1MSGCTL(2)                  Linux Programmer's Manual                 MSGCTL(2)
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NAME

6       msgctl - 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 message
17       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 (non-standard) */
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 in <sys/ipc.h> as follows (the  high‐
37       lighted fields 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_IPC_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.
75
76       IPC_INFO (Linux-specific)
77              Returns  information  about system-wide message queue limits and
78              parameters in the structure pointed to by buf.   This  structure
79              is  of  type  msginfo  (thus,  a  cast  is required), defined in
80              <sys/msg.h> if the _GNU_SOURCE feature test macro is defined:
81
82                  struct msginfo {
83                      int msgpool; /* Size in kibibytes of buffer pool
84                                      used to hold message data;
85                                      unused within kernel */
86                      int msgmap;  /* Maximum number of entries in message
87                                      map; unused within kernel */
88                      int msgmax;  /* Maximum number of bytes that can be
89                                      written in a single message */
90                      int msgmnb;  /* Maximum number of bytes that can be
91                                      written to queue; used to initialize
92                                      msg_qbytes during queue creation
93                                      (msgget(2)) */
94                      int msgmni;  /* Maximum number of message queues */
95                      int msgssz;  /* Message segment size;
96                                      unused within kernel */
97                      int msgtql;  /* Maximum number of messages on all queues
98                                      in system; unused within kernel */
99                      unsigned short int msgseg;
100                                   /* Maximum number of segments;
101                                      unused within kernel */
102                  };
103
104              The msgmni, msgmax, and msgmnb settings can be changed via /proc
105              files of the same name; see proc(5) for details.
106
107       MSG_INFO (Linux-specific)
108              Returns  a  msginfo structure containing the same information as
109              for IPC_INFO, except that the following fields are returned with
110              information  about  system resources consumed by message queues:
111              the msgpool field returns the number of message queues that cur‐
112              rently  exist  on the system; the msgmap field returns the total
113              number of messages in all queues on the system; and  the  msgtql
114              field  returns  the total number of bytes in all messages in all
115              queues on the system.
116
117       MSG_STAT (Linux-specific)
118              Returns a msqid_ds structure  as  for  IPC_STAT.   However,  the
119              msqid  argument  is not a queue identifier, but instead an index
120              into the kernel's  internal  array  that  maintains  information
121              about all message queues on the system.
122

RETURN VALUE

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

ERRORS

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

CONFORMING TO

157       SVr4, POSIX.1-2001.
158

NOTES

160       The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the  ipcs(8)
161       program  to  provide information on allocated resources.  In the future
162       these may modified or moved to a /proc file system interface.
163
164       Various fields in the struct msqid_ds were typed as short  under  Linux
165       2.2 and have become long under Linux 2.4.  To take advantage of this, a
166       recompilation under glibc-2.1.91 or later should suffice.  (The  kernel
167       distinguishes old and new calls by an IPC_64 flag in cmd.)
168

SEE ALSO

170       msgget(2),   msgrcv(2),   msgsnd(2),  capabilities(7),  mq_overview(7),
171       svipc(7)
172

COLOPHON

174       This page is part of release 3.22 of the Linux  man-pages  project.   A
175       description  of  the project, and information about reporting bugs, can
176       be found at http://www.kernel.org/doc/man-pages/.
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180Linux                             2008-08-06                         MSGCTL(2)
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