man7/svipc.7

   1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
   2 .\"
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  23 .\" FIXME There is now duplication of some of the information
  24 .\" below in semctl.2, msgctl.2, and shmctl.2 -- MTK, Nov 04
  25 .TH SVIPC 7 1993-11-01 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 svipc \- System V interprocess communication mechanisms
  28 .SH SYNOPSIS
  29 .nf
  30 .B
  31 # include <sys/types.h>
  32 .B
  33 # include <sys/ipc.h>
  34 .B
  35 # include <sys/msg.h>
  36 .B
  37 # include <sys/sem.h>
  38 .B
  39 # include <sys/shm.h>
  40 .SH DESCRIPTION
  41 This manual page refers to the Linux implementation of the System V
  42 interprocess communication mechanisms:
  43 message queues, semaphore sets, and shared memory segments.
  44 In the following, the word
  45 .I resource
  46 means an instantiation of one among such mechanisms.
  47 .SS Resource Access Permissions
  48 For each resource, the system uses a common structure of type
  49 .I "struct ipc_perm"
  50 to store information needed in determining permissions to perform an
  51 ipc operation.
  52 The
  53 .I ipc_perm
  54 structure, defined by the
  55 .I <sys/ipc.h>
  56 system header file, includes the following members:
  57 .in +0.5i
  58 .nf
  59
  60 struct ipc_perm {
  61     uid_t  cuid;   /* creator user ID */
  62     gid_t  cgid;   /* creator group ID */
  63     uid_t  uid;    /* owner user ID */
  64     gid_t  gid;    /* owner group ID */
  65     ushort mode;   /* r/w permissions */
  66 };
  67 .fi
  68 .in
  69 .PP
  70 The
  71 .I mode
  72 member of the
  73 .I ipc_perm
  74 structure defines, with its lower 9 bits, the access permissions to the
  75 resource for a process executing an ipc system call.
  76 The permissions are interpreted as follows:
  77 .sp
  78 .nf
  79     0400    Read by user.
  80     0200    Write by user.
  81 .sp .5
  82     0040    Read by group.
  83     0020    Write by group.
  84 .sp .5
  85     0004    Read by others.
  86     0002    Write by others.
  87 .fi
  88 .PP
  89 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
  90 Furthermore,
  91 "write"
  92 effectively means
  93 "alter"
  94 for a semaphore set.
  95 .PP
  96 The same system header file also defines the following symbolic
  97 constants:
  98 .TP 14
  99 .B IPC_CREAT
 100 Create entry if key doesn't exist.
 101 .TP
 102 .B IPC_EXCL
 103 Fail if key exists.
 104 .TP
 105 .B IPC_NOWAIT
 106 Error if request must wait.
 107 .TP
 108 .B IPC_PRIVATE
 109 Private key.
 110 .TP
 111 .B IPC_RMID
 112 Remove resource.
 113 .TP
 114 .B IPC_SET
 115 Set resource options.
 116 .TP
 117 .B IPC_STAT
 118 Get resource options.
 119 .PP
 120 Note that
 121 .B IPC_PRIVATE
 122 is a
 123 .I key_t
 124 type, while all the other symbolic constants are flag fields and can
 125 be OR'ed into an
 126 .I int
 127 type variable.
 128 .SS Message Queues
 129 A message queue is uniquely identified by a positive integer
 130 .RI "(its " msqid )
 131 and has an associated data structure of type
 132 .IR "struct msqid_ds" ,
 133 defined in
 134 .IR <sys/msg.h> ,
 135 containing the following members:
 136 .in +0.5i
 137 .nf
 138
 139 struct msqid_ds {
 140     struct ipc_perm msg_perm;
 141     msgqnum_t       msg_qnum;    /* no of messages on queue */
 142     msglen_t        msg_qbytes;  /* bytes max on a queue */
 143     pid_t           msg_lspid;   /* PID of last msgsnd(2) call */
 144     pid_t           msg_lrpid;   /* PID of last msgrcv(2) call */
 145     time_t          msg_stime;   /* last msgsnd(2) time */
 146     time_t          msg_rtime;   /* last msgrcv(2) time */
 147     time_t          msg_ctime;   /* last change time */
 148 };
 149 .fi
 150 .in
 151 .TP 11
 152 .I msg_perm
 153 .I ipc_perm
 154 structure that specifies the access permissions on the message
 155 queue.
 156 .TP
 157 .I msg_qnum
 158 Number of messages currently on the message queue.
 159 .TP
 160 .I msg_qbytes
 161 Maximum number of bytes of message text allowed on the message
 162 queue.
 163 .TP
 164 .I msg_lspid
 165 ID of the process that performed the last
 166 .BR msgsnd (2)
 167 system call.
 168 .TP
 169 .I msg_lrpid
 170 ID of the process that performed the last
 171 .BR msgrcv (2)
 172 system call.
 173 .TP
 174 .I msg_stime
 175 Time of the last
 176 .BR msgsnd (2)
 177 system call.
 178 .TP
 179 .I msg_rtime
 180 Time of the last
 181 .BR msgrcv (2)
 182 system call.
 183 .TP
 184 .I msg_ctime
 185 Time of the last
 186 system call that changed a member of the
 187 .I msqid_ds
 188 structure.
 189 .SS Semaphore Sets
 190 A semaphore set is uniquely identified by a positive integer
 191 .RI "(its " semid )
 192 and has an associated data structure of type
 193 .IR "struct semid_ds" ,
 194 defined in
 195 .IR <sys/sem.h> ,
 196 containing the following members:
 197 .in +0.5i
 198 .nf
 199
 200 struct semid_ds {
 201     struct ipc_perm sem_perm;
 202     time_t          sem_otime;   /* last operation time */
 203     time_t          sem_ctime;   /* last change time */
 204     ulong           sem_nsems;   /* count of sems in set */
 205 };
 206 .fi
 207 .in
 208 .TP 11
 209 .I sem_perm
 210 .I ipc_perm
 211 structure that specifies the access permissions on the semaphore
 212 set.
 213 .TP
 214 .I sem_otime
 215 Time of last
 216 .BR semop (2)
 217 system call.
 218 .TP
 219 .I sem_ctime
 220 Time of last
 221 .BR semctl (2)
 222 system call that changed a member of the above structure or of one
 223 semaphore belonging to the set.
 224 .TP
 225 .I sem_nsems
 226 Number of semaphores in the set.
 227 Each semaphore of the set is referenced by a non-negative integer
 228 ranging from
 229 .B 0
 230 to
 231 .IR sem_nsems\-1 .
 232 .PP
 233 A semaphore is a data structure of type
 234 .I "struct sem"
 235 containing the following members:
 236 .in +0.5i
 237 .nf
 238
 239 struct sem {
 240     int semval;  /* semaphore value */
 241     int sempid;  /* PID for last operation */
 242 .\"    ushort semncnt; /* nr awaiting semval to increase */
 243 .\"    ushort semzcnt; /* nr awaiting semval = 0 */
 244 }
 245 .fi
 246 .in
 247 .TP 11
 248 .I semval
 249 Semaphore value: a non-negative integer.
 250 .TP
 251 .I sempid
 252 ID of the last process that performed a semaphore operation
 253 on this semaphore.
 254 .\".TP
 255 .\".I semncnt
 256 .\"Number of processes suspended awaiting for
 257 .\".I semval
 258 .\"to increase.
 259 .\".TP
 260 .\".I semznt
 261 .\"Number of processes suspended awaiting for
 262 .\".I semval
 263 .\"to become zero.
 264 .SS Shared Memory Segments
 265 A shared memory segment is uniquely identified by a positive integer
 266 .RI "(its " shmid )
 267 and has an associated data structure of type
 268 .IR "struct shmid_ds" ,
 269 defined in
 270 .IR <sys/shm.h> ,
 271 containing the following members:
 272 .in +0.5i
 273 .nf
 274
 275 struct shmid_ds {
 276     struct ipc_perm shm_perm;
 277     size_t          shm_segsz;   /* size of segment */
 278     pid_t           shm_cpid;    /* PID of creator */
 279     pid_t           shm_lpid;    /* PID, last operation */
 280     shmatt_t        shm_nattch;  /* no. of current attaches */
 281     time_t          shm_atime;   /* time of last attach */
 282     time_t          shm_dtime;   /* time of last detach */
 283     time_t          shm_ctime;   /* time of last change */
 284 };
 285 .fi
 286 .in
 287 .TP 11
 288 .I shm_perm
 289 .I ipc_perm
 290 structure that specifies the access permissions on the shared memory
 291 segment.
 292 .TP
 293 .I shm_segsz
 294 Size in bytes of the shared memory segment.
 295 .TP
 296 .I shm_cpid
 297 ID of the process that created the shared memory segment.
 298 .TP
 299 .I shm_lpid
 300 ID of the last process that executed a
 301 .BR shmat (2)
 302 or
 303 .BR shmdt (2)
 304 system call.
 305 .TP
 306 .I shm_nattch
 307 Number of current alive attaches for this shared memory segment.
 308 .TP
 309 .I shm_atime
 310 Time of the last
 311 .BR shmat (2)
 312 system call.
 313 .TP
 314 .I shm_dtime
 315 Time of the last
 316 .BR shmdt (2)
 317 system call.
 318 .TP
 319 .I shm_ctime
 320 Time of the last
 321 .BR shmctl (2)
 322 system call that changed
 323 .IR shmid_ds .
 324 .SH "SEE ALSO"
 325 .BR ipc (2),
 326 .BR msgctl (2),
 327 .BR msgget (2),
 328 .BR msgrcv (2),
 329 .BR msgsnd (2),
 330 .BR semctl (2),
 331 .BR semget (2),
 332 .BR semop (2),
 333 .BR shmat (2),
 334 .BR shmctl (2),
 335 .BR shmdt (2),
 336 .BR shmget (2),
 337 .BR ftok (3)