man2/msgctl.2

   1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
   2 .\" and Copyright 2004, 2005 Michael Kerrisk <mtk-manpages@gmx.net>
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  24 .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
  25 .\" Modified Sun Feb 18 01:59:29 2001 by Andries E. Brouwer <aeb@cwi.nl>
  26 .\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
  27 .\"     Added notes on CAP_IPC_OWNER requirement
  28 .\" Modified, 17 Jun 2004, Michael Kerrisk <mtk-manpages@gmx.net>
  29 .\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
  30 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk-manpages@gmx.net>
  31 .\"     Language and formatting clean-ups
  32 .\"     Added msqid_ds and ipc_perm structure definitions
  33 .\" 2005-08-02, mtk: Added IPC_INFO, MSG_INFO, MSG_STAT descriptions
  34 .\"
  35 .TH MSGCTL 2 2004-11-10 "Linux 2.6.9" "Linux Programmer's Manual"
  36 .SH NAME
  37 msgctl \- message control operations
  38 .SH SYNOPSIS
  39 .nf
  40 .B
  41 #include <sys/types.h>
  42 .B
  43 #include <sys/ipc.h>
  44 .B
  45 #include <sys/msg.h>
  46 .fi
  47 .sp
  48 .BI "int msgctl(int " msqid ,
  49 .BI "int " cmd ,
  50 .BI "struct msqid_ds *" buf );
  51 .SH DESCRIPTION
  52 .BR msgctl ()
  53 performs the control operation specified by
  54 .I cmd
  55 on the message queue with identifier
  56 .IR msqid .
  57 .PP
  58 The
  59 .I msqid_ds
  60 data structure is defined in <sys/msg.h> as follows:
  61 .nf
  62 .in +4n
  63
  64 struct msqid_ds {
  65     struct ipc_perm msg_perm;     /* Ownership and permissions
  66     time_t         msg_stime;    /* Time of last msgsnd() */
  67     time_t         msg_rtime;    /* Time of last msgrcv() */
  68     time_t         msg_ctime;    /* Time of last change */
  69     unsigned long  __msg_cbytes; /* Current number of bytes in
  70                                     queue (non-standard) */
  71     msgqnum_t      msg_qnum;     /* Current number of messages
  72                                     in queue */
  73     msglen_t       msg_qbytes;   /* Maximum number of bytes
  74                                     allowed in queue */
  75     pid_t          msg_lspid;    /* PID of last msgsnd() */
  76     pid_t          msg_lrpid;    /* PID of last msgrcv() */
  77 };
  78 .in -4n
  79 .fi
  80 .PP
  81 The
  82 .I ipc_perm
  83 structure is defined in <sys/ipc.h> as follows
  84 (the highlighted fields are settable using
  85 .BR IPC_SET ):
  86 .PP
  87 .nf
  88 .in +4n
  89 struct ipc_perm {
  90     key_t key;            /* Key supplied to msgget() */
  91     uid_t \fBuid\fP;            /* Effective UID of owner */
  92     gid_t \fBgid\fP;            /* Effective GID of owner */
  93     uid_t cuid;           /* Effective UID of creator */
  94     gid_t cgid;           /* Effective GID of creator */
  95     unsigned short \fBmode\fP;  /* Permissions */
  96     unsigned short seq;   /* Sequence number */
  97 };
  98 .in -4n
  99 .fi
 100 .PP
 101 Valid values for
 102 .I cmd
 103 are:
 104 .TP
 105 .B IPC_STAT
 106 Copy information from the kernel data structure associated with
 107 .I msqid
 108 into the
 109 .I msqid_ds
 110 structure pointed to by
 111 .IR buf .
 112 The caller must have read permission on the message queue.
 113 .TP
 114 .B IPC_SET
 115 Write the values of some members of the
 116 .I msqid_ds
 117 structure pointed to by
 118 .I buf
 119 to the kernel data structure associated with this message queue,
 120 updating also its
 121 .I msg_ctime
 122 member.
 123 The following members of the structure are updated:
 124 .IR msg_qbytes ,
 125 .IR msg_perm.uid ,
 126 .IR msg_perm.gid ,
 127 and (the least significant 9 bits of)
 128 .IR msg_perm.mode .
 129 The effective UID of the calling process must match the owner
 130 .RI ( msg_perm.uid )
 131 or creator
 132 .RI ( msg_perm.cuid )
 133 of the message queue, or the caller must be privileged.
 134 Appropriate privilege (Linux: the
 135 .B CAP_IPC_RESOURCE
 136 capability) is required to raise the
 137 .I msg_qbytes
 138 value beyond the system parameter
 139 .BR MSGMNB .
 140 .TP
 141 .B IPC_RMID
 142 Immediately remove the message queue,
 143 awakening all waiting reader and writer processes (with an error
 144 return and
 145 .I errno
 146 set to
 147 .BR EIDRM ).
 148 The calling process must have appropriate privileges
 149 or its effective user ID must be either that of the creator or owner
 150 of the message queue.
 151 .TP
 152 .BR IPC_INFO " (Linux specific)"
 153 Returns information about system-wide message queue limits and
 154 parameters in the structure pointed to by
 155 .IR buf .
 156 This structure is of type
 157 .IR msginfo
 158 (thus, a cast is required),
 159 defined in
 160 .I <sys/msg.h>
 161 if the _GNU_SOURCE feature test macro is defined:
 162 .nf
 163 .in +2n
 164
 165 struct msginfo {
 166     int msgpool; /* Size in bytes of buffer pool used
 167                     to hold message data; unused */
 168     int msgmap;  /* Max. # of entries in message
 169                     map; unused */
 170     int msgmax;  /* Max. # of bytes that can be
 171                     written in a single message */
 172     int msgmnb;  /* Max. # of bytes that can be written to
 173                     queue; used to initialize msg_qbytes
 174                     during queue creation (msgget()) */
 175     int msgmni;  /* Max. # of message queues */
 176     int msgssz;  /* Message segment size; unused */
 177     int msgtql;  /* Max. # of messages on all queues
 178                     in system; unused */
 179     unsigned short int msgseg;
 180                  /* Max. # of segments; unused */
 181 };
 182
 183 .in -2n
 184 .fi
 185 The
 186 .IR msgmni ,
 187 .IR msgmax ,
 188 and
 189 .I msgmnb
 190 settings can be changed via
 191 .I /proc
 192 files of the same name; see
 193 .BR proc (5)
 194 for details.
 195 .TP
 196 .BR MSG_INFO " (Linux specific)"
 197 Returns a
 198 .I msginfo
 199 structure containing the same information as for
 200 .BR IPC_INFO ,
 201 except that the following fields are returned with information
 202 about system resources consumed by message queues: the
 203 .I msgpool
 204 field returns the number of message queues that currently exist
 205 on the system; the
 206 .I msgmap
 207 field returns the total number of messages in all queues
 208 on the system; and the
 209 .I msgtql
 210 field returns the total number of bytes in all messages
 211 in all queues on the system.
 212 .TP
 213 .BR MSG_STAT " (Linux specific)"
 214 Returns a
 215 .I msqid_ds
 216 structure as for
 217 .BR IPC_STAT .
 218 However, the
 219 .I msqid
 220 argument is not a queue identifier, but instead an index into
 221 the kernel's internal array that maintains information about
 222 all message queues on the system.
 223 .SH "RETURN VALUE"
 224 On success,
 225 .BR IPC_STAT ,
 226 .BR IPC_SET ,
 227 and
 228 .BR IPC_RMID
 229 return 0.
 230 A successful
 231 .B IPC_INFO
 232 or
 233 .B MSG_INFO
 234 operation returns the index of the highest used entry in the
 235 kernel's internal array recording information about all
 236 message queues.
 237 (This information can be used with repeated
 238 .B MSG_STAT
 239 operations to obtain information about all queues on the system.)
 240 A successful
 241 .B MSG_STAT
 242 operation returns the identifier of the queue whose index was given in
 243 .IR msqid .
 244
 245 On error, \-1 is returned with
 246 .I errno
 247 indicating the error.
 248 .SH ERRORS
 249 On failure,
 250 .I errno
 251 is set to one of the following:
 252 .TP 11
 253 .B EACCES
 254 The argument
 255 .I cmd
 256 is equal to
 257 .BR IPC_STAT
 258 or
 259 .BR MSG_STAT ,
 260 but the calling process does not have read permission on the message queue
 261 .IR msqid ,
 262 and does not have the
 263 .B CAP_IPC_OWNER
 264 capability.
 265 .TP
 266 .B EFAULT
 267 The argument
 268 .I cmd
 269 has the value
 270 .B IPC_SET
 271 or
 272 .BR IPC_STAT ,
 273 but the address pointed to by
 274 .I buf
 275 isn't accessible.
 276 .TP
 277 .B EIDRM
 278 The message queue was removed.
 279 .TP
 280 .B EINVAL
 281 Invalid value for
 282 .I cmd
 283 or
 284 .IR msqid .
 285 Or: for a
 286 .B MSG_STAT
 287 operation, the index value specified in
 288 .I msqid
 289 referred to an array slot that is currently unused.
 290 .TP
 291 .B EPERM
 292 The argument
 293 .I cmd
 294 has the value
 295 .B IPC_SET
 296 or
 297 .BR IPC_RMID ,
 298 but the effective user ID of the calling process is not the creator
 299 (as found in
 300 .IR msg_perm.cuid )
 301 or the owner
 302 (as found in
 303 .IR msg_perm.uid )
 304 of the message queue,
 305 and the process is not privileged (Linux: it does not have the
 306 .B CAP_SYS_ADMIN
 307 capability).
 308 .SH NOTES
 309 The
 310 .BR IPC_INFO ,
 311 .BR MSG_STAT
 312 and
 313 .B MSG_INFO
 314 operations are used by the
 315 .BR ipcs (8)
 316 program to provide information on allocated resources.
 317 In the future these may modified or moved to a /proc file system
 318 interface.
 319
 320 Various fields in the \fIstruct msqid_ds\fP were shorts under Linux 2.2
 321 and have become longs under Linux 2.4. To take advantage of this,
 322 a recompilation under glibc-2.1.91 or later should suffice.
 323 (The kernel distinguishes old and new calls by an IPC_64 flag in
 324 .IR cmd .)
 325 .SH "CONFORMING TO"
 326 SVr4, SVID.  SVID does not document the EIDRM error condition.
 327 .SH "SEE ALSO"
 328 .BR msgget (2),
 329 .BR msgrcv (2),
 330 .BR msgsnd (2),
 331 .BR capabilities (7),
 332 .BR ipc (7),
 333 .BR mq_overview (7)