semctl.3p (8364B)
- '\" et
- .TH SEMCTL "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
- semctl
- \(em XSI semaphore control operations
- .SH SYNOPSIS
- .LP
- .nf
- #include\ <sys/sem.h>
- .P
- int semctl(int \fIsemid\fP, int \fIsemnum\fP, int \fIcmd\fP, ...);
- .fi
- .SH DESCRIPTION
- The
- \fIsemctl\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
- \fIsemctl\fR()
- function provides a variety of semaphore control operations as
- specified by
- .IR cmd .
- The fourth argument is optional and depends upon the operation
- requested. If required, it is of type
- .BR "union semun" ,
- which the application shall explicitly declare:
- .sp
- .RS 4
- .nf
- union semun {
- int val;
- struct semid_ds *buf;
- unsigned short *array;
- } arg;
- .fi
- .P
- .RE
- .P
- Each operation shall be performed atomically.
- .P
- The following semaphore control operations as specified by
- .IR cmd
- are executed with respect to the semaphore specified by
- .IR semid
- and
- .IR semnum .
- The level of permission required for each operation is shown with each
- command; see
- .IR "Section 2.7" ", " "XSI Interprocess Communication".
- The symbolic names for the values of
- .IR cmd
- are defined in the
- .IR <sys/sem.h>
- header:
- .IP GETVAL 12
- Return the value of
- .IR semval ;
- see
- .IR <sys/sem.h> .
- Requires read permission.
- .IP SETVAL 12
- Set the value of
- .IR semval
- to
- .IR arg.val ,
- where
- .IR arg
- is the value of the fourth argument to
- \fIsemctl\fR().
- When this command is successfully executed, the
- .IR semadj
- value corresponding to the specified semaphore in all processes is
- cleared. Also, the
- .IR sem_ctime
- timestamp shall be set to the current time, as described in
- .IR "Section 2.7.1" ", " "IPC General Description".
- Requires alter permission; see
- .IR "Section 2.7" ", " "XSI Interprocess Communication".
- .IP GETPID 12
- Return the value of
- .IR sempid .
- Requires read permission.
- .IP GETNCNT 12
- Return the value of
- .IR semncnt .
- Requires read permission.
- .IP GETZCNT 12
- Return the value of
- .IR semzcnt .
- Requires read permission.
- .P
- The following values of
- .IR cmd
- operate on each
- .IR semval
- in the set of semaphores:
- .IP GETALL 12
- Return the value of
- .IR semval
- for each semaphore in the semaphore set and place into the array
- pointed to by
- .IR arg.array ,
- where
- .IR arg
- is the fourth argument to
- \fIsemctl\fR().
- Requires read permission.
- .IP SETALL 12
- Set the value of
- .IR semval
- for each semaphore in the semaphore set according to the array pointed
- to by
- .IR arg.array ,
- where
- .IR arg
- is the fourth argument to
- \fIsemctl\fR().
- When this command is successfully executed, the
- .IR semadj
- values corresponding to each specified semaphore in all processes are
- cleared. Also, the
- .IR sem_ctime
- timestamp shall be set to the current time, as described in
- .IR "Section 2.7.1" ", " "IPC General Description".
- Requires alter permission.
- .br
- .P
- The following values of
- .IR cmd
- are also available:
- .IP IPC_STAT 12
- Place the current value of each member of the
- .BR semid_ds
- data structure associated with
- .IR semid
- into the structure pointed to by
- .IR arg.buf ,
- where
- .IR arg
- is the fourth argument to
- \fIsemctl\fR().
- The contents of this structure are defined in
- .IR <sys/sem.h> .
- Requires read permission.
- .IP IPC_SET 12
- Set the value of the following members of the
- .BR semid_ds
- data structure associated with
- .IR semid
- to the corresponding value found in the structure pointed to by
- .IR arg.buf ,
- where
- .IR arg
- is the fourth argument to
- \fIsemctl\fR():
- .RS 12
- .sp
- .RS 4
- .nf
- sem_perm.uid
- sem_perm.gid
- sem_perm.mode
- .fi
- .P
- .RE
- .P
- The mode bits specified in
- .IR "Section 2.7.1" ", " "IPC General Description"
- are copied into the corresponding bits of the
- .IR sem_perm.mode
- associated with
- .IR semid .
- The stored values of any other bits are unspecified. The
- .IR sem_ctime
- timestamp shall be set to the current time, as described in
- .IR "Section 2.7.1" ", " "IPC General Description".
- .P
- This command can only be executed by a process that has an effective
- user ID equal to either that of a process with appropriate privileges
- or to the value of
- .IR sem_perm.cuid
- or
- .IR sem_perm.uid
- in the
- .BR semid_ds
- data structure associated with
- .IR semid .
- .RE
- .IP IPC_RMID 12
- Remove the semaphore identifier specified by
- .IR semid
- from the system and destroy the set of semaphores and
- .BR semid_ds
- data structure associated with it. This command can only be executed
- by a process that has an effective user ID equal to either that of a
- process with appropriate privileges or to the value of
- .IR sem_perm.cuid
- or
- .IR sem_perm.uid
- in the
- .BR semid_ds
- data structure associated with
- .IR semid .
- .SH "RETURN VALUE"
- If successful, the value returned by
- \fIsemctl\fR()
- depends on
- .IR cmd
- as follows:
- .IP GETVAL 12
- The value of
- .IR semval .
- .IP GETPID 12
- The value of
- .IR sempid .
- .IP GETNCNT 12
- The value of
- .IR semncnt .
- .IP GETZCNT 12
- The value of
- .IR semzcnt .
- .IP "All others" 12
- 0.
- .P
- Otherwise,
- \fIsemctl\fR()
- shall return \-1 and set
- .IR errno
- to indicate the error.
- .SH ERRORS
- The
- \fIsemctl\fR()
- function shall fail if:
- .TP
- .BR EACCES
- Operation permission is denied to the calling process; see
- .IR "Section 2.7" ", " "XSI Interprocess Communication".
- .TP
- .BR EINVAL
- The value of
- .IR semid
- is not a valid semaphore identifier, or the value of
- .IR semnum
- is less than 0 or greater than or equal to
- .IR sem_nsems ,
- or the value of
- .IR cmd
- is not a valid command.
- .TP
- .BR EPERM
- The argument
- .IR cmd
- is equal to IPC_RMID or IPC_SET
- and the effective user ID of the calling process is not equal to that
- of a process with appropriate privileges and it is not equal to the
- value of
- .IR sem_perm.cuid
- or
- .IR sem_perm.uid
- in the data structure associated with
- .IR semid .
- .TP
- .BR ERANGE
- The argument
- .IR cmd
- is equal to SETVAL or SETALL and the value to which
- .IR semval
- is to be set is greater than the system-imposed maximum.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- Refer to
- .IR "\fIsemop\fR\^(\|)".
- .SH "APPLICATION USAGE"
- The fourth parameter in the SYNOPSIS section is now specified as
- .BR \(dq...\(dq
- in order to avoid a clash with the ISO\ C standard when referring to the union
- .IR semun
- (as defined in Issue 3) and for backwards-compatibility.
- .P
- 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"
- None.
- .SH "SEE ALSO"
- .ad l
- .IR "Section 2.7" ", " "XSI Interprocess Communication",
- .IR "Section 2.8" ", " "Realtime",
- .IR "\fIsemget\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\^(\|)"
- .ad b
- .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 .