1SHMOP(2) Linux Programmer's Manual SHMOP(2)
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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() attaches the System V shared memory segment identified by shmid
18 to the address space of the calling process. The attaching address is
19 specified by shmaddr with one of the following criteria:
20 If shmaddr is NULL, the system chooses a suitable (unused) address at
22 which to attach the segment.
23 If shmaddr isn't NULL and SHM_RND is specified in shmflg, the attach
25 occurs at the address equal to shmaddr rounded down to the nearest mul‐
26 tiple of SHMLBA. Otherwise shmaddr must be a page-aligned address at
27 which the attach occurs.
28 If SHM_RDONLY is specified in shmflg, the segment is attached for read‐
30 ing and the process must have read permission for the segment. Other‐
31 wise the segment is attached for read and write and the process must
32 have read and write permission for the segment. There is no notion of
33 a write-only shared memory segment.
34 The (Linux-specific) SHM_REMAP flag may be specified in shmflg to indi‐
36 cate that the mapping of the segment should replace any existing map‐
37 ping in the range starting at shmaddr and continuing for the size of
38 the segment. (Normally an EINVAL error would result if a mapping
39 already exists in this address range.) In this case, shmaddr must not
40 be NULL.
41 The brk(2) value of the calling process is not altered by the attach.
43 The segment will automatically be detached at process exit. The same
44 segment may be attached as a read and as a read-write one, and more
45 than once, in the process's address space.
46 A successful shmat() call updates the members of the shmid_ds structure
48 (see shmctl(2)) associated with the shared memory segment as follows:
49 shm_atime is set to the current time.
51 shm_lpid is set to the process-ID of the calling process.
53 shm_nattch is incremented by one.
55 shmdt() detaches the shared memory segment located at the address spec‐
57 ified by shmaddr from the address space of the calling process. The
58 to-be-detached segment must be currently attached with shmaddr equal to
59 the value returned by the attaching shmat() call.
60 On a successful shmdt() call the system updates the members of the
62 shmid_ds structure associated with the shared memory segment as fol‐
63 lows:
64 shm_dtime is set to the current time.
66 shm_lpid is set to the process-ID of the calling process.
68 shm_nattch is decremented by one. If it becomes 0 and the seg‐
70 ment is marked for deletion, the segment is deleted.
71 After a fork(2) the child inherits the attached shared memory segments.
73 After an execve(2) all attached shared memory segments are detached
75 from the process.
76 Upon _exit(2) all attached shared memory segments are detached from the
78 process.
79
81 On success shmat() returns the address of the attached shared memory
82 segment; on error (void *) -1 is returned, and errno is set to indicate
83 the cause of the error.
84 On success shmdt() returns 0; on error -1 is returned, and errno is set
86 to indicate the cause of the error.
87
89 When shmat() fails, errno is set to one of the following:
90 EACCES The calling process does not have the required permissions for
92 the requested attach type, and does not have the CAP_IPC_OWNER
93 capability.
94 EIDRM shmid points to a removed identifier.
96 EINVAL Invalid shmid value, unaligned (i.e., not page-aligned and
98 SHM_RND was not specified) or invalid shmaddr value, or can't
99 attach segment at shmaddr, or SHM_REMAP was specified and
100 shmaddr was NULL.
101 ENOMEM Could not allocate memory for the descriptor or for the page
103 tables.
104 When shmdt() fails, errno is set as follows:
106 EINVAL There is no shared memory segment attached at shmaddr; or,
108 shmaddr is not aligned on a page boundary.
109
111 SVr4, POSIX.1-2001.
112 In SVID 3 (or perhaps earlier) the type of the shmaddr argument was
114 changed from char * into const void *, and the returned type of shmat()
115 from char * into void *. (Linux libc4 and libc5 have the char * proto‐
116 types; glibc2 has void *.)
117
119 Using shmat() with shmaddr equal to NULL is the preferred, portable way
120 of attaching a shared memory segment. Be aware that the shared memory
121 segment attached in this way may be attached at different addresses in
122 different processes. Therefore, any pointers maintained within the
123 shared memory must be made relative (typically to the starting address
124 of the segment), rather than absolute.
125 On Linux, it is possible to attach a shared memory segment even if it
127 is already marked to be deleted. However, POSIX.1-2001 does not spec‐
128 ify this behavior and many other implementations do not support it.
129 The following system parameter affects shmat():
131 SHMLBA Segment low boundary address multiple. Must be page aligned.
133 For the current implementation the SHMLBA value is PAGE_SIZE.
134 The implementation places no intrinsic limit on the per-process maximum
136 number of shared memory segments (SHMSEG).
137
139 brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_over‐
140 view(7), svipc(7)
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143 This page is part of release 3.53 of the Linux man-pages project. A
144 description of the project, and information about reporting bugs, can
145 be found at http://www.kernel.org/doc/man-pages/.
146Linux 2013-02-12 SHMOP(2)