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23.Dd June 8, 2000
24.Dt SEM_OPEN 2
25.Os Darwin
26.Sh NAME
27.Nm sem_open
28.Nd initialize and open a named semaphore
29.Sh SYNOPSIS
30.Fd #include <semaphore.h>
31.Ft sem_t *
32.Fo sem_open
33.Fa "const char *name"
34.Fa "int oflag"
35.Fa "..."
36.Fc
37.Pp
38The parameters "mode_t mode" and "unsigned int value"
39are optional.
40.Sh DESCRIPTION
41The named semaphore named
42.Fa name
43is initialized and opened as specified by the argument
44.Fa oflag
45and a semaphore descriptor is returned to the calling process.
46.Pp
47The value of
48.Fa oflag
49is formed by
50.Em or Ns 'ing
51the following values:
52.Pp
53.Bd -literal -offset indent -compact
54O_CREAT create the semaphore if it does not exist
55O_EXCL error if create and semaphore exists
56.Ed
57.Pp
58If
59.Dv O_CREAT
60is specified,
61.Fn sem_open
62requires an additional two arguments.
63.Fa mode
64specifies the permissions for the semaphore as described in
65.Xr chmod 2
66and modified by the process' umask value (see
67.Xr umask 2 ) .
68The semaphore is created with an initial
69.Fa value ,
70which must be less than or equal to
71.Dv SEM_VALUE_MAX .
72.Pp
73If
74.Dv O_EXCL
75is specified and the semaphore exists,
76.Fn sem_open
77fails. The check for the existence of the semaphore and the creation
78of the semaphore are atomic with respect to all processes calling
79.Fn sem_open
80with
81.Dv O_CREAT
82and
83.Dv O_EXCL
84set.
85.Pp
86When a new semaphore is created, it is given the user ID and group ID
87which correspond to the effective user and group IDs of the calling
88process. There is no visible entry in the file system for the created
89object in this implementation.
90.Pp
91The returned semaphore descriptor is available to the calling process
92until it is closed with
93.Fn sem_close ,
94or until the caller exits or execs.
95.Pp
96If a process makes repeated calls to
97.Fn sem_open ,
98with the same
99.Fa name
100argument, the same descriptor is returned for each successful call,
101unless
102.Fn sem_unlink
103has been called on the semaphore in the interim.
104.Pp
105If
106.Fn sem_open
107fails for any reason, it will return a value of
108.Dv SEM_FAILED
109and sets
110.Va errno .
111On success, it returns a semaphore descriptor.
112.Sh ERRORS
113The named semaphore is opened unless:
114.Bl -tag -width Er
115.It Bq Er EACCES
116The required permissions (for reading and/or writing)
117are denied for the given flags; or
118.Dv O_CREAT
119is specified, the object does not exist, and permission to
120create the semaphore is denied.
121.It Bq Er EEXIST
122.Dv O_CREAT
123and
124.Dv O_EXCL
125were specified and the semaphore exists.
126.It Bq Er EINTR
127The
128.Fn sem_open
129operation was interrupted by a signal.
130.It Bq Er EINVAL
131The
132.Fn shm_open
133operation is not supported; or
134.Dv O_CREAT
135is specified and
136.Fa value
137exceeds
138.Dv SEM_VALUE_MAX .
139.It Bq Er EMFILE
140The process has already reached its limit for semaphores or file
141descriptors in use.
142.It Bq Er ENAMETOOLONG
143.Fa name
144exceeded
145.Dv SEM_NAME_LEN
146characters.
147.It Bq Er ENFILE
148Too many semaphores or file descriptors are open on the system.
149.It Bq Er ENOENT
150.Dv O_CREAT
151is not set and the named semaphore does not exist.
152.It Bq Er ENOSPC
153.Dv O_CREAT
154is specified, the file does not exist, and there is insufficient
155space available to create the semaphore.
156.El
157.Sh SEE ALSO
158.Xr sem_close 2 ,
159.Xr sem_post 2 ,
160.Xr sem_trywait 2 ,
161.Xr sem_unlink 2 ,
162.Xr sem_wait 2 ,
163.Xr semctl 2 ,
164.Xr semget 2 ,
165.Xr semop 2 ,
166.Xr umask 2
167.Sh HISTORY
168.Fn sem_open
169is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).