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<html><!-- This HTML file has been created by texi2html 1.29
from syscalls.texi on 4 June 1994 -->
<TITLE>Syscall specifications of Linux - semctl</TITLE>
<P>Go to the <A HREF="syscalls_76.html" tppabs="http://www.infran.ru/TechInfo/syscalls/syscalls_76.html">previous</A>, <A HREF="syscalls_78.html" tppabs="http://www.infran.ru/TechInfo/syscalls/syscalls_78.html">next</A> section.<P>
<H2><A NAME="SEC77" HREF="syscalls_toc.html#SEC77" tppabs="http://www.infran.ru/TechInfo/syscalls/syscalls_toc.html#SEC77">semctl</A></H2>
<P>
<H3>SYNOPSIS</H3>
<P>
<CODE>int semctl(int <VAR>semid</VAR>, int <VAR>semnun</VAR>, int <VAR>cmd</VAR>, union
semun <VAR>arg</VAR>);</CODE>
<P>
<H3>PARAMETERS</H3>
<P>
<VAR>semid</VAR>: [in] the semaphore set to manipulate.
<P>
<VAR>semnum</VAR>: [in] the semaphore in the set to manipulate (0 is the first).
<P>
<VAR>cmd</VAR>: [in] the operation to perform.
<P>
<VAR>arg</VAR>: [in out] an argument to the operation (see description).
<P>
<H3>DESCRIPTION</H3>
<P>
Manipulates a semaphore set or members of a semaphore set. The possible
values for <VAR>cmd</VAR> are:
<P>
<DL COMPACT>
<DT><CODE>IPC_STAT</CODE>
<DD>gets some information on the semaphore set. The calling task must have read
access to the semaphore set.
<P>
<DT><CODE>IPC_SET</CODE>
<DD>modify some members of the <CODE>semid_ds</CODE> structure of a semaphore in
the set. The members that can be modified are: <CODE>sem_perm.uid</CODE>,
<CODE>sem_perm.gid</CODE>, and the lower 9 bits of <CODE>sem_perm.mode</CODE>. The
calling task must be the owner or the creator of the semaphore set or the
superuser. The <CODE>sem_ctime</CODE> field of the set is updated to the
current time.
<P>
<DT><CODE>IPC_RMID</CODE>
<DD>removes the semaphore set and awakens the process blocked on operations
performed on that set. Only the creator or the owner of the set or the
superuser may use this option.
<P>
<DT><CODE>GETALL</CODE>
<DD>returns the <CODE>semval</CODE> value of all semaphore in an array.
<P>
<DT><CODE>GETNCNT</CODE>
<DD>returns the number of processes waiting for the value of a specific
semaphore in the semaphore set to increase. The calling task must have
read privileges on the semaphore set.
<P>
<DT><CODE>GETPPID</CODE>
<DD>returns the pid of the last process to have executed a semaphore
opreation a specific semaphore of the semaphore set. The calling task
must have read privileges on the semaphore set.
<P>
<DT><CODE>GETVAL</CODE>
<DD>returns the value of a specific semaphore in the semaphore set. The
calling task must have read privileges on the semaphore set.
<P>
<DT><CODE>GETZCNT</CODE>
<DD>returns the number of processes waiting for the value of a specific
semaphore in the semaphore set to be zero. The calling task must have
read privileges on the semaphore set.
<P>
<DT><CODE>SETALL</CODE>
<DD>sets the value of all semaphore in the semaphore set. The calling task
must have alter privileges on the semaphore set.
<P>
<DT><CODE>SETVAL</CODE>
<DD>sets the value of a specific semaphore in the semaphore set. The calling
task must have alter privileges on the semaphore set.
</DL>
<P>
<H3>RETURN VALUE</H3>
<P>
On success, with <VAR>cmd</VAR> equals to:
<P>
<DL COMPACT>
<DT><CODE>GETNCNT</CODE>
<DD>the number of waiting tasks (see description).
<P>
<DT><CODE>GETPID</CODE>
<DD>the task pid (see description).
<P>
<DT><CODE>GETVAL</CODE>
<DD>the value of the semaphore.
<P>
<DT><CODE>GETZCNT</CODE>
<DD>the number of waiting tasks (see description).
</DL>
<P>
For other values of <VAR>cmd</VAR>, it returns zero. On error, the call
returns -1 and <CODE>errno</CODE> is set to one of the following values:
<P>
<UL>
<LI><CODE>EACCESS</CODE>, <CODE>EFAULT</CODE>, <CODE>EIDRM</CODE>
<LI><CODE>EINVAL</CODE>: invalid value for <VAR>cmd</VAR> or <VAR>semid</VAR>.
<LI><CODE>EPERM</CODE>: the calling task tried <CODE>IPC_SET</CODE> or
<CODE>IPC_RMID</CODE> but is not the creator or the owner of the semaphore set
or the superuser.
<LI><CODE>ERANGE</CODE>: the specified value for the semaphore is out of
range.
</UL>
<P>
<P>Go to the <A HREF="syscalls_76.html" tppabs="http://www.infran.ru/TechInfo/syscalls/syscalls_76.html">previous</A>, <A HREF="syscalls_78.html" tppabs="http://www.infran.ru/TechInfo/syscalls/syscalls_78.html">next</A> section.<P>