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d748a962 | 1 | .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it) |
2 | .\" | |
3 | .\" Permission is granted to make and distribute verbatim copies of this | |
4 | .\" manual provided the copyright notice and this permission notice are | |
5 | .\" preserved on all copies. | |
6 | .\" | |
7 | .\" Permission is granted to copy and distribute modified versions of this | |
8 | .\" manual under the conditions for verbatim copying, provided that the | |
9 | .\" entire resulting derived work is distributed under the terms of a | |
10 | .\" permission notice identical to this one | |
11 | .\" | |
12 | .\" Since the Linux kernel and libraries are constantly changing, this | |
13 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
14 | .\" responsibility for errors or omissions, or for damages resulting from | |
15 | .\" the use of the information contained herein. The author(s) may not | |
16 | .\" have taken the same level of care in the production of this manual, | |
17 | .\" which is licensed free of charge, as they might when working | |
18 | .\" professionally. | |
19 | .\" | |
20 | .\" Formatted or processed versions of this manual, if unaccompanied by | |
21 | .\" the source, must acknowledge the copyright and authors of this work. | |
22 | .\" | |
23 | .\" Modified Sun Nov 28 17:06:19 1993, Rik Faith (faith@cs.unc.edu) | |
24 | .\" with material from Luigi P. Bai (lpb@softint.com) | |
25 | .\" Portions Copyright 1993 Luigi P. Bai | |
26 | .\" Modified Tue Oct 22 22:04:23 1996 by Eric S. Raymond <esr@thyrsus.com> | |
27 | .\" Modified, 5 Jan 2002, Michael Kerrisk <mtk16@ext.canterbury.ac.nz> | |
28 | .\" | |
29 | .TH SHMOP 2 2002-01-05 "Linux 2.5" "Linux Programmer's Manual" | |
30 | .SH NAME | |
31 | shmop \- shared memory operations | |
32 | .SH SYNOPSIS | |
33 | .nf | |
34 | .B | |
35 | #include <sys/types.h> | |
36 | .B | |
37 | #include <sys/shm.h> | |
38 | .fi | |
39 | .sp | |
40 | .BI "void *shmat(int " shmid , | |
41 | .BI "const void *" shmaddr , | |
42 | .BI "int " shmflg ); | |
43 | .sp | |
44 | .BI "int shmdt(const void *" shmaddr ); | |
45 | .SH DESCRIPTION | |
46 | The function | |
47 | .B shmat | |
48 | attaches the shared memory segment identified by | |
49 | .B shmid | |
50 | to the address space of the calling process. | |
51 | The attaching address is specified by | |
52 | .I shmaddr | |
53 | with one of the following criteria: | |
54 | .LP | |
55 | If | |
56 | .I shmaddr | |
57 | is | |
58 | .BR NULL , | |
59 | the system chooses a suitable (unused) address at which to attach | |
60 | the segment. | |
61 | .LP | |
62 | If | |
63 | .I shmaddr | |
64 | isn't | |
65 | .B NULL | |
66 | and | |
67 | .B SHM_RND | |
68 | is asserted in | |
69 | .IR shmflg , | |
70 | the attach occurs at the address equal to | |
71 | .I shmaddr | |
72 | rounded down to the nearest multiple of | |
73 | .BR SHMLBA . | |
74 | Otherwise | |
75 | .I shmaddr | |
76 | must be a page aligned address at which the attach occurs. | |
77 | .PP | |
78 | If | |
79 | .B SHM_RDONLY | |
80 | is asserted in | |
81 | .IR shmflg , | |
82 | the segment is attached for reading and the process must have | |
83 | read permission for the segment. | |
84 | Otherwise the segment is attached for read and write | |
85 | and the process must have read and write permission for the segment. | |
86 | There is no notion of write-only shared memory segment. | |
87 | .PP | |
88 | The | |
89 | .B brk | |
90 | value of the calling process is not altered by the attach. | |
91 | The segment will automatically be detached at process exit. | |
92 | The same segment may be attached as a read and as a read-write | |
93 | one, and more than once, in the process's address space. | |
94 | .PP | |
95 | On a successful | |
96 | .B shmat | |
97 | call the system updates the members of the | |
98 | .B shmid_ds | |
99 | structure associated to the shared memory segment as follows: | |
100 | .IP | |
101 | .B shm_atime | |
102 | is set to the current time. | |
103 | .IP | |
104 | .B shm_lpid | |
105 | is set to the process-ID of the calling process. | |
106 | .IP | |
107 | .B shm_nattch | |
108 | is incremented by one. | |
109 | .PP | |
110 | Note that the attach succeeds also if the shared memory segment is | |
111 | marked to be deleted. | |
112 | .PP | |
113 | The function | |
114 | .B shmdt | |
115 | detaches the shared memory segment located at the address specified by | |
116 | .I shmaddr | |
117 | from the address space of the calling process. | |
118 | The to\-be\-detached segment must be currently | |
119 | attached with | |
120 | .I shmaddr | |
121 | equal to the value returned by the its attaching | |
122 | .B shmat | |
123 | call. | |
124 | .PP | |
125 | On a successful | |
126 | .B shmdt | |
127 | call the system updates the members of the | |
128 | .B shmid_ds | |
129 | structure associated with the shared memory segment as follows: | |
130 | .IP | |
131 | .B shm_dtime | |
132 | is set to the current time. | |
133 | .IP | |
134 | .B shm_lpid | |
135 | is set to the process-ID of the calling process. | |
136 | .IP | |
137 | .B shm_nattch | |
138 | is decremented by one. | |
139 | If it becomes 0 and the segment is marked for deletion, | |
140 | the segment is deleted. | |
141 | .PP | |
142 | The occupied region in the user space of the calling process is | |
143 | unmapped. | |
144 | .SH "SYSTEM CALLS" | |
145 | .TP | |
146 | .B fork() | |
147 | After a | |
148 | .B fork() | |
149 | the child inherits the attached shared memory segments. | |
150 | .TP | |
151 | .B exec() | |
152 | After an | |
153 | .B exec() | |
154 | all attached shared memory segments are detached from the process. | |
155 | .TP | |
156 | .B exit() | |
157 | Upon | |
158 | .B exit() | |
159 | all attached shared memory segments are detached from the process. | |
160 | .SH "RETURN VALUE" | |
161 | On failure both functions return | |
162 | .B \-1 | |
163 | with | |
164 | .I errno | |
165 | indicating the error. | |
166 | On success | |
167 | .B shmat | |
168 | returns the address of the attached shared memory segment, and | |
169 | .B shmdt | |
170 | returns | |
171 | .BR 0 . | |
172 | .SH ERRORS | |
173 | When | |
174 | .B shmat | |
175 | fails, | |
176 | .I errno | |
177 | is set to one of the following: | |
178 | .TP 11 | |
179 | .B EACCES | |
180 | The calling process has no access permissions for the requested attach | |
181 | type. | |
182 | .TP | |
183 | .B EINVAL | |
184 | Invalid | |
185 | .I shmid | |
186 | value, unaligned (i.e., not page-aligned and \fBSHM_RND\fP was not | |
187 | specified) or invalid | |
188 | .I shmaddr | |
189 | value, or failing attach at | |
190 | .BR brk . | |
191 | .TP | |
192 | .B ENOMEM | |
193 | Could not allocate memory for the descriptor or for the page tables. | |
194 | .PP | |
195 | The function | |
196 | .B shmdt | |
197 | can fail only if there is no shared memory segment attached at | |
198 | .IR shmaddr , | |
199 | in such a case at return | |
200 | .I errno | |
201 | will be set to | |
202 | .BR EINVAL . | |
203 | .\" Actually the above is what *should* be done, according to POSIX. | |
204 | .\" However as at kernel 2.2.19, and 2.4.15, shmdt() never returns an | |
205 | .\" error, even if shmaddr is invalid. (MTK, Jan 2002) | |
206 | .SH NOTES | |
207 | Using | |
208 | .B shmat | |
209 | with | |
210 | .I shmaddr | |
211 | equal to | |
212 | .B NULL | |
213 | is the preferred, portable way of attaching a shared memory segment. | |
214 | Be aware that the shared memory segment attached in this way | |
215 | may be attached at different addresses in different processes. | |
216 | Therefore, any pointers maintained within the shared memory must be | |
217 | made relative (typically to the starting address of the segment), | |
218 | rather than absolute. | |
219 | .LP | |
220 | The following system parameter affects a | |
221 | .B shmat | |
222 | system call: | |
223 | .TP 11 | |
224 | .B SHMLBA | |
225 | Segment low boundary address multiple. | |
226 | Must be page aligned. | |
227 | For the current implementation the | |
228 | .B SHMBLA | |
229 | value is | |
230 | .BR PAGE_SIZE . | |
231 | .PP | |
232 | The implementation has no intrinsic limit to the per\-process maximum | |
233 | number of shared memory segments | |
234 | .RB ( SHMSEG ) | |
235 | .SH "CONFORMING TO" | |
236 | SVr4, SVID. SVr4 documents an additional error condition EMFILE. | |
237 | In SVID-v4 the type of the \fIshmaddr\fP argument was changed from | |
238 | .B "char *" | |
239 | into | |
240 | .BR "const void *" , | |
241 | and the returned type of \fIshmat\fP() from | |
242 | .B "char *" | |
243 | into | |
244 | .BR "void *" . | |
245 | (Linux libc4 and libc5 have the | |
246 | .B "char *" | |
247 | prototypes; glibc2 has | |
248 | .BR "void *" .) | |
249 | .SH "SEE ALSO" | |
250 | .BR ipc (5), | |
251 | .BR shmctl (2), | |
252 | .BR shmget (2) |