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Linux系统调用

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<TITLE>Syscall specifications of Linux - shmctl</TITLE>
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<H2><A NAME="SEC81" HREF="syscalls_toc.html#SEC81" tppabs="http://personal.xfol.com/%7erezaie/syscall/syscalls_toc.html#SEC81">shmctl</A></H2>
<P>
<H3>SYNOPSIS</H3>
<P>
<CODE>int shmctl(int <VAR>shmid</VAR>, int <VAR>cmd</VAR>, struct shmid_ds
*<VAR>buf</VAR>);</CODE>
<P>
<H3>PARAMETERS</H3>
<P>
<VAR>shmid</VAR>: [in] the memory segment to manipulate.
<P>
<VAR>cmd</VAR>: [in] the operation to perform.
<P>
<VAR>buf</VAR>: [in out] parameter for the operation (see description).
 
<H3>DESCRIPTION</H3>
<P>
This calls manipulates some operational parameters of shared memory
segments. The <CODE>shmid_ds</CODE> structure has the following layout:
<P>
<PRE>
struct shmid_ds {
        struct  ipc_perm shm_perm;      /* operation perms */
        int     shm_segsz;              /* size of segment (bytes) */
        time_t  shm_atime;              /* last attach time */
        time_t  shm_dtime;              /* last detach time */
        time_t  shm_ctime;              /* last change time */
        unsigned short  shm_cpid;       /* pid of creator */
        unsigned short  shm_lpid;       /* pid of last operator */
        short   shm_nattch;             /* no. of current attaches */
        /* the following are private */
        unsigned short   shm_npages;  /* size of segment (pages) */
        unsigned long   *shm_pages;   /* array of ptrs to frames -&#62; SHMMAX */ 
        struct shm_desc *attaches;    /* descriptors for attaches */
};
</PRE>
<P>
<VAR>cmd</VAR> can take the following values:
<P>
<DL COMPACT>
<DT><CODE>IPC_STAT</CODE>
<DD>retreives the <CODE>shmid_ds</CODE> structure of the memory segment. The
calling taks must have read privileges on the segment.
<P>
<DT><CODE>IPC_SET</CODE>
<DD>sets the <CODE>shmid_ds</CODE> structure of the memory segment. The calling
task must have alter privileges on the segment. Only the members
<CODE>uid</CODE>, <CODE>gid</CODE> and the lower 9 bits of <CODE>mode</CODE> of
<CODE>shm_perm</CODE> can be modified.
<P>
<DT><CODE>IPC_RMID</CODE>
<DD>mark the shared memory segment as destroyed. The calling taks must have
the same uid of the creator or the owner of the memory segment or be the
superuser.
<P>
<DT><CODE>SHM_LOCK</CODE>
<DD>prevents the memory segment to be swaped out to disk. Only a task with
superuser privileges may use this command.
<P>
<DT><CODE>SHM_UNLOCK</CODE>
<DD>allows the memory segment to be swaped out to disk. Only a task with
superuser privileges may use this command.
<P>
</DL>
<P>
<H3>RETURN VALUE</H3>
<P>
On success, the call returns zero. On error -1 is returned and
<CODE>errno</CODE> is set to one of the following values:
<P>
<UL>
<LI><CODE>EINVAL</CODE>: <VAR>shmid</VAR> or <VAR>cmd</VAR> are not valid.
<LI><CODE>EPERM</CODE>: the calling task does not have enough privileges on
the memory segment to perform the operation.
<LI><CODE>EFAULT</CODE> or <CODE>EIDRM</CODE>.
</UL>
<P>
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