sem_open(3)

1sem_open(3)                Library Functions Manual                sem_open(3)
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NAME

6       sem_open - initialize and open a named semaphore
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LIBRARY

9       POSIX threads library (libpthread, -lpthread)
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SYNOPSIS

12       #include <fcntl.h>           /* For O_* constants */
13       #include <sys/stat.h>        /* For mode constants */
14       #include <semaphore.h>
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16       sem_t *sem_open(const char *name, int oflag);
17       sem_t *sem_open(const char *name, int oflag,
18                       mode_t mode, unsigned int value);
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DESCRIPTION

21       sem_open()  creates  a  new  POSIX semaphore or opens an existing sema‐
22       phore.  The semaphore is identified by name.  For details of  the  con‐
23       struction of name, see sem_overview(7).
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25       The  oflag  argument  specifies flags that control the operation of the
26       call.  (Definitions of the flags values can be  obtained  by  including
27       <fcntl.h>.)   If  O_CREAT  is specified in oflag, then the semaphore is
28       created if it does not already exist.  The owner (user ID) of the sema‐
29       phore  is  set  to  the  effective user ID of the calling process.  The
30       group ownership (group ID) is set to the  effective  group  ID  of  the
31       calling  process.   If  both O_CREAT and O_EXCL are specified in oflag,
32       then an error is returned if a semaphore with the  given  name  already
33       exists.
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35       If O_CREAT is specified in oflag, then two additional arguments must be
36       supplied.  The mode argument specifies the permissions to be placed  on
37       the  new semaphore, as for open(2).  (Symbolic definitions for the per‐
38       missions bits can be obtained by including <sys/stat.h>.)  The  permis‐
39       sions  settings  are  masked  against the process umask.  Both read and
40       write permission should be granted to each class of user that will  ac‐
41       cess the semaphore.  The value argument specifies the initial value for
42       the new semaphore.  If O_CREAT is specified, and a semaphore  with  the
43       given name already exists, then mode and value are ignored.
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RETURN VALUE

46       On  success,  sem_open() returns the address of the new semaphore; this
47       address is used when calling other semaphore-related functions.  On er‐
48       ror,  sem_open() returns SEM_FAILED, with errno set to indicate the er‐
49       ror.
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ERRORS

52       EACCES The semaphore exists, but the caller does not have permission to
53              open it.
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55       EEXIST Both O_CREAT and O_EXCL were specified in oflag, but a semaphore
56              with this name already exists.
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58       EINVAL value was greater than SEM_VALUE_MAX.
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60       EINVAL name consists of just "/", followed by no other characters.
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62       EMFILE The per-process limit on the number of open file descriptors has
63              been reached.
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65       ENAMETOOLONG
66              name was too long.
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68       ENFILE The system-wide limit on the total number of open files has been
69              reached.
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71       ENOENT The O_CREAT flag was not specified in  oflag  and  no  semaphore
72              with  this  name  exists;  or,  O_CREAT  was specified, but name
73              wasn't well formed.
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75       ENOMEM Insufficient memory.
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ATTRIBUTES

78       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
79       tributes(7).
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81       ┌────────────────────────────────────────────┬───────────────┬─────────┐
82       │Interface                                   │ Attribute     │ Value   │
83       ├────────────────────────────────────────────┼───────────────┼─────────┤
84       │sem_open()                                  │ Thread safety │ MT-Safe │
85       └────────────────────────────────────────────┴───────────────┴─────────┘
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STANDARDS

88       POSIX.1-2008.
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HISTORY

91       POSIX.1-2001.
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