1SHM_OPEN(3) Linux Programmer's Manual SHM_OPEN(3)
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6 shm_open, shm_unlink - create/open or unlink POSIX shared memory
7 objects
8
10 #include <sys/mman.h>
11 #include <sys/stat.h> /* For mode constants */
12 #include <fcntl.h> /* For O_* constants */
13 int shm_open(const char *name, int oflag, mode_t mode);
15 int shm_unlink(const char *name);
17 Link with -lrt.
19
21 shm_open() creates and opens a new, or opens an existing, POSIX shared
22 memory object. A POSIX shared memory object is in effect a handle
23 which can be used by unrelated processes to mmap(2) the same region of
24 shared memory. The shm_unlink() function performs the converse opera‐
25 tion, removing an object previously created by shm_open().
26 The operation of shm_open() is analogous to that of open(2). name
28 specifies the shared memory object to be created or opened. For porta‐
29 ble use, a shared memory object should be identified by a name of the
30 form /somename; that is, a null-terminated string of up to NAME_MAX
31 (i.e., 255) characters consisting of an initial slash, followed by one
32 or more characters, none of which are slashes.
33 oflag is a bit mask created by ORing together exactly one of O_RDONLY
35 or O_RDWR and any of the other flags listed here:
36 O_RDONLY Open the object for read access. A shared memory object
38 opened in this way can be mmap(2)ed only for read
39 (PROT_READ) access.
40 O_RDWR Open the object for read-write access.
42 O_CREAT Create the shared memory object if it does not exist. The
44 user and group ownership of the object are taken from the
45 corresponding effective IDs of the calling process, and the
46 object's permission bits are set according to the low-order
47 9 bits of mode, except that those bits set in the process
48 file mode creation mask (see umask(2)) are cleared for the
49 new object. A set of macro constants which can be used to
50 define mode is listed in open(2). (Symbolic definitions of
51 these constants can be obtained by including <sys/stat.h>.)
52 A new shared memory object initially has zero length—the
54 size of the object can be set using ftruncate(2). The newly
55 allocated bytes of a shared memory object are automatically
56 initialized to 0.
57 O_EXCL If O_CREAT was also specified, and a shared memory object
59 with the given name already exists, return an error. The
60 check for the existence of the object, and its creation if
61 it does not exist, are performed atomically.
62 O_TRUNC If the shared memory object already exists, truncate it to
64 zero bytes.
65 Definitions of these flag values can be obtained by including
67 <fcntl.h>.
68 On successful completion shm_open() returns a new file descriptor
70 referring to the shared memory object. This file descriptor is guaran‐
71 teed to be the lowest-numbered file descriptor not previously opened
72 within the process. The FD_CLOEXEC flag (see fcntl(2)) is set for the
73 file descriptor.
74 The file descriptor is normally used in subsequent calls to ftrun‐
76 cate(2) (for a newly created object) and mmap(2). After a call to
77 mmap(2) the file descriptor may be closed without affecting the memory
78 mapping.
79 The operation of shm_unlink() is analogous to unlink(2): it removes a
81 shared memory object name, and, once all processes have unmapped the
82 object, de-allocates and destroys the contents of the associated memory
83 region. After a successful shm_unlink(), attempts to shm_open() an
84 object with the same name fail (unless O_CREAT was specified, in which
85 case a new, distinct object is created).
86
88 On success, shm_open() returns a nonnegative file descriptor. On fail‐
89 ure, shm_open() returns -1. shm_unlink() returns 0 on success, or -1
90 on error.
91
93 On failure, errno is set to indicate the cause of the error. Values
94 which may appear in errno include the following:
95 EACCES Permission to shm_unlink() the shared memory object was denied.
97 EACCES Permission was denied to shm_open() name in the specified mode,
99 or O_TRUNC was specified and the caller does not have write per‐
100 mission on the object.
101 EEXIST Both O_CREAT and O_EXCL were specified to shm_open() and the
103 shared memory object specified by name already exists.
104 EINVAL The name argument to shm_open() was invalid.
106 EMFILE The per-process limit on the number of open file descriptors has
108 been reached.
109 ENAMETOOLONG
111 The length of name exceeds PATH_MAX.
112 ENFILE The system-wide limit on the total number of open files has been
114 reached.
115 ENOENT An attempt was made to shm_open() a name that did not exist, and
117 O_CREAT was not specified.
118 ENOENT An attempt was to made to shm_unlink() a name that does not
120 exist.
121
123 These functions are provided in glibc 2.2 and later.
124
126 For an explanation of the terms used in this section, see
127 attributes(7).
128 ┌─────────────────────────┬───────────────┬────────────────┐
130 │Interface │ Attribute │ Value │
131 ├─────────────────────────┼───────────────┼────────────────┤
132 │shm_open(), shm_unlink() │ Thread safety │ MT-Safe locale │
133 └─────────────────────────┴───────────────┴────────────────┘
134
136 POSIX.1-2001, POSIX.1-2008.
137 POSIX.1-2001 says that the group ownership of a newly created shared
139 memory object is set to either the calling process's effective group ID
140 or "a system default group ID". POSIX.1-2008 says that the group own‐
141 ership may be set to either the calling process's effective group ID
142 or, if the object is visible in the filesystem, the group ID of the
143 parent directory.
144
146 POSIX leaves the behavior of the combination of O_RDONLY and O_TRUNC
147 unspecified. On Linux, this will successfully truncate an existing
148 shared memory object—this may not be so on other UNIX systems.
149 The POSIX shared memory object implementation on Linux makes use of a
151 dedicated tmpfs(5) filesystem that is normally mounted under /dev/shm.
152
154 close(2), fchmod(2), fchown(2), fcntl(2), fstat(2), ftruncate(2),
155 memfd_create(2), mmap(2), open(2), umask(2), shm_overview(7)
156
158 This page is part of release 4.16 of the Linux man-pages project. A
159 description of the project, information about reporting bugs, and the
160 latest version of this page, can be found at
161 https://www.kernel.org/doc/man-pages/.
162Linux 2017-09-15 SHM_OPEN(3)