shmdt(2) - osol9

1shmop(2)                         System Calls                         shmop(2)
2
3
4

NAME

6       shmop, shmat, shmdt - shared memory operations
7

SYNOPSIS

9       #include <sys/types.h>
10       #include <sys/shm.h>
11
12       void *shmat(int shmid, const void *shmaddr, int shmflg);
13
14
15       int shmdt(char *shmaddr);
16
17
18   Standard conforming
19       int shmdt(const void *shmaddr);
20
21

DESCRIPTION

23       The shmat() function attaches the shared memory segment associated with
24       the shared memory identifier specified by shmid to the data segment  of
25       the calling process.
26
27
28       The  permission required for a shared memory control operation is given
29       as {token}, where token is the type of permission needed. The types  of
30       permission are interpreted as follows:
31
32         00400    READ by user
33         00200    WRITE by user
34         00040    READ by group
35         00020    WRITE by group
36         00004    READ by others
37         00002    WRITE by others
38
39
40
41       See  the  Shared  Memory  Operation Permissions section of Intro(2) for
42       more information.
43
44
45       For shared memory segments created with the SHM_SHARE_MMU or  SHM_PAGE‐
46       ABLE  flags, the default protections cannot be changed so as to prevent
47       a single process from affecting other processes sharing the same shared
48       segment.
49
50
51       When  (shmflg&SHM_SHARE_MMU) is true, virtual memory resources in addi‐
52       tion to shared memory itself are shared among processes  that  use  the
53       same shared memory.
54
55
56       When (shmflg&SHM_PAGEABLE) is true, virtual memory resources are shared
57       and the dynamic shared memory (DISM) framework is created. The  dynamic
58       shared  memory  can be resized dynamically within the specified size in
59       shmget(2). The DISM shared memory is pageable unless it is locked.
60
61
62       The shared memory segment is attached to the data segment of the  call‐
63       ing  process  at  the  address specified based on one of the  following
64       criteria:
65
66           o      If shmaddr is equal to (void *) 0, the segment  is  attached
67                  to the first available address as selected by the system.
68
69           o      If    shmaddr   is   equal   to   (void  *)  0  and  (  shm‐
70                  flg&SHM_SHARE_MMU) or (shmflg&SHM_PAGEABLE)  is  true,  then
71                  the  segment  is  attached  to  the first available suitably
72                  aligned  address.  When  (shmflg&SHM_SHARE_MMU)   or   (shm‐
73                  flg&SHM_PAGEABLE)  is  set, however, the permission given by
74                  shmget() determines whether  the  segment  is  attached  for
75                  reading or reading and writing.
76
77           o      If  shmaddr  is not equal to (void *) 0 and (shmflg&SHM_RND)
78                  is true, the segment is attached to  the  address  given  by
79                  (shmaddr-  (shmaddr modulus SHMLBA)).
80
81           o      If  shmaddr  is not equal to (void *) 0 and (shmflg&SHM_RND)
82                  is false, the segment is attached to the  address  given  by
83                  shmaddr.
84
85           o      The  segment  is attached for reading if (shmflg&SHM_RDONLY)
86                  is true {READ}, otherwise it is  attached  for  reading  and
87                  writing {READ/WRITE}.
88
89
90       The  shmdt()  function detaches from the calling process's data segment
91       the shared memory segment located at the address specified by  shmaddr.
92       If  the  application  is  standard-conforming  (see  standards(5)), the
93       shmaddr argument is of type const void *. Otherwise it is of type  char
94       *.
95
96
97       Shared memory segments must be explicitly removed after the last refer‐
98       ence to them has been removed.
99

RETURN VALUES

101       Upon successful completion, shmat()  returns  the  data  segment  start
102       address  of the attached shared memory segment; shmdt() returns 0. Oth‐
103       erwise, −1 is returned, the shared memory segment is not attached,  and
104       errno is set to indicate the error.
105

ERRORS

107       The shmat() function will fail if:
108
109       EACCES    Operation  permission  is  denied to the calling process (see
110                 Intro(2)).
111
112
113       EINVAL    The shmid argument is not a valid shared memory identifier.
114
115                 The shmaddr argument is not equal to  0,  and  the  value  of
116                 (shmaddr-  (shmaddr modulus SHMLBA)) is an illegal address.
117
118                 The  shmaddr  argument  is  not  equal  to  0,  is an illegal
119                 address, and (shmflg&SHM_RND) is false.
120
121                 The shmaddr argument is not  equal  to  0,  is  not  properly
122                 aligned, and (shmfg&SHM_SHARE_MMU) is true.
123
124                 SHM_SHARE_MMU is not supported in certain architectures.
125
126                 Both  (shmflg&SHM_SHARE_MMU)  and  (shmflg&SHM_PAGEABLE)  are
127                 true.
128
129                 (shmflg&SHM_SHARE_MMU) is true and the shared memory  segment
130                 specified  by  shmid() had previously been attached by a call
131                 to shmat() in which (shmflg&SHM_PAGEABLE) was true.
132
133                 (shmflg&SHM_PAGEABLE) is true and the shared  memory  segment
134                 specified  by  shmid() had previously been attached by a call
135                 to shmat() in which (shmflg&SHM_SHARE_MMU) was true.
136
137
138       EMFILE    The number of shared memory segments attached to the  calling
139                 process would exceed the system-imposed limit.
140
141
142       ENOMEM    The  available data space  is not large enough to accommodate
143                 the shared memory segment.
144
145
146
147       The shmdt() function will fail if:
148
149       EINVAL    The shmaddr argument is not the data segment start address of
150                 a shared memory segment.
151
152
153       ENOMEM    (shmflg&SHM_SHARE_MMU)  is  true  and attaching to the shared
154                 memory segment would exceed a limit or  resource  control  on
155                 locked memory.
156
157

WARNINGS

159       Using  a fixed value for the shmaddr argument can adversely affect per‐
160       formance on certain platforms due to D-cache aliasing.
161

ATTRIBUTES

163       See attributes(5) for descriptions of the following attributes:
164
165
166
167
168       ┌─────────────────────────────┬─────────────────────────────┐
169       │      ATTRIBUTE TYPE         │      ATTRIBUTE VALUE        │
170       ├─────────────────────────────┼─────────────────────────────┤
171       │Interface Stability          │Committed                    │
172       ├─────────────────────────────┼─────────────────────────────┤
173       │MT-Level                     │Async-Signal-Safe            │
174       ├─────────────────────────────┼─────────────────────────────┤
175       │Standard                     │See standards(5).            │
176       └─────────────────────────────┴─────────────────────────────┘
177