1SEM_OPEN(3P) POSIX Programmer's Manual SEM_OPEN(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
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12 sem_open — initialize and open a named semaphore
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15 #include <semaphore.h>
16 sem_t *sem_open(const char *name, int oflag, ...);
18
20 The sem_open() function shall establish a connection between a named
21 semaphore and a process. Following a call to sem_open() with semaphore
22 name name, the process may reference the semaphore associated with name
23 using the address returned from the call. This semaphore may be used in
24 subsequent calls to sem_wait(), sem_timedwait(), sem_trywait(),
25 sem_post(), and sem_close(). The semaphore remains usable by this
26 process until the semaphore is closed by a successful call to
27 sem_close(), _exit(), or one of the exec functions.
28 The oflag argument controls whether the semaphore is created or merely
30 accessed by the call to sem_open(). The following flag bits may be set
31 in oflag:
32 O_CREAT This flag is used to create a semaphore if it does not
34 already exist. If O_CREAT is set and the semaphore already
35 exists, then O_CREAT has no effect, except as noted under
36 O_EXCL. Otherwise, sem_open() creates a named semaphore. The
37 O_CREAT flag requires a third and a fourth argument: mode,
38 which is of type mode_t, and value, which is of type
39 unsigned. The semaphore is created with an initial value of
40 value. Valid initial values for semaphores are less than or
41 equal to {SEM_VALUE_MAX}.
42 The user ID of the semaphore shall be set to the effective
44 user ID of the process. The group ID of the semaphore shall
45 be set to the effective group ID of the process; however, if
46 the name argument is visible in the file system, the group ID
47 may be set to the group ID of the containing directory. The
48 permission bits of the semaphore are set to the value of the
49 mode argument except those set in the file mode creation mask
50 of the process. When bits in mode other than the file permis‐
51 sion bits are specified, the effect is unspecified.
52 After the semaphore named name has been created by sem_open()
54 with the O_CREAT flag, other processes can connect to the
55 semaphore by calling sem_open() with the same value of name.
56 O_EXCL If O_EXCL and O_CREAT are set, sem_open() fails if the sema‐
58 phore name exists. The check for the existence of the sema‐
59 phore and the creation of the semaphore if it does not exist
60 are atomic with respect to other processes executing
61 sem_open() with O_EXCL and O_CREAT set. If O_EXCL is set and
62 O_CREAT is not set, the effect is undefined.
63 If flags other than O_CREAT and O_EXCL are specified in the
65 oflag parameter, the effect is unspecified.
66 The name argument points to a string naming a semaphore object. It is
68 unspecified whether the name appears in the file system and is visible
69 to functions that take pathnames as arguments. The name argument con‐
70 forms to the construction rules for a pathname, except that the inter‐
71 pretation of <slash> characters other than the leading <slash> charac‐
72 ter in name is implementation-defined, and that the length limits for
73 the name argument are implementation-defined and need not be the same
74 as the pathname limits {PATH_MAX} and {NAME_MAX}. If name begins with
75 the <slash> character, then processes calling sem_open() with the same
76 value of name shall refer to the same semaphore object, as long as that
77 name has not been removed. If name does not begin with the <slash>
78 character, the effect is implementation-defined.
79 If a process makes multiple successful calls to sem_open() with the
81 same value for name, the same semaphore address shall be returned for
82 each such successful call, provided that there have been no calls to
83 sem_unlink() for this semaphore, and at least one previous successful
84 sem_open() call for this semaphore has not been matched with a
85 sem_close() call.
86 References to copies of the semaphore produce undefined results.
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90 Upon successful completion, the sem_open() function shall return the
91 address of the semaphore. Otherwise, it shall return a value of
92 SEM_FAILED and set errno to indicate the error. The symbol SEM_FAILED
93 is defined in the <semaphore.h> header. No successful return from
94 sem_open() shall return the value SEM_FAILED.
95
97 If any of the following conditions occur, the sem_open() function shall
98 return SEM_FAILED and set errno to the corresponding value:
99 EACCES The named semaphore exists and the permissions specified by
101 oflag are denied, or the named semaphore does not exist and per‐
102 mission to create the named semaphore is denied.
103 EEXIST O_CREAT and O_EXCL are set and the named semaphore already
105 exists.
106 EINTR The sem_open() operation was interrupted by a signal.
108 EINVAL The sem_open() operation is not supported for the given name, or
110 O_CREAT was specified in oflag and value was greater than
111 {SEM_VALUE_MAX}.
112 EMFILE Too many semaphore descriptors or file descriptors are currently
114 in use by this process.
115 ENFILE Too many semaphores are currently open in the system.
117 ENOENT O_CREAT is not set and the named semaphore does not exist.
119 ENOMEM There is insufficient memory for the creation of the new named
121 semaphore.
122 ENOSPC There is insufficient space on a storage device for the creation
124 of the new named semaphore.
125 If any of the following conditions occur, the sem_open() function may
127 return SEM_FAILED and set errno to the corresponding value:
128 ENAMETOOLONG
130 The length of the name argument exceeds {_POSIX_PATH_MAX} on
131 systems that do not support the XSI option or exceeds
132 {_XOPEN_PATH_MAX} on XSI systems, or has a pathname component
133 that is longer than {_POSIX_NAME_MAX} on systems that do not
134 support the XSI option or longer than {_XOPEN_NAME_MAX} on XSI
135 systems.
136 The following sections are informative.
138
140 None.
141
143 None.
144
146 Early drafts required an error return value of -1 with the type sem_t *
147 for the sem_open() function, which is not guaranteed to be portable
148 across implementations. The revised text provides the symbolic error
149 code SEM_FAILED to eliminate the type conflict.
150
152 A future version might require the sem_open() and sem_unlink() func‐
153 tions to have semantics similar to normal file system operations.
154
156 semctl(), semget(), semop(), sem_close(), sem_post(), sem_timedwait(),
157 sem_trywait(), sem_unlink()
158 The Base Definitions volume of POSIX.1‐2017, <semaphore.h>
160
162 Portions of this text are reprinted and reproduced in electronic form
163 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
164 table Operating System Interface (POSIX), The Open Group Base Specifi‐
165 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
166 Electrical and Electronics Engineers, Inc and The Open Group. In the
167 event of any discrepancy between this version and the original IEEE and
168 The Open Group Standard, the original IEEE and The Open Group Standard
169 is the referee document. The original Standard can be obtained online
170 at http://www.opengroup.org/unix/online.html .
171 Any typographical or formatting errors that appear in this page are
173 most likely to have been introduced during the conversion of the source
174 files to man page format. To report such errors, see https://www.ker‐
175 nel.org/doc/man-pages/reporting_bugs.html .
176IEEE/The Open Group 2017 SEM_OPEN(3P)