) or creator (msg_perm.cuid)
64              of the message queue, or the caller must be privileged.   Appro‐
65              priate  privilege  (Linux:  the  CAP_SYS_RESOURCE capability) is
66              required to raise the msg_qbytes value beyond the system parame‐
67              ter MSGMNB.
68
69       IPC_RMID
70              Immediately  remove  the  message  queue,  awakening all waiting
71              reader and writer processes (with an error return and errno  set
72              to EIDRM).  The calling process must have appropriate privileges
73              or its effective user ID must be either that of the  creator  or
74              owner  of  the message queue.  The third argument to msgctl() is
75              ignored in this case.
76
77       IPC_INFO (Linux-specific)
78              Return information about system-wide message  queue  limits  and
79              parameters  in  the structure pointed to by buf.  This structure
80              is of type msginfo  (thus,  a  cast  is  required),  defined  in
81              <sys/msg.h> if the _GNU_SOURCE feature test macro is defined:
82
83                  struct msginfo {
84                      int msgpool; /* Size in kibibytes of buffer pool
85                                      used to hold message data;
86                                      unused within kernel */
87                      int msgmap;  /* Maximum number of entries in message
88                                      map; unused within kernel */
89                      int msgmax;  /* Maximum number of bytes that can be
90                                      written in a single message */
91                      int msgmnb;  /* Maximum number of bytes that can be
92                                      written to queue; used to initialize
93                                      msg_qbytes during queue creation
94                                      (msgget(2)) */
95                      int msgmni;  /* Maximum number of message queues */
96                      int msgssz;  /* Message segment size;
97                                      unused within kernel */
98                      int msgtql;  /* Maximum number of messages on all queues
99                                      in system; unused within kernel */
100                      unsigned short int msgseg;
101                                   /* Maximum number of segments;
102                                      unused within kernel */
103                  };
104
105              The msgmni, msgmax, and msgmnb settings can be changed via /proc
106              files of the same name; see proc(5) for details.
107
108       MSG_INFO (Linux-specific)
109              Return a msginfo structure containing the  same  information  as
110              for IPC_INFO, except that the following fields are returned with
111              information about system resources consumed by  message  queues:
112              the msgpool field returns the number of message queues that cur‐
113              rently exist on the system; the msgmap field returns  the  total
114              number  of  messages in all queues on the system; and the msgtql
115              field returns the total number of bytes in all messages  in  all
116              queues on the system.
117
118       MSG_STAT (Linux-specific)
119              Return a msqid_ds structure as for IPC_STAT.  However, the msqid
120              argument is not a queue identifier, but instead  an  index  into
121              the kernel's internal array that maintains information about all
122              message queues on the system.
123

RETURN VALUE

125       On success, IPC_STAT, IPC_SET, and IPC_RMID  return  0.   A  successful
126       IPC_INFO  or  MSG_INFO  operation returns the index of the highest used
127       entry in the kernel's internal array recording  information  about  all
128       message  queues.   (This information can be used with repeated MSG_STAT
129       operations to obtain information about all queues on  the  system.)   A
130       successful MSG_STAT operation returns the identifier of the queue whose
131       index was given in msqid.
132
133       On error, -1 is returned with errno indicating the error.
134

ERRORS

136       On failure, errno is set to one of the following:
137
138       EACCES The argument cmd is equal to IPC_STAT or MSG_STAT, but the call‐
139              ing  process  does not have read permission on the message queue
140              msqid, and does not have the  CAP_IPC_OWNER  capability  in  the
141              user namespace that governs its IPC namespace.
142
143       EFAULT The  argument  cmd  has  the  value IPC_SET or IPC_STAT, but the
144              address pointed to by buf isn't accessible.
145
146       EIDRM  The message queue was removed.
147
148       EINVAL Invalid value for cmd or msqid.  Or: for a  MSG_STAT  operation,
149              the  index  value  specified  in msqid referred to an array slot
150              that is currently unused.
151
152       EPERM  The argument cmd has the value  IPC_SET  or  IPC_RMID,  but  the
153              effective  user ID of the calling process is not the creator (as
154              found in msg_perm.cuid) or the owner (as found in  msg_perm.uid)
155              of  the  message queue, and the caller is not privileged (Linux:
156              does not have the CAP_SYS_ADMIN capability).
157
158       EPERM  An attempt (IPC_SET) was made to increase msg_qbytes beyond  the
159              system  parameter  MSGMNB,  but  the  caller  is  not privileged
160              (Linux: does not have the CAP_SYS_RESOURCE capability).
161

CONFORMING TO

163       POSIX.1-2001, POSIX.1-2008, SVr4.
164

NOTES

166       The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on  Linux
167       or by any version of POSIX.  However, some old implementations required
168       the inclusion of these header files, and the SVID also documented their
169       inclusion.   Applications  intended  to be portable to such old systems
170       may need to include these header files.
171
172       The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the  ipcs(1)
173       program  to  provide information on allocated resources.  In the future
174       these may modified or moved to a /proc filesystem interface.
175
176       Various fields in the struct msqid_ds were typed as short  under  Linux
177       2.2 and have become long under Linux 2.4.  To take advantage of this, a
178       recompilation under glibc-2.1.91 or later should suffice.  (The  kernel
179       distinguishes old and new calls by an IPC_64 flag in cmd.)
180

SEE ALSO

182       msgget(2),   msgrcv(2),   msgsnd(2),  capabilities(7),  mq_overview(7),
183       svipc(7)
184

COLOPHON

186       This page is part of release 4.16 of the Linux  man-pages  project.   A
187       description  of  the project, information about reporting bugs, and the
188       latest    version    of    this    page,    can     be     found     at
189       https://www.kernel.org/doc/man-pages/.
190
191
192
193Linux                             2017-09-15                         MSGCTL(2)
Impressum