semget.3p (5679B)
- '\" et
- .TH SEMGET "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH PROLOG
- This manual page is part of the POSIX Programmer's Manual.
- The Linux implementation of this interface may differ (consult
- the corresponding Linux manual page for details of Linux behavior),
- or the interface may not be implemented on Linux.
- .\"
- .SH NAME
- semget
- \(em get set of XSI semaphores
- .SH SYNOPSIS
- .LP
- .nf
- #include <sys/sem.h>
- .P
- int semget(key_t \fIkey\fP, int \fInsems\fP, int \fIsemflg\fP);
- .fi
- .SH DESCRIPTION
- The
- \fIsemget\fR()
- function operates on XSI semaphores (see the Base Definitions volume of POSIX.1\(hy2017,
- .IR "Section 4.17" ", " "Semaphore").
- It is unspecified whether this function interoperates with the
- realtime interprocess communication facilities defined in
- .IR "Section 2.8" ", " "Realtime".
- .P
- The
- \fIsemget\fR()
- function shall return the semaphore identifier associated with
- .IR key .
- .P
- A semaphore identifier with its associated
- .BR semid_ds
- data structure and its associated set of
- .IR nsems
- semaphores (see
- .IR <sys/sem.h> )
- is created for
- .IR key
- if one of the following is true:
- .IP " *" 4
- The argument
- .IR key
- is equal to IPC_PRIVATE.
- .IP " *" 4
- The argument
- .IR key
- does not already have a semaphore identifier associated with it and
- (\fIsemflg\fP &IPC_CREAT) is non-zero.
- .P
- Upon creation, the
- .BR semid_ds
- data structure associated with the new semaphore identifier is
- initialized as follows:
- .IP " *" 4
- In the operation permissions structure
- .IR sem_perm.cuid ,
- .IR sem_perm.uid ,
- .IR sem_perm.cgid ,
- and
- .IR sem_perm.gid
- shall be set to the effective user ID and effective group ID,
- respectively, of the calling process.
- .IP " *" 4
- The low-order 9 bits of
- .IR sem_perm.mode
- shall be set to the low-order 9 bits of
- .IR semflg .
- .IP " *" 4
- The variable
- .IR sem_nsems
- shall be set to the value of
- .IR nsems .
- .IP " *" 4
- The variable
- .IR sem_otime
- shall be set to 0 and
- .IR sem_ctime
- shall be set to the current time, as described in
- .IR "Section 2.7.1" ", " "IPC General Description".
- .IP " *" 4
- The data structure associated with each semaphore in the set need not
- be initialized. The
- \fIsemctl\fR()
- function with the command SETVAL or SETALL
- can be used to initialize each semaphore.
- .SH "RETURN VALUE"
- Upon successful completion,
- \fIsemget\fR()
- shall return a non-negative integer, namely a semaphore identifier;
- otherwise, it shall return \-1 and set
- .IR errno
- to indicate the error.
- .SH ERRORS
- The
- \fIsemget\fR()
- function shall fail if:
- .TP
- .BR EACCES
- A semaphore identifier exists for
- .IR key ,
- but operation permission as specified by the low-order 9 bits of
- .IR semflg
- would not be granted; see
- .IR "Section 2.7" ", " "XSI Interprocess Communication".
- .TP
- .BR EEXIST
- A semaphore identifier exists for the argument
- .IR key
- but ((\fIsemflg\fP &IPC_CREAT) &&(\fIsemflg\fP &IPC_EXCL))
- is non-zero.
- .TP
- .BR EINVAL
- The value of
- .IR nsems
- is either less than or equal to 0 or greater than the system-imposed
- limit, or a semaphore identifier exists for the argument
- .IR key ,
- but the number of semaphores in the set associated with it is less than
- .IR nsems
- and
- .IR nsems
- is not equal to 0.
- .TP
- .BR ENOENT
- A semaphore identifier does not exist for the argument
- .IR key
- and (\fIsemflg\fP &IPC_CREAT) is equal to 0.
- .TP
- .BR ENOSPC
- A semaphore identifier is to be created but the system-imposed limit on
- the maximum number of allowed semaphores system-wide would be
- exceeded.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- Refer to
- .IR "\fIsemop\fR\^(\|)".
- .SH "APPLICATION USAGE"
- The POSIX Realtime Extension defines alternative interfaces for interprocess
- communication. Application developers who need to use IPC should
- design their applications so that modules using the IPC routines
- described in
- .IR "Section 2.7" ", " "XSI Interprocess Communication"
- can be easily modified to use the alternative interfaces.
- .SH RATIONALE
- None.
- .SH "FUTURE DIRECTIONS"
- A future version may require that the value of the
- .IR semval ,
- .IR sempid ,
- .IR semncnt ,
- and
- .IR semzcnt
- members of all semaphores in a semaphore set be initialized to zero when
- a call to
- \fIsemget\fR()
- creates a semaphore set. Many semaphore implementations already do this
- and it greatly simplifies what an application must do to initialize a
- semaphore set.
- .SH "SEE ALSO"
- .IR "Section 2.7" ", " "XSI Interprocess Communication",
- .IR "Section 2.8" ", " "Realtime",
- .IR "\fIftok\fR\^(\|)",
- .IR "\fIsemctl\fR\^(\|)",
- .IR "\fIsemop\fR\^(\|)",
- .IR "\fIsem_close\fR\^(\|)",
- .IR "\fIsem_destroy\fR\^(\|)",
- .IR "\fIsem_getvalue\fR\^(\|)",
- .IR "\fIsem_init\fR\^(\|)",
- .IR "\fIsem_open\fR\^(\|)",
- .IR "\fIsem_post\fR\^(\|)",
- .IR "\fIsem_trywait\fR\^(\|)",
- .IR "\fIsem_unlink\fR\^(\|)"
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "Section 4.17" ", " "Semaphore",
- .IR "\fB<sys_sem.h>\fP"
- .\"
- .SH COPYRIGHT
- Portions of this text are reprinted and reproduced in electronic form
- from IEEE Std 1003.1-2017, Standard for Information Technology
- -- Portable Operating System Interface (POSIX), The Open Group Base
- Specifications Issue 7, 2018 Edition,
- Copyright (C) 2018 by the Institute of
- Electrical and Electronics Engineers, Inc and The Open Group.
- In the event of any discrepancy between this version and the original IEEE and
- The Open Group Standard, the original IEEE and The Open Group Standard
- is the referee document. The original Standard can be obtained online at
- http://www.opengroup.org/unix/online.html .
- .PP
- Any typographical or formatting errors that appear
- in this page are most likely
- to have been introduced during the conversion of the source files to
- man page format. To report such errors, see
- https://www.kernel.org/doc/man-pages/reporting_bugs.html .