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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  25 .\"
  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 .\" 2018-03-20, dbueso: Added MSG_STAT_ANY description.
  37 .\"
  38 .TH MSGCTL 2 2020-04-11 "Linux" "Linux Programmer's Manual"
  39 .SH NAME
  40 msgctl \- System V message control operations
  41 .SH SYNOPSIS
  42 .nf
  43 .B #include <sys/types.h>
  44 .B #include <sys/ipc.h>
  45 .B #include <sys/msg.h>
  46 .PP
  47 .BI "int msgctl(int " msqid ", int " cmd ", struct msqid_ds *" buf );
  48 .fi
  49 .SH DESCRIPTION
  50 .BR msgctl ()
  51 performs the control operation specified by
  52 .I cmd
  53 on the System\ V message queue with identifier
  54 .IR msqid .
  55 .PP
  56 The
  57 .I msqid_ds
  58 data structure is defined in \fI<sys/msg.h>\fP as follows:
  59 .PP
  60 .in +4n
  61 .EX
  62 struct msqid_ds {
  63     struct ipc_perm msg_perm;     /* Ownership and permissions */
  64     time_t          msg_stime;    /* Time of last msgsnd(2) */
  65     time_t          msg_rtime;    /* Time of last msgrcv(2) */
  66     time_t          msg_ctime;    /* Creation time/time of last
  67                                      modification via msgctl() */
  68     unsigned long   __msg_cbytes; /* Current number of bytes in
  69                                      queue (nonstandard) */
  70     msgqnum_t       msg_qnum;     /* Current number of messages
  71                                      in queue */
  72     msglen_t        msg_qbytes;   /* Maximum number of bytes
  73                                      allowed in queue */
  74     pid_t           msg_lspid;    /* PID of last msgsnd(2) */
  75     pid_t           msg_lrpid;    /* PID of last msgrcv(2) */
  76 };
  77 .EE
  78 .in
  79 .PP
  80 The fields of the
  81 .I msgid_ds
  82 structure are as follows:
  83 .TP 11
  84 .I msg_perm
  85 This is an
  86 .I ipc_perm
  87 structure (see below) that specifies the access permissions on the message
  88 queue.
  89 .TP
  90 .I msg_qnum
  91 Number of messages currently on the message queue.
  92 .TP
  93 .I msg_qbytes
  94 Maximum number of bytes of message text allowed on the message
  95 queue.
  96 .TP
  97 .I msg_lspid
  98 ID of the process that performed the last
  99 .BR msgsnd (2)
 100 system call.
 101 .TP
 102 .I msg_lrpid
 103 ID of the process that performed the last
 104 .BR msgrcv (2)
 105 system call.
 106 .TP
 107 .I msg_stime
 108 Time of the last
 109 .BR msgsnd (2)
 110 system call.
 111 .TP
 112 .I msg_rtime
 113 Time of the last
 114 .BR msgrcv (2)
 115 system call.
 116 .TP
 117 .I msg_ctime
 118 Time of creation of queue or time of last
 119 .BR msgctl ()
 120 .BR IPC_SET
 121 operation.
 122 .PP
 123 The
 124 .I ipc_perm
 125 structure is defined as follows
 126 (the highlighted fields are settable using
 127 .BR IPC_SET ):
 128 .PP
 129 .in +4n
 130 .EX
 131 struct ipc_perm {
 132     key_t          __key;       /* Key supplied to msgget(2) */
 133     uid_t          \fBuid\fP;         /* Effective UID of owner */
 134     gid_t          \fBgid\fP;         /* Effective GID of owner */
 135     uid_t          cuid;        /* Effective UID of creator */
 136     gid_t          cgid;        /* Effective GID of creator */
 137     unsigned short \fBmode\fP;        /* Permissions */
 138     unsigned short __seq;       /* Sequence number */
 139 };
 140 .EE
 141 .in
 142 .PP
 143 The least significant 9 bits of the
 144 .I mode
 145 field of the
 146 .I ipc_perm
 147 structure define the access permissions for the message queue.
 148 The permission bits are as follows:
 149 .TS
 150 l l.
 151 0400    Read by user
 152 0200    Write by user
 153 0040    Read by group
 154 0020    Write by group
 155 0004    Read by others
 156 0002    Write by others
 157 .TE
 158 .PP
 159 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
 160 .PP
 161 Valid values for
 162 .I cmd
 163 are:
 164 .TP
 165 .B IPC_STAT
 166 Copy information from the kernel data structure associated with
 167 .I msqid
 168 into the
 169 .I msqid_ds
 170 structure pointed to by
 171 .IR buf .
 172 The caller must have read permission on the message queue.
 173 .TP
 174 .B IPC_SET
 175 Write the values of some members of the
 176 .I msqid_ds
 177 structure pointed to by
 178 .I buf
 179 to the kernel data structure associated with this message queue,
 180 updating also its
 181 .I msg_ctime
 182 member.
 183 The following members of the structure are updated:
 184 .IR msg_qbytes ,
 185 .IR msg_perm.uid ,
 186 .IR msg_perm.gid ,
 187 and (the least significant 9 bits of)
 188 .IR msg_perm.mode .
 189 The effective UID of the calling process must match the owner
 190 .RI ( msg_perm.uid )
 191 or creator
 192 .RI ( msg_perm.cuid )
 193 of the message queue, or the caller must be privileged.
 194 Appropriate privilege (Linux: the
 195 .B CAP_SYS_RESOURCE
 196 capability) is required to raise the
 197 .I msg_qbytes
 198 value beyond the system parameter
 199 .BR MSGMNB .
 200 .TP
 201 .B IPC_RMID
 202 Immediately remove the message queue,
 203 awakening all waiting reader and writer processes (with an error
 204 return and
 205 .I errno
 206 set to
 207 .BR EIDRM ).
 208 The calling process must have appropriate privileges
 209 or its effective user ID must be either that of the creator or owner
 210 of the message queue.
 211 The third argument to
 212 .BR msgctl ()
 213 is ignored in this case.
 214 .TP
 215 .BR IPC_INFO " (Linux-specific)"
 216 Return information about system-wide message queue limits and
 217 parameters in the structure pointed to by
 218 .IR buf .
 219 This structure is of type
 220 .I msginfo
 221 (thus, a cast is required),
 222 defined in
 223 .I <sys/msg.h>
 224 if the
 225 .B _GNU_SOURCE
 226 feature test macro is defined:
 227 .IP
 228 .in +4n
 229 .EX
 230 struct msginfo {
 231     int msgpool; /* Size in kibibytes of buffer pool
 232                     used to hold message data;
 233                     unused within kernel */
 234     int msgmap;  /* Maximum number of entries in message
 235                     map; unused within kernel */
 236     int msgmax;  /* Maximum number of bytes that can be
 237                     written in a single message */
 238     int msgmnb;  /* Maximum number of bytes that can be
 239                     written to queue; used to initialize
 240                     msg_qbytes during queue creation
 241                     (msgget(2)) */
 242     int msgmni;  /* Maximum number of message queues */
 243     int msgssz;  /* Message segment size;
 244                     unused within kernel */
 245     int msgtql;  /* Maximum number of messages on all queues
 246                     in system; unused within kernel */
 247     unsigned short int msgseg;
 248                  /* Maximum number of segments;
 249                     unused within kernel */
 250 };
 251 .EE
 252 .in
 253 .IP
 254 The
 255 .IR msgmni ,
 256 .IR msgmax ,
 257 and
 258 .I msgmnb
 259 settings can be changed via
 260 .I /proc
 261 files of the same name; see
 262 .BR proc (5)
 263 for details.
 264 .TP
 265 .BR MSG_INFO " (Linux-specific)"
 266 Return a
 267 .I msginfo
 268 structure containing the same information as for
 269 .BR IPC_INFO ,
 270 except that the following fields are returned with information
 271 about system resources consumed by message queues: the
 272 .I msgpool
 273 field returns the number of message queues that currently exist
 274 on the system; the
 275 .I msgmap
 276 field returns the total number of messages in all queues
 277 on the system; and the
 278 .I msgtql
 279 field returns the total number of bytes in all messages
 280 in all queues on the system.
 281 .TP
 282 .BR MSG_STAT " (Linux-specific)"
 283 Return a
 284 .I msqid_ds
 285 structure as for
 286 .BR IPC_STAT .
 287 However, the
 288 .I msqid
 289 argument is not a queue identifier, but instead an index into
 290 the kernel's internal array that maintains information about
 291 all message queues on the system.
 292 .TP
 293 .BR MSG_STAT_ANY " (Linux-specific, since Linux 4.17)"
 294 Return a
 295 .I msqid_ds
 296 structure as for
 297 .BR MSG_STAT .
 298 However,
 299 .I msg_perm.mode
 300 is not checked for read access for
 301 .IR msqid
 302 meaning that any user can employ this operation (just as any user may read
 303 .IR /proc/sysvipc/msg
 304 to obtain the same information).
 305 .SH RETURN VALUE
 306 On success,
 307 .BR IPC_STAT ,
 308 .BR IPC_SET ,
 309 and
 310 .B IPC_RMID
 311 return 0.
 312 A successful
 313 .B IPC_INFO
 314 or
 315 .B MSG_INFO
 316 operation returns the index of the highest used entry in the
 317 kernel's internal array recording information about all
 318 message queues.
 319 (This information can be used with repeated
 320 .B MSG_STAT
 321 or
 322 .B MSG_STAT_ANY
 323 operations to obtain information about all queues on the system.)
 324 A successful
 325 .B MSG_STAT
 326 or
 327 .B MSG_STAT_ANY
 328 operation returns the identifier of the queue whose index was given in
 329 .IR msqid .
 330 .PP
 331 On error, \-1 is returned with
 332 .I errno
 333 indicating the error.
 334 .SH ERRORS
 335 On failure,
 336 .I errno
 337 is set to one of the following:
 338 .TP
 339 .B EACCES
 340 The argument
 341 .I cmd
 342 is equal to
 343 .B IPC_STAT
 344 or
 345 .BR MSG_STAT ,
 346 but the calling process does not have read permission on the message queue
 347 .IR msqid ,
 348 and does not have the
 349 .B CAP_IPC_OWNER
 350 capability in the user namespace that governs its IPC namespace.
 351 .TP
 352 .B EFAULT
 353 The argument
 354 .I cmd
 355 has the value
 356 .B IPC_SET
 357 or
 358 .BR IPC_STAT ,
 359 but the address pointed to by
 360 .I buf
 361 isn't accessible.
 362 .TP
 363 .B EIDRM
 364 The message queue was removed.
 365 .TP
 366 .B EINVAL
 367 Invalid value for
 368 .I cmd
 369 or
 370 .IR msqid .
 371 Or: for a
 372 .B MSG_STAT
 373 operation, the index value specified in
 374 .I msqid
 375 referred to an array slot that is currently unused.
 376 .TP
 377 .B EPERM
 378 The argument
 379 .I cmd
 380 has the value
 381 .B IPC_SET
 382 or
 383 .BR IPC_RMID ,
 384 but the effective user ID of the calling process is not the creator
 385 (as found in
 386 .IR msg_perm.cuid )
 387 or the owner
 388 (as found in
 389 .IR msg_perm.uid )
 390 of the message queue,
 391 and the caller is not privileged (Linux: does not have the
 392 .B CAP_SYS_ADMIN
 393 capability).
 394 .TP
 395 .B EPERM
 396 An attempt
 397 .RB ( IPC_SET )
 398 was made to increase
 399 .I msg_qbytes
 400 beyond the system parameter
 401 .BR MSGMNB ,
 402 but the caller is not privileged (Linux: does not have the
 403 .B CAP_SYS_RESOURCE
 404 capability).
 405 .SH CONFORMING TO
 406 POSIX.1-2001, POSIX.1-2008, SVr4.
 407 .\" SVID does not document the EIDRM error condition.
 408 .SH NOTES
 409 The inclusion of
 410 .I <sys/types.h>
 411 and
 412 .I <sys/ipc.h>
 413 isn't required on Linux or by any version of POSIX.
 414 However,
 415 some old implementations required the inclusion of these header files,
 416 and the SVID also documented their inclusion.
 417 Applications intended to be portable to such old systems may need
 418 to include these header files.
 419 .\" Like Linux, the FreeBSD man pages still document
 420 .\" the inclusion of these header files.
 421 .PP
 422 The
 423 .BR IPC_INFO ,
 424 .B MSG_STAT
 425 and
 426 .B MSG_INFO
 427 operations are used by the
 428 .BR ipcs (1)
 429 program to provide information on allocated resources.
 430 In the future these may modified or moved to a
 431 .I /proc
 432 filesystem interface.
 433 .PP
 434 Various fields in the \fIstruct msqid_ds\fP were
 435 typed as
 436 .I short
 437 under Linux 2.2
 438 and have become
 439 .I long
 440 under Linux 2.4.
 441 To take advantage of this,
 442 a recompilation under glibc-2.1.91 or later should suffice.
 443 (The kernel distinguishes old and new calls by an
 444 .B IPC_64
 445 flag in
 446 .IR cmd .)
 447 .SH SEE ALSO
 448 .BR msgget (2),
 449 .BR msgrcv (2),
 450 .BR msgsnd (2),
 451 .BR capabilities (7),
 452 .BR mq_overview (7),
 453 .BR sysvipc (7)