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25 .\" FIXME . Add an example to this page
26 .TH SHM_OPEN 3 2017-09-15 "Linux" "Linux Programmer's Manual"
28 shm_open, shm_unlink \- create/open or unlink POSIX shared memory objects
30 .B #include <sys/mman.h>
32 .BR "#include <sys/stat.h>" " /* For mode constants */"
34 .BR "#include <fcntl.h>" " /* For O_* constants */"
36 .BI "int shm_open(const char *" name ", int " oflag ", mode_t " mode );
38 .BI "int shm_unlink(const char *" name );
40 Link with \fI\-lrt\fP.
43 creates and opens a new, or opens an existing, POSIX shared memory object.
44 A POSIX shared memory object is in effect a handle which can
45 be used by unrelated processes to
47 the same region of shared memory.
50 function performs the converse operation,
51 removing an object previously created by
56 is analogous to that of
59 specifies the shared memory object to be created or opened.
61 a shared memory object should be identified by a name of the form
63 that is, a null-terminated string of up to
65 (i.e., 255) characters consisting of an initial slash,
66 .\" glibc allows the initial slash to be omitted, and makes
67 .\" multiple initial slashes equivalent to a single slash.
68 .\" This differs from the implementation of POSIX message queues.
69 followed by one or more characters, none of which are slashes.
70 .\" glibc allows subdirectory components in the name, in which
71 .\" case the subdirectory must exist under /dev/shm, and allow the
72 .\" required permissions if a user wants to create a shared memory
73 .\" object in that subdirectory.
76 is a bit mask created by ORing together exactly one of
80 and any of the other flags listed here:
83 Open the object for read access.
84 A shared memory object opened in this way can be
91 Open the object for read-write access.
94 Create the shared memory object if it does not exist.
95 The user and group ownership of the object are taken
96 from the corresponding effective IDs of the calling process,
97 .\" In truth it is actually the filesystem IDs on Linux, but these
98 .\" are nearly always the same as the effective IDs. (MTK, Jul 05)
100 permission bits are set according to the low-order 9 bits of
102 except that those bits set in the process file mode
105 are cleared for the new object.
106 A set of macro constants which can be used to define
110 (Symbolic definitions of these constants can be obtained by including
113 A new shared memory object initially has zero length\(emthe size of the
114 object can be set using
116 The newly allocated bytes of a shared memory
117 object are automatically initialized to 0.
122 was also specified, and a shared memory object with the given
124 already exists, return an error.
125 The check for the existence of the object, and its creation if it
126 does not exist, are performed atomically.
129 If the shared memory object already exists, truncate it to zero bytes.
131 Definitions of these flag values can be obtained by including
134 On successful completion
136 returns a new file descriptor referring to the shared memory object.
137 This file descriptor is guaranteed to be the lowest-numbered file descriptor
138 not previously opened within the process.
143 is set for the file descriptor.
145 The file descriptor is normally used in subsequent calls
148 (for a newly created object) and
152 the file descriptor may be closed without affecting the memory mapping.
159 it removes a shared memory object name, and, once all processes
160 have unmapped the object, de-allocates and
161 destroys the contents of the associated memory region.
166 an object with the same
170 was specified, in which case a new, distinct object is created).
174 returns a nonnegative file descriptor.
179 returns 0 on success, or \-1 on error.
183 is set to indicate the cause of the error.
184 Values which may appear in
186 include the following:
191 the shared memory object was denied.
194 Permission was denied to
201 was specified and the caller does not have write permission on the object.
210 and the shared memory object specified by
222 The per-process limit on the number of open file descriptors has been reached.
231 The system-wide limit on the total number of open files has been reached.
234 An attempt was made to
238 that did not exist, and
243 An attempt was to made to
249 These functions are provided in glibc 2.2 and later.
251 For an explanation of the terms used in this section, see
257 Interface Attribute Value
261 T} Thread safety MT-Safe locale
265 POSIX.1-2001, POSIX.1-2008.
267 POSIX.1-2001 says that the group ownership of a newly created shared
268 memory object is set to either the calling process's effective group ID
269 or "a system default group ID".
270 POSIX.1-2008 says that the group ownership
271 may be set to either the calling process's effective group ID
272 or, if the object is visible in the filesystem,
273 the group ID of the parent directory.
276 POSIX leaves the behavior of the combination of
281 On Linux, this will successfully truncate an existing
282 shared memory object\(emthis may not be so on other UNIX systems.
284 The POSIX shared memory object implementation on Linux makes use
287 filesystem that is normally mounted under
296 .BR memfd_create (2),