1SHMOP(2) Linux Programmer's Manual SHMOP(2)
2
6 shmat, shmdt - System V shared memory operations
7
9 #include <sys/types.h>
10 #include <sys/shm.h>
11 void *shmat(int shmid, const void *shmaddr, int shmflg);
13 int shmdt(const void *shmaddr);
15
17 shmat()
18 shmat() attaches the System V shared memory segment identified by shmid
19 to the address space of the calling process. The attaching address is
20 specified by shmaddr with one of the following criteria:
21 • If shmaddr is NULL, the system chooses a suitable (unused) page-
23 aligned address to attach the segment.
24 • If shmaddr isn't NULL and SHM_RND is specified in shmflg, the attach
26 occurs at the address equal to shmaddr rounded down to the nearest
27 multiple of SHMLBA.
28 • Otherwise, shmaddr must be a page-aligned address at which the attach
30 occurs.
31 In addition to SHM_RND, the following flags may be specified in the
33 shmflg bit-mask argument:
34 SHM_EXEC (Linux-specific; since Linux 2.6.9)
36 Allow the contents of the segment to be executed. The caller
37 must have execute permission on the segment.
38 SHM_RDONLY
40 Attach the segment for read-only access. The process must have
41 read permission for the segment. If this flag is not specified,
42 the segment is attached for read and write access, and the
43 process must have read and write permission for the segment.
44 There is no notion of a write-only shared memory segment.
45 SHM_REMAP (Linux-specific)
47 This flag specifies that the mapping of the segment should re‐
48 place any existing mapping in the range starting at shmaddr and
49 continuing for the size of the segment. (Normally, an EINVAL
50 error would result if a mapping already exists in this address
51 range.) In this case, shmaddr must not be NULL.
52 The brk(2) value of the calling process is not altered by the attach.
54 The segment will automatically be detached at process exit. The same
55 segment may be attached as a read and as a read-write one, and more
56 than once, in the process's address space.
57 A successful shmat() call updates the members of the shmid_ds structure
59 (see shmctl(2)) associated with the shared memory segment as follows:
60 • shm_atime is set to the current time.
62 • shm_lpid is set to the process-ID of the calling process.
64 • shm_nattch is incremented by one.
66 shmdt()
68 shmdt() detaches the shared memory segment located at the address spec‐
69 ified by shmaddr from the address space of the calling process. The
70 to-be-detached segment must be currently attached with shmaddr equal to
71 the value returned by the attaching shmat() call.
72 On a successful shmdt() call, the system updates the members of the
74 shmid_ds structure associated with the shared memory segment as fol‐
75 lows:
76 • shm_dtime is set to the current time.
78 • shm_lpid is set to the process-ID of the calling process.
80 • shm_nattch is decremented by one. If it becomes 0 and the segment is
82 marked for deletion, the segment is deleted.
83
85 On success, shmat() returns the address of the attached shared memory
86 segment; on error, (void *) -1 is returned, and errno is set to indi‐
87 cate the cause of the error.
88 On success, shmdt() returns 0; on error -1 is returned, and errno is
90 set to indicate the cause of the error.
91
93 When shmat() fails, errno is set to one of the following:
94 EACCES The calling process does not have the required permissions for
96 the requested attach type, and does not have the CAP_IPC_OWNER
97 capability in the user namespace that governs its IPC namespace.
98 EIDRM shmid points to a removed identifier.
100 EINVAL Invalid shmid value, unaligned (i.e., not page-aligned and
102 SHM_RND was not specified) or invalid shmaddr value, or can't
103 attach segment at shmaddr, or SHM_REMAP was specified and
104 shmaddr was NULL.
105 ENOMEM Could not allocate memory for the descriptor or for the page ta‐
107 bles.
108 When shmdt() fails, errno is set as follows:
110 EINVAL There is no shared memory segment attached at shmaddr; or,
112 shmaddr is not aligned on a page boundary.
113
115 POSIX.1-2001, POSIX.1-2008, SVr4.
116 In SVID 3 (or perhaps earlier), the type of the shmaddr argument was
118 changed from char * into const void *, and the returned type of shmat()
119 from char * into void *.
120
122 After a fork(2), the child inherits the attached shared memory seg‐
123 ments.
124 After an execve(2), all attached shared memory segments are detached
126 from the process.
127 Upon _exit(2), all attached shared memory segments are detached from
129 the process.
130 Using shmat() with shmaddr equal to NULL is the preferred, portable way
132 of attaching a shared memory segment. Be aware that the shared memory
133 segment attached in this way may be attached at different addresses in
134 different processes. Therefore, any pointers maintained within the
135 shared memory must be made relative (typically to the starting address
136 of the segment), rather than absolute.
137 On Linux, it is possible to attach a shared memory segment even if it
139 is already marked to be deleted. However, POSIX.1 does not specify
140 this behavior and many other implementations do not support it.
141 The following system parameter affects shmat():
143 SHMLBA Segment low boundary address multiple. When explicitly specify‐
145 ing an attach address in a call to shmat(), the caller should
146 ensure that the address is a multiple of this value. This is
147 necessary on some architectures, in order either to ensure good
148 CPU cache performance or to ensure that different attaches of
149 the same segment have consistent views within the CPU cache.
150 SHMLBA is normally some multiple of the system page size. (On
151 many Linux architectures, SHMLBA is the same as the system page
152 size.)
153 The implementation places no intrinsic per-process limit on the number
155 of shared memory segments (SHMSEG).
156
158 The two programs shown below exchange a string using a shared memory
159 segment. Further details about the programs are given below. First,
160 we show a shell session demonstrating their use.
161 In one terminal window, we run the "reader" program, which creates a
163 System V shared memory segment and a System V semaphore set. The pro‐
164 gram prints out the IDs of the created objects, and then waits for the
165 semaphore to change value.
166 $ ./svshm_string_read
168 shmid = 1114194; semid = 15
169 In another terminal window, we run the "writer" program. The "writer"
171 program takes three command-line arguments: the IDs of the shared mem‐
172 ory segment and semaphore set created by the "reader", and a string.
173 It attaches the existing shared memory segment, copies the string to
174 the shared memory, and modifies the semaphore value.
175 $ ./svshm_string_write 1114194 15 'Hello, world'
177 Returning to the terminal where the "reader" is running, we see that
179 the program has ceased waiting on the semaphore and has printed the
180 string that was copied into the shared memory segment by the writer:
181 Hello, world
183 Program source: svshm_string.h
185 The following header file is included by the "reader" and "writer" pro‐
186 grams.
187 #include <sys/types.h>
189 #include <sys/ipc.h>
190 #include <sys/shm.h>
191 #include <sys/sem.h>
192 #include <stdio.h>
193 #include <stdlib.h>
194 #include <string.h>
195 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
197 } while (0)
198 union semun { /* Used in calls to semctl() */
200 int val;
201 struct semid_ds * buf;
202 unsigned short * array;
203 #if defined(__linux__)
204 struct seminfo * __buf;
205 #endif
206 };
207 #define MEM_SIZE 4096
209 Program source: svshm_string_read.c
211 The "reader" program creates a shared memory segment and a semaphore
212 set containing one semaphore. It then attaches the shared memory ob‐
213 ject into its address space and initializes the semaphore value to 1.
214 Finally, the program waits for the semaphore value to become 0, and af‐
215 terwards prints the string that has been copied into the shared memory
216 segment by the "writer".
217 /* svshm_string_read.c
219 Licensed under GNU General Public License v2 or later.
221 */
222 #include "svshm_string.h"
223 int
225 main(int argc, char *argv[])
226 {
227 int semid, shmid;
228 union semun arg, dummy;
229 struct sembuf sop;
230 char *addr;
231 /* Create shared memory and semaphore set containing one
233 semaphore */
234 shmid = shmget(IPC_PRIVATE, MEM_SIZE, IPC_CREAT | 0600);
236 if (shmid == -1)
237 errExit("shmget");
238 semid = semget(IPC_PRIVATE, 1, IPC_CREAT | 0600);
240 if (shmid == -1)
241 errExit("shmget");
242 /* Attach shared memory into our address space */
244 addr = shmat(shmid, NULL, SHM_RDONLY);
246 if (addr == (void *) -1)
247 errExit("shmat");
248 /* Initialize semaphore 0 in set with value 1 */
250 arg.val = 1;
252 if (semctl(semid, 0, SETVAL, arg) == -1)
253 errExit("semctl");
254 printf("shmid = %d; semid = %d\n", shmid, semid);
256 /* Wait for semaphore value to become 0 */
258 sop.sem_num = 0;
260 sop.sem_op = 0;
261 sop.sem_flg = 0;
262 if (semop(semid, &sop, 1) == -1)
264 errExit("semop");
265 /* Print the string from shared memory */
267 printf("%s\n", addr);
269 /* Remove shared memory and semaphore set */
271 if (shmctl(shmid, IPC_RMID, NULL) == -1)
273 errExit("shmctl");
274 if (semctl(semid, 0, IPC_RMID, dummy) == -1)
275 errExit("semctl");
276 exit(EXIT_SUCCESS);
278 }
279 Program source: svshm_string_write.c
281 The writer program takes three command-line arguments: the IDs of the
282 shared memory segment and semaphore set that have already been created
283 by the "reader", and a string. It attaches the shared memory segment
284 into its address space, and then decrements the semaphore value to 0 in
285 order to inform the "reader" that it can now examine the contents of
286 the shared memory.
287 /* svshm_string_write.c
289 Licensed under GNU General Public License v2 or later.
291 */
292 #include "svshm_string.h"
293 int
295 main(int argc, char *argv[])
296 {
297 int semid, shmid;
298 struct sembuf sop;
299 char *addr;
300 size_t len;
301 if (argc != 4) {
303 fprintf(stderr, "Usage: %s shmid semid string\n", argv[0]);
304 exit(EXIT_FAILURE);
305 }
306 len = strlen(argv[3]) + 1; /* +1 to include trailing '\0' */
308 if (len > MEM_SIZE) {
309 fprintf(stderr, "String is too big!\n");
310 exit(EXIT_FAILURE);
311 }
312 /* Get object IDs from command-line */
314 shmid = atoi(argv[1]);
316 semid = atoi(argv[2]);
317 /* Attach shared memory into our address space and copy string
319 (including trailing null byte) into memory. */
320 addr = shmat(shmid, NULL, 0);
322 if (addr == (void *) -1)
323 errExit("shmat");
324 memcpy(addr, argv[3], len);
326 /* Decrement semaphore to 0 */
328 sop.sem_num = 0;
330 sop.sem_op = -1;
331 sop.sem_flg = 0;
332 if (semop(semid, &sop, 1) == -1)
334 errExit("semop");
335 exit(EXIT_SUCCESS);
337 }
338
340 brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_over‐
341 view(7), sysvipc(7)
342
344 This page is part of release 5.10 of the Linux man-pages project. A
345 description of the project, information about reporting bugs, and the
346 latest version of this page, can be found at
347 https://www.kernel.org/doc/man-pages/.
348Linux 2020-04-11 SHMOP(2)