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1 perry 1 SHMOP
2 !!!SHMOP
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 SYSTEM CALLS
7 RETURN VALUE
8 ERRORS
9 NOTES
10 CONFORMING TO
11 SEE ALSO
12 ----
13 !!NAME
14
15
16 shmop - shared memory operations
17 !!SYNOPSIS
18
19
20 __# include
21 ____void *shmat ( int__ ''shmid''__, const void *__''shmaddr''__, int__ ''shmflg'' __)__
22
23
24 __int shmdt ( const void
25 *__''shmaddr''__)__
26 !!DESCRIPTION
27
28
29 The function __shmat__ attaches the shared memory segment
30 identified by __shmid__ to the data segment of the
31 calling process. The attaching address is specified by
32 ''shmaddr'' with one of the following
33 criteria:
34
35
36 If ''shmaddr'' is __0__, the system tries to find an
37 unmapped region in the range 1 - 1.5G starting from the
38 upper value and coming down from there.
39
40
41 If ''shmaddr'' isn't __0__ and __SHM_RND__ is
42 asserted in ''shmflg'', the attach occurs at address
43 equal to the rounding down of ''shmaddr'' to a multiple
44 of __SHMLBA__. Otherwise ''shmaddr'' must be a page
45 aligned address at which the attach occurs.
46
47
48 If __SHM_RDONLY__ is asserted in ''shmflg'', the
49 segment is attached for reading and the process must have
50 read access permissions to the segment. Otherwise the
51 segment is attached for read and write and the process must
52 have read and write access permissions to the segment. There
53 is no notion of write-only shared memory
54 segment.
55
56
57 The __brk__ value of the calling process is not altered
58 by the attach. The segment will automatically detached at
59 process exit. The same segment may be attached as a read and
60 as a read-write one, and more than once, in the process's
61 address space.
62
63
64 On a successful __shmat__ call the system updates the
65 members of the structure __shmid_ds__ associated to the
66 shared memory segment as follows:
67
68
69 __shm_atime__ is set to the current time.
70
71
72 __shm_lpid__ is set to the process-ID of the calling
73 process.
74
75
76 __shm_nattch__ is incremented by one.
77
78
79 Note that the attach succeeds also if the shared memory
80 segment is marked as to be deleted.
81
82
83 The function __shmdt__ detaches from the calling
84 process's data segment the shared memory segment located at
85 the address specified by ''shmaddr''. The detaching
86 shared memory segment must be one among the currently
87 attached ones (to the process's address space) with
88 ''shmaddr'' equal to the value returned by the its
89 attaching __shmat__ call.
90
91
92 On a successful __shmdt__ call the system updates the
93 members of the structure __shmid_ds__ associated to the
94 shared memory segment as follows:
95
96
97 __shm_dtime__ is set to the current time.
98
99
100 __shm_lpid__ is set to the process-ID of the calling
101 process.
102
103
104 __shm_nattch__ is decremented by one. If it becomes 0 and
105 the segment is marked for deletion, the segment is
106 deleted.
107
108
109 The occupied region in the user space of the calling process
110 is unmapped.
111 !!SYSTEM CALLS
112
113
114 __fork()__
115
116
117 After a __fork()__ the child inherits the attached shared
118 memory segments.
119
120
121 __exec()__
122
123
124 After an __exec()__ all attached shared memory segments
125 are detached (not destroyed).
126
127
128 __exit()__
129
130
131 Upon __exit()__ all attached shared memory segments are
132 detached (not destroyed).
133 !!RETURN VALUE
134
135
136 On a failure both functions return __-1__ with
137 __errno__ indicating the error, otherwise __shmat__
138 returns the address of the attached shared memory segment,
139 and __shmdt__ returns __0__.
140 !!ERRORS
141
142
143 When __shmat__ fails, at return __errno__ will be set
144 to one among the following values:
145
146
147 __EACCES__
148
149
150 The calling process has no access permissions for the
151 requested attach type.
152
153
154 __EINVAL__
155
156
157 Invalid ''shmid'' value, unaligned (i.e., not
158 page-aligned and __SHM_RND__ was not specified) or
159 invalid ''shmaddr'' value, or failing attach at
160 __brk__.
161
162
163 __ENOMEM__
164
165
166 Could not allocate memory for the descriptor or for the page
167 tables.
168
169
170 The function __shmdt__ can fails only if there is no
171 shared memory segment attached at ''shmaddr'', in such a
172 case at return __errno__ will be set to
173 __EINVAL__.
174 !!NOTES
175
176
177 On executing a fork(2) system call, the child
178 inherits all the attached shared memory
179 segments.
180
181
182 The shared memory segments attached to a process executing
183 an execve(2) system call will not be attached to the
184 resulting process.
185
186
187 The following is a system parameter affecting a __shmat__
188 system call:
189
190
191 __SHMLBA__
192
193
194 Segment low boundary address multiple. Must be page aligned.
195 For the current implementation the __SHMBLA__ value is
196 __PAGE_SIZE__.
197
198
199 The implementation has no intrinsic limit to the per process
200 maximum number of shared memory segments
201 (__SHMSEG__)
202 !!CONFORMING TO
203
204
205 SVr4, SVID. SVr4 documents an additional error condition
206 EMFILE. In SVID-v4 the type of the ''shmaddr'' argument
207 was changed from __char *__ into __const void *__, and
208 the returned type of ''shmat''() from __char *__ into
209 __void *__. (Linux libc4 and libc5 have the __char *__
210 prototypes; glibc2 has __void *__.)
211 !!SEE ALSO
212
213
214 ipc(5), shmctl(2),
215 shmget(2)
216 ----
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