1SEM_OPEN(P) POSIX Programmer's Manual SEM_OPEN(P)
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6 sem_open - initialize and open a named semaphore (REALTIME)
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9 #include <semaphore.h>
10 sem_t *sem_open(const char *name, int oflag, ...);
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15 The sem_open() function shall establish a connection between a named
16 semaphore and a process. Following a call to sem_open() with semaphore
17 name name, the process may reference the semaphore associated with name
18 using the address returned from the call. This semaphore may be used in
19 subsequent calls to sem_wait(), sem_trywait(), sem_post(), and
20 sem_close(). The semaphore remains usable by this process until the
21 semaphore is closed by a successful call to sem_close(), _exit(), or
22 one of the exec functions.
23 The oflag argument controls whether the semaphore is created or merely
25 accessed by the call to sem_open(). The following flag bits may be set
26 in oflag:
27 O_CREAT
29 This flag is used to create a semaphore if it does not already
30 exist. If O_CREAT is set and the semaphore already exists, then
31 O_CREAT has no effect, except as noted under O_EXCL. Otherwise,
32 sem_open() creates a named semaphore. The O_CREAT flag requires
33 a third and a fourth argument: mode, which is of type mode_t,
34 and value, which is of type unsigned. The semaphore is created
35 with an initial value of value. Valid initial values for sema‐
36 phores are less than or equal to {SEM_VALUE_MAX}.
37 The user ID of the semaphore is set to the effective user ID of the
39 process; the group ID of the semaphore is set to a system default group
40 ID or to the effective group ID of the process. The permission bits of
41 the semaphore are set to the value of the mode argument except those
42 set in the file mode creation mask of the process. When bits in mode
43 other than the file permission bits are specified, the effect is
44 unspecified.
45 After the semaphore named name has been created by sem_open() with the
47 O_CREAT flag, other processes can connect to the semaphore by calling
48 sem_open() with the same value of name.
49 O_EXCL If O_EXCL and O_CREAT are set, sem_open() fails if the semaphore
51 name exists. The check for the existence of the semaphore and
52 the creation of the semaphore if it does not exist are atomic
53 with respect to other processes executing sem_open() with O_EXCL
54 and O_CREAT set. If O_EXCL is set and O_CREAT is not set, the
55 effect is undefined.
56 If flags other than O_CREAT and O_EXCL are specified in the oflag
58 parameter, the effect is unspecified.
59 The name argument points to a string naming a semaphore object. It is
62 unspecified whether the name appears in the file system and is visible
63 to functions that take pathnames as arguments. The name argument con‐
64 forms to the construction rules for a pathname. If name begins with the
65 slash character, then processes calling sem_open() with the same value
66 of name shall refer to the same semaphore object, as long as that name
67 has not been removed. If name does not begin with the slash character,
68 the effect is implementation-defined. The interpretation of slash char‐
69 acters other than the leading slash character in name is implementa‐
70 tion-defined.
71 If a process makes multiple successful calls to sem_open() with the
73 same value for name, the same semaphore address shall be returned for
74 each such successful call, provided that there have been no calls to
75 sem_unlink() for this semaphore.
76 References to copies of the semaphore produce undefined results.
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80 Upon successful completion, the sem_open() function shall return the
81 address of the semaphore. Otherwise, it shall return a value of
82 SEM_FAILED and set errno to indicate the error. The symbol SEM_FAILED
83 is defined in the <semaphore.h> header. No successful return from
84 sem_open() shall return the value SEM_FAILED.
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87 If any of the following conditions occur, the sem_open() function shall
88 return SEM_FAILED and set errno to the corresponding value:
89 EACCES The named semaphore exists and the permissions specified by
91 oflag are denied, or the named semaphore does not exist and per‐
92 mission to create the named semaphore is denied.
93 EEXIST O_CREAT and O_EXCL are set and the named semaphore already
95 exists.
96 EINTR The sem_open() operation was interrupted by a signal.
98 EINVAL The sem_open() operation is not supported for the given name, or
100 O_CREAT was specified in oflag and value was greater than
101 {SEM_VALUE_MAX}.
102 EMFILE Too many semaphore descriptors or file descriptors are currently
104 in use by this process.
105 ENAMETOOLONG
107 The length of the name argument exceeds {PATH_MAX} or a pathname
108 component is longer than {NAME_MAX}.
109 ENFILE Too many semaphores are currently open in the system.
111 ENOENT O_CREAT is not set and the named semaphore does not exist.
113 ENOSPC There is insufficient space for the creation of the new named
115 semaphore.
116 The following sections are informative.
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121 None.
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124 The sem_open() function is part of the Semaphores option and need not
125 be available on all implementations.
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128 Early drafts required an error return value of -1 with the type sem_t *
129 for the sem_open() function, which is not guaranteed to be portable
130 across implementations. The revised text provides the symbolic error
131 code SEM_FAILED to eliminate the type conflict.
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134 None.
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137 semctl() , semget() , semop() , sem_close() , sem_post() , sem_timed‐
138 wait() , sem_trywait() , sem_unlink() , sem_wait() , the Base Defini‐
139 tions volume of IEEE Std 1003.1-2001, <semaphore.h>
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142 Portions of this text are reprinted and reproduced in electronic form
143 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
144 -- Portable Operating System Interface (POSIX), The Open Group Base
145 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
146 Electrical and Electronics Engineers, Inc and The Open Group. In the
147 event of any discrepancy between this version and the original IEEE and
148 The Open Group Standard, the original IEEE and The Open Group Standard
149 is the referee document. The original Standard can be obtained online
150 at http://www.opengroup.org/unix/online.html .
151IEEE/The Open Group 2003 SEM_OPEN(P)