man2/msgctl.2

   1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
   2 .\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
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  23 .\"
  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@gmail.com>
  27 .\"     Added notes on CAP_IPC_OWNER requirement
  28 .\" Modified, 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  29 .\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
  30 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  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 2008-05-20 "Linux" "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 \fI<sys/msg.h>\fP 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(2) */
  67     time_t         msg_rtime;    /* Time of last msgrcv(2) */
  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(2) */
  76     pid_t          msg_lrpid;    /* PID of last msgrcv(2) */
  77 };
  78 .in
  79 .fi
  80 .PP
  81 The
  82 .I ipc_perm
  83 structure is defined in \fI<sys/ipc.h>\fP 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(2) */
  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
  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 .I msginfo
 158 (thus, a cast is required),
 159 defined in
 160 .I <sys/msg.h>
 161 if the
 162 .B _GNU_SOURCE
 163 feature test macro is defined:
 164 .nf
 165 .in +4n
 166
 167 struct msginfo {
 168     int msgpool; /* Size in kibibytes of buffer pool
 169                     used to hold message data;
 170                     unused within kernel */
 171     int msgmap;  /* Maximum number of entries in message
 172                     map; unused within kernel */
 173     int msgmax;  /* Maximum number of bytes that can be
 174                     written in a single message */
 175     int msgmnb;  /* Maximum number of bytes that can be
 176                     written to queue; used to initialize
 177                     msg_qbytes during queue creation
 178                     (msgget(2)) */
 179     int msgmni;  /* Maximum number of message queues */
 180     int msgssz;  /* Message segment size;
 181                     unused within kernel */
 182     int msgtql;  /* Maximum number of messages on all queues
 183                     in system; unused within kernel */
 184     unsigned short int msgseg;
 185                  /* Maximum number of segments;
 186                     unused within kernel */
 187 };
 188
 189 .in
 190 .fi
 191 The
 192 .IR msgmni ,
 193 .IR msgmax ,
 194 and
 195 .I msgmnb
 196 settings can be changed via
 197 .I /proc
 198 files of the same name; see
 199 .BR proc (5)
 200 for details.
 201 .TP
 202 .BR MSG_INFO " (Linux-specific)"
 203 Returns a
 204 .I msginfo
 205 structure containing the same information as for
 206 .BR IPC_INFO ,
 207 except that the following fields are returned with information
 208 about system resources consumed by message queues: the
 209 .I msgpool
 210 field returns the number of message queues that currently exist
 211 on the system; the
 212 .I msgmap
 213 field returns the total number of messages in all queues
 214 on the system; and the
 215 .I msgtql
 216 field returns the total number of bytes in all messages
 217 in all queues on the system.
 218 .TP
 219 .BR MSG_STAT " (Linux-specific)"
 220 Returns a
 221 .I msqid_ds
 222 structure as for
 223 .BR IPC_STAT .
 224 However, the
 225 .I msqid
 226 argument is not a queue identifier, but instead an index into
 227 the kernel's internal array that maintains information about
 228 all message queues on the system.
 229 .SH "RETURN VALUE"
 230 On success,
 231 .BR IPC_STAT ,
 232 .BR IPC_SET ,
 233 and
 234 .B IPC_RMID
 235 return 0.
 236 A successful
 237 .B IPC_INFO
 238 or
 239 .B MSG_INFO
 240 operation returns the index of the highest used entry in the
 241 kernel's internal array recording information about all
 242 message queues.
 243 (This information can be used with repeated
 244 .B MSG_STAT
 245 operations to obtain information about all queues on the system.)
 246 A successful
 247 .B MSG_STAT
 248 operation returns the identifier of the queue whose index was given in
 249 .IR msqid .
 250
 251 On error, \-1 is returned with
 252 .I errno
 253 indicating the error.
 254 .SH ERRORS
 255 On failure,
 256 .I errno
 257 is set to one of the following:
 258 .TP 11
 259 .B EACCES
 260 The argument
 261 .I cmd
 262 is equal to
 263 .B IPC_STAT
 264 or
 265 .BR MSG_STAT ,
 266 but the calling process does not have read permission on the message queue
 267 .IR msqid ,
 268 and does not have the
 269 .B CAP_IPC_OWNER
 270 capability.
 271 .TP
 272 .B EFAULT
 273 The argument
 274 .I cmd
 275 has the value
 276 .B IPC_SET
 277 or
 278 .BR IPC_STAT ,
 279 but the address pointed to by
 280 .I buf
 281 isn't accessible.
 282 .TP
 283 .B EIDRM
 284 The message queue was removed.
 285 .TP
 286 .B EINVAL
 287 Invalid value for
 288 .I cmd
 289 or
 290 .IR msqid .
 291 Or: for a
 292 .B MSG_STAT
 293 operation, the index value specified in
 294 .I msqid
 295 referred to an array slot that is currently unused.
 296 .TP
 297 .B EPERM
 298 The argument
 299 .I cmd
 300 has the value
 301 .B IPC_SET
 302 or
 303 .BR IPC_RMID ,
 304 but the effective user ID of the calling process is not the creator
 305 (as found in
 306 .IR msg_perm.cuid )
 307 or the owner
 308 (as found in
 309 .IR msg_perm.uid )
 310 of the message queue,
 311 and the process is not privileged (Linux: it does not have the
 312 .B CAP_SYS_ADMIN
 313 capability).
 314 .SH "CONFORMING TO"
 315 SVr4, POSIX.1-2001.
 316 .\" SVID does not document the EIDRM error condition.
 317 .SH NOTES
 318 The
 319 .BR IPC_INFO ,
 320 .B MSG_STAT
 321 and
 322 .B MSG_INFO
 323 operations are used by the
 324 .BR ipcs (8)
 325 program to provide information on allocated resources.
 326 In the future these may modified or moved to a /proc file system
 327 interface.
 328
 329 Various fields in the \fIstruct msqid_ds\fP were
 330 typed as
 331 .I short
 332 under Linux 2.2
 333 and have become
 334 .I long
 335 under Linux 2.4.
 336 To take advantage of this,
 337 a recompilation under glibc-2.1.91 or later should suffice.
 338 (The kernel distinguishes old and new calls by an
 339 .B IPC_64
 340 flag in
 341 .IR cmd .)
 342 .SH "SEE ALSO"
 343 .BR msgget (2),
 344 .BR msgrcv (2),
 345 .BR msgsnd (2),
 346 .BR capabilities (7),
 347 .BR mq_overview (7),
 348 .BR svipc (7)