1shm_open(3C) Standard C Library Functions shm_open(3C)
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6 shm_open - open a shared memory object
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9 #include <sys/mman.h>
10 int shm_open(const char *name, int oflag, mode_t mode);
12
15 The shm_open() function establishes a connection between a shared mem‐
16 ory object and a file descriptor. It creates an open file description
17 that refers to the shared memory object and a file descriptor that
18 refers to that open file description. The file descriptor is used by
19 other functions to refer to that shared memory object. The name argu‐
20 ment points to a string naming a shared memory object. It is unspeci‐
21 fied whether the name appears in the file system and is visible to
22 other functions that take pathnames as arguments. The name argument
23 conforms to the construction rules for a pathname. The first character
24 of name must be a slash (/) character and the remaining characters of
25 name cannot include any slash characters. For maximum portability,
26 name should include no more than 14 characters, but this limit is not
27 enforced.
28 If successful, shm_open() returns a file descriptor for the shared mem‐
31 ory object that is the lowest numbered file descriptor not currently
32 open for that process. The open file description is new, and therefore
33 the file descriptor does not share it with any other processes. It is
34 unspecified whether the file offset is set. The FD_CLOEXEC file
35 descriptor flag associated with the new file descriptor is set.
36 The file status flags and file access modes of the open file descrip‐
39 tion are according to the value of oflag. The oflag argument is the
40 bitwise inclusive OR of the following flags defined in the header
41 <fcntl.h>. Applications specify exactly one of the first two values
42 (access modes) below in the value of oflag:
43 O_RDONLY Open for read access only.
45 O_RDWR Open for read or write access.
48 Any combination of the remaining flags may be specified in the value of
52 oflag:
53 O_CREAT If the shared memory object exists, this flag has no
55 effect, except as noted under O_EXCL below. Otherwise the
56 shared memory object is created; the user ID of the shared
57 memory object will be set to the effective user ID of the
58 process; the group ID of the shared memory object will be
59 set to a system default group ID or to the effective group
60 ID of the process. The permission bits of the shared memory
61 object will be set to the value of the mode argument except
62 those set in the file mode creation mask of the process.
63 When bits in mode other than the file permission bits are
64 set, the effect is unspecified. The mode argument does not
65 affect whether the shared memory object is opened for read‐
66 ing, for writing, or for both. The shared memory object has
67 a size of zero.
68 O_EXCL If O_EXCL and O_CREAT are set, shm_open() fails if the
71 shared memory object exists. The check for the existence of
72 the shared memory object and the creation of the object if
73 it does not exist is atomic with respect to other processes
74 executing shm_open() naming the same shared memory object
75 with O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT
76 is not set, the result is undefined.
77 O_TRUNC If the shared memory object exists, and it is successfully
80 opened O_RDWR, the object will be truncated to zero length
81 and the mode and owner will be unchanged by this function
82 call. The result of using O_TRUNC with O_RDONLY is unde‐
83 fined.
84 When a shared memory object is created, the state of the shared memory
88 object, including all data associated with the shared memory object,
89 persists until the shared memory object is unlinked and all other ref‐
90 erences are gone. It is unspecified whether the name and shared memory
91 object state remain valid after a system reboot.
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94 Upon successful completion, the shm_open() function returns a non-nega‐
95 tive integer representing the lowest numbered unused file descriptor.
96 Otherwise, it returns −1 and sets errno to indicate the error condi‐
97 tion.
98
100 The shm_open() function will fail if:
101 EACCES The shared memory object exists and the permissions
103 specified by oflag are denied, or the shared memory
104 object does not exist and permission to create the
105 shared memory object is denied, or O_TRUNC is speci‐
106 fied and write permission is denied.
107 EEXIST O_CREAT and O_EXCL are set and the named shared memory
110 object already exists.
111 EINTR The shm_open() operation was interrupted by a signal.
114 EINVAL The shm_open() operation is not supported for the
117 given name.
118 EMFILE Too many file descriptors are currently in use by this
121 process.
122 ENAMETOOLONG The length of the name string exceeds PATH_MAX, or a
125 pathname component is longer than NAME_MAX while
126 _POSIX_NO_TRUNC is in effect.
127 ENFILE Too many shared memory objects are currently open in
130 the system.
131 ENOENT O_CREAT is not set and the named shared memory object
134 does not exist.
135 ENOSPC There is insufficient space for the creation of the
138 new shared memory object.
139 ENOSYS The shm_open() function is not supported by the sys‐
142 tem.
143
146 See attributes(5) for descriptions of the following attributes:
147 ┌─────────────────────────────┬─────────────────────────────┐
152 │ATTRIBUTE TYPE │ATTRIBUTE VALUE │
153 ├─────────────────────────────┼─────────────────────────────┤
154 │Interface Stability │Committed │
155 ├─────────────────────────────┼─────────────────────────────┤
156 │MT-Level │MT-Safe │
157 ├─────────────────────────────┼─────────────────────────────┤
158 │Standard │See standards(5). │
159 └─────────────────────────────┴─────────────────────────────┘
160
162 close(2), dup(2), exec(2), fcntl(2), mmap(2), umask(2), shm_unlink(3C),
163 sysconf(3C), fcntl.h(3HEAD), attributes(5), standards(5)
164
166 Solaris 2.6 was the first release to support the Asynchronous Input and
167 Output option. Prior to this release, this function always returned −1
168 and set errno to ENOSYS.
169SunOS 5.11 5 Feb 2008 shm_open(3C)