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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  26 .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
  27 .\" Modified Sun Feb 18 01:59:29 2001 by Andries E. Brouwer <aeb@cwi.nl>
  28 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  29 .\"     Added notes on CAP_IPC_OWNER requirement
  30 .\" Modified, 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  31 .\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
  32 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  33 .\"     Language and formatting clean-ups
  34 .\"     Added msqid_ds and ipc_perm structure definitions
  35 .\" 2005-08-02, mtk: Added IPC_INFO, MSG_INFO, MSG_STAT descriptions
  36 .\"
  37 .TH MSGCTL 2 2012-05-31 "Linux" "Linux Programmer's Manual"
  38 .SH NAME
  39 msgctl \- message control operations
  40 .SH SYNOPSIS
  41 .nf
  42 .B #include <sys/types.h>
  43 .B #include <sys/ipc.h>
  44 .B #include <sys/msg.h>
  45
  46 .BI "int msgctl(int " msqid ", int " cmd ", struct msqid_ds *" buf );
  47 .fi
  48 .SH DESCRIPTION
  49 .BR msgctl ()
  50 performs the control operation specified by
  51 .I cmd
  52 on the message queue with identifier
  53 .IR msqid .
  54 .PP
  55 The
  56 .I msqid_ds
  57 data structure is defined in \fI<sys/msg.h>\fP as follows:
  58 .nf
  59 .in +4n
  60
  61 struct msqid_ds {
  62     struct ipc_perm msg_perm;     /* Ownership and permissions */
  63     time_t          msg_stime;    /* Time of last msgsnd(2) */
  64     time_t          msg_rtime;    /* Time of last msgrcv(2) */
  65     time_t          msg_ctime;    /* Time of last change */
  66     unsigned long   __msg_cbytes; /* Current number of bytes in
  67                                      queue (nonstandard) */
  68     msgqnum_t       msg_qnum;     /* Current number of messages
  69                                      in queue */
  70     msglen_t        msg_qbytes;   /* Maximum number of bytes
  71                                      allowed in queue */
  72     pid_t           msg_lspid;    /* PID of last msgsnd(2) */
  73     pid_t           msg_lrpid;    /* PID of last msgrcv(2) */
  74 };
  75 .in
  76 .fi
  77 .PP
  78 The
  79 .I ipc_perm
  80 structure is defined as follows
  81 (the highlighted fields are settable using
  82 .BR IPC_SET ):
  83 .PP
  84 .nf
  85 .in +4n
  86 struct ipc_perm {
  87     key_t          __key;       /* Key supplied to msgget(2) */
  88     uid_t          \fBuid\fP;         /* Effective UID of owner */
  89     gid_t          \fBgid\fP;         /* Effective GID of owner */
  90     uid_t          cuid;        /* Effective UID of creator */
  91     gid_t          cgid;        /* Effective GID of creator */
  92     unsigned short \fBmode\fP;        /* Permissions */
  93     unsigned short __seq;       /* Sequence number */
  94 };
  95 .in
  96 .fi
  97 .PP
  98 Valid values for
  99 .I cmd
 100 are:
 101 .TP
 102 .B IPC_STAT
 103 Copy information from the kernel data structure associated with
 104 .I msqid
 105 into the
 106 .I msqid_ds
 107 structure pointed to by
 108 .IR buf .
 109 The caller must have read permission on the message queue.
 110 .TP
 111 .B IPC_SET
 112 Write the values of some members of the
 113 .I msqid_ds
 114 structure pointed to by
 115 .I buf
 116 to the kernel data structure associated with this message queue,
 117 updating also its
 118 .I msg_ctime
 119 member.
 120 The following members of the structure are updated:
 121 .IR msg_qbytes ,
 122 .IR msg_perm.uid ,
 123 .IR msg_perm.gid ,
 124 and (the least significant 9 bits of)
 125 .IR msg_perm.mode .
 126 The effective UID of the calling process must match the owner
 127 .RI ( msg_perm.uid )
 128 or creator
 129 .RI ( msg_perm.cuid )
 130 of the message queue, or the caller must be privileged.
 131 Appropriate privilege (Linux: the
 132 .B CAP_IPC_RESOURCE
 133 capability) is required to raise the
 134 .I msg_qbytes
 135 value beyond the system parameter
 136 .BR MSGMNB .
 137 .TP
 138 .B IPC_RMID
 139 Immediately remove the message queue,
 140 awakening all waiting reader and writer processes (with an error
 141 return and
 142 .I errno
 143 set to
 144 .BR EIDRM ).
 145 The calling process must have appropriate privileges
 146 or its effective user ID must be either that of the creator or owner
 147 of the message queue.
 148 .TP
 149 .BR IPC_INFO " (Linux-specific)"
 150 Returns information about system-wide message queue limits and
 151 parameters in the structure pointed to by
 152 .IR buf .
 153 This structure is of type
 154 .I msginfo
 155 (thus, a cast is required),
 156 defined in
 157 .I <sys/msg.h>
 158 if the
 159 .B _GNU_SOURCE
 160 feature test macro is defined:
 161 .nf
 162 .in +4n
 163
 164 struct msginfo {
 165     int msgpool; /* Size in kibibytes of buffer pool
 166                     used to hold message data;
 167                     unused within kernel */
 168     int msgmap;  /* Maximum number of entries in message
 169                     map; unused within kernel */
 170     int msgmax;  /* Maximum number of bytes that can be
 171                     written in a single message */
 172     int msgmnb;  /* Maximum number of bytes that can be
 173                     written to queue; used to initialize
 174                     msg_qbytes during queue creation
 175                     (msgget(2)) */
 176     int msgmni;  /* Maximum number of message queues */
 177     int msgssz;  /* Message segment size;
 178                     unused within kernel */
 179     int msgtql;  /* Maximum number of messages on all queues
 180                     in system; unused within kernel */
 181     unsigned short int msgseg;
 182                  /* Maximum number of segments;
 183                     unused within kernel */
 184 };
 185
 186 .in
 187 .fi
 188 The
 189 .IR msgmni ,
 190 .IR msgmax ,
 191 and
 192 .I msgmnb
 193 settings can be changed via
 194 .I /proc
 195 files of the same name; see
 196 .BR proc (5)
 197 for details.
 198 .TP
 199 .BR MSG_INFO " (Linux-specific)"
 200 Returns a
 201 .I msginfo
 202 structure containing the same information as for
 203 .BR IPC_INFO ,
 204 except that the following fields are returned with information
 205 about system resources consumed by message queues: the
 206 .I msgpool
 207 field returns the number of message queues that currently exist
 208 on the system; the
 209 .I msgmap
 210 field returns the total number of messages in all queues
 211 on the system; and the
 212 .I msgtql
 213 field returns the total number of bytes in all messages
 214 in all queues on the system.
 215 .TP
 216 .BR MSG_STAT " (Linux-specific)"
 217 Returns a
 218 .I msqid_ds
 219 structure as for
 220 .BR IPC_STAT .
 221 However, the
 222 .I msqid
 223 argument is not a queue identifier, but instead an index into
 224 the kernel's internal array that maintains information about
 225 all message queues on the system.
 226 .SH RETURN VALUE
 227 On success,
 228 .BR IPC_STAT ,
 229 .BR IPC_SET ,
 230 and
 231 .B IPC_RMID
 232 return 0.
 233 A successful
 234 .B IPC_INFO
 235 or
 236 .B MSG_INFO
 237 operation returns the index of the highest used entry in the
 238 kernel's internal array recording information about all
 239 message queues.
 240 (This information can be used with repeated
 241 .B MSG_STAT
 242 operations to obtain information about all queues on the system.)
 243 A successful
 244 .B MSG_STAT
 245 operation returns the identifier of the queue whose index was given in
 246 .IR msqid .
 247
 248 On error, \-1 is returned with
 249 .I errno
 250 indicating the error.
 251 .SH ERRORS
 252 On failure,
 253 .I errno
 254 is set to one of the following:
 255 .TP
 256 .B EACCES
 257 The argument
 258 .I cmd
 259 is equal to
 260 .B IPC_STAT
 261 or
 262 .BR MSG_STAT ,
 263 but the calling process does not have read permission on the message queue
 264 .IR msqid ,
 265 and does not have the
 266 .B CAP_IPC_OWNER
 267 capability.
 268 .TP
 269 .B EFAULT
 270 The argument
 271 .I cmd
 272 has the value
 273 .B IPC_SET
 274 or
 275 .BR IPC_STAT ,
 276 but the address pointed to by
 277 .I buf
 278 isn't accessible.
 279 .TP
 280 .B EIDRM
 281 The message queue was removed.
 282 .TP
 283 .B EINVAL
 284 Invalid value for
 285 .I cmd
 286 or
 287 .IR msqid .
 288 Or: for a
 289 .B MSG_STAT
 290 operation, the index value specified in
 291 .I msqid
 292 referred to an array slot that is currently unused.
 293 .TP
 294 .B EPERM
 295 The argument
 296 .I cmd
 297 has the value
 298 .B IPC_SET
 299 or
 300 .BR IPC_RMID ,
 301 but the effective user ID of the calling process is not the creator
 302 (as found in
 303 .IR msg_perm.cuid )
 304 or the owner
 305 (as found in
 306 .IR msg_perm.uid )
 307 of the message queue,
 308 and the process is not privileged (Linux: it does not have the
 309 .B CAP_SYS_ADMIN
 310 capability).
 311 .SH CONFORMING TO
 312 SVr4, POSIX.1-2001.
 313 .\" SVID does not document the EIDRM error condition.
 314 .SH NOTES
 315 The inclusion of
 316 .I <sys/types.h>
 317 and
 318 .I <sys/ipc.h>
 319 isn't required on Linux or by any version of POSIX.
 320 However,
 321 some old implementations required the inclusion of these header files,
 322 and the SVID also documented their inclusion.
 323 Applications intended to be portable to such old systems may need
 324 to include these header files.
 325 .\" Like Linux, the FreeBSD man pages still document
 326 .\" the inclusion of these header files.
 327
 328 The
 329 .BR IPC_INFO ,
 330 .B MSG_STAT
 331 and
 332 .B MSG_INFO
 333 operations are used by the
 334 .BR ipcs (1)
 335 program to provide information on allocated resources.
 336 In the future these may modified or moved to a /proc file system
 337 interface.
 338
 339 Various fields in the \fIstruct msqid_ds\fP were
 340 typed as
 341 .I short
 342 under Linux 2.2
 343 and have become
 344 .I long
 345 under Linux 2.4.
 346 To take advantage of this,
 347 a recompilation under glibc-2.1.91 or later should suffice.
 348 (The kernel distinguishes old and new calls by an
 349 .B IPC_64
 350 flag in
 351 .IR cmd .)
 352 .SH SEE ALSO
 353 .BR msgget (2),
 354 .BR msgrcv (2),
 355 .BR msgsnd (2),
 356 .BR capabilities (7),
 357 .BR mq_overview (7),
 358 .BR svipc (7)