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Linux系统调用

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<TITLE>Syscall specifications of Linux - shmat</TITLE>
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<H2><A NAME="SEC80" HREF="syscalls_toc.html#SEC80" tppabs="http://personal.xfol.com/%7erezaie/syscall/syscalls_toc.html#SEC80">shmat and shmdt</A></H2>
<P>
<H3>SYNOPSIS</H3>
<P>
<CODE>char *shmat(int <VAR>shmid</VAR>, char *<VAR>shmaddr</VAR>, int <VAR>shmflg</VAR>);</CODE>
<P>
<CODE>int shmdt(char *<VAR>shmaddr</VAR>);</CODE>
<P>
<H3>PARAMETERS</H3>
<P>
<VAR>shmid</VAR>: [in] the shared memory identificator.
<P>
<VAR>shmaddr</VAR>: [in] the start of the shared memory.
<P>
<VAR>shmflg</VAR>: [in] some flags (see description).
<P>
<H3>DESCRIPTION</H3>
<P>
<CODE>shmat</CODE> attaches a shared memory identificator to a memory range in
the task address space. If <VAR>shmaddr</VAR> is zero the range where to
attach the shared memory is choosen by the system. If <VAR>shmaddr</VAR> is
not zero and the flag <CODE>SHM_RND</CODE> is specified, the address is
rounded down at a multiple of <CODE>SHMBLA</CODE>. In any other cases, the
address must be page alligned.
<P>
If the flag <CODE>SHM_RDONLY</CODE> is specified the task must have read
privileges to the segment and it is attached for reading only.
Otherwise, the task must have read and write privileges on the segment
and it is attached for reading and writing. The same segment may be
attached more than once is the same address space with different flags.
<P>
On success, the following members of the <CODE>shmid_ds</CODE> structure of
the segment are modified:
<P>
<DL COMPACT>
<DT><CODE>shm_atime</CODE>
<DD>is set to the current time.
<P>
<DT><CODE>shm_lpid</CODE>
<DD>is set to the current pid.
<P>
<DT><CODE>shm_nattch</CODE>
<DD>is incremented by one.
</DL>
<P>
<CODE>shmdt</CODE> detaches shared memory segments attached by <CODE>shmat</CODE>.
The <VAR>shmaddr</VAR> parameter must be the same value returned by the
<CODE>shmat</CODE> that attached the segment.
<P>
On success, the following members of the <CODE>shmid_ds</CODE> structure of
the segment are modified:
<P>
<DL COMPACT>
<DT><CODE>shm_dtime</CODE>
<DD>is set to the current time.
<P>
<DT><CODE>shm_lpid</CODE>
<DD>is set to the current pid.
<P>
<DT><CODE>shm_nattch</CODE>
<DD>is decremented by one. If it becomes 0 and the segment is marked for
deletionm, the segment is deleted.
</DL>
<P>
Attached memory segments are inherited through the <CODE>fork</CODE> call.
They are detached after a <CODE>exec</CODE> or <CODE>exit</CODE> call.
<P>
<H3>RETURN VALUE</H3>
<P>
On success <CODE>shmat</CODE> returns the address of the new memory segment
and <CODE>shmdt</CODE> returns zero. On error, both calls return -1, and set
<CODE>errno</CODE>. The possibles values of <CODE>errno</CODE> are:
<P>
for <CODE>shmat</CODE>:
<P>
<UL>
<LI><CODE>EACCESS</CODE>: not enough privileges on the segment.
<LI><CODE>EINVAL</CODE>: <VAR>shmid</VAR> or <VAR>shmaddr</VAR> invalid.
<LI><CODE>ENOMEM</CODE>.
</UL>
<P>
for <CODE>shmdt</CODE>: the only possible value is <CODE>EINVAL</CODE> for an
invalid value of <VAR>shmaddr</VAR>.
<P>
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