C++利用shmget實(shí)現(xiàn)start，stop交替執(zhí)行

C++利用共享內(nèi)存實(shí)現(xiàn)start執(zhí)行之后已經(jīng)要執(zhí)行stop才能在執(zhí)行start，考試大把mian的源代碼貼出來說明這么個問題
    int main(int argc, char *argv[])
    {
    int iShmId;
    if( argc <= 1)
    {
    DisplayHelp();
    }
    else if( !strcmp( argv[1], "start" ) )
    {
    iShmId = shmget( ftok("dvrcollector",1), 1 , IPC_CREAT|IPC_EXCL|0666 );
    if( -1 == iShmId && errno == EEXIST )
    {
    printf( "dvrcollector already running.\n");
    return -1;
    }
    // printf( "start...\n");
    StartDownFile();
    }
    else if( !strcmp( argv[1], "restart" ) )
    {
    iShmId = shmget( ftok("dvrcollector",1), 1 , IPC_CREAT|IPC_EXCL|0666 );
    if( -1 == iShmId && errno == EEXIST )
    {
    StopDownFile();
    // printf( "stop...\n");
    sleep(1);
    }
    // printf( "start...\n");
    StartDownFile();
    }
    else if( !strcmp( argv[1], "stop" ) )
    {
    iShmId = shmget( ftok("dvrcollector",1), 1 , IPC_CREAT|IPC_EXCL|0666 );
    if( -1 == iShmId && errno == EEXIST )
    {
    if( -1 != (iShmId = shmget( ftok("dvrcollector",1), 1 , IPC_CREAT | 0666 ) ) )
    shmctl( iShmId, IPC_RMID, NULL);
    // printf( "stop...\n");
    StopDownFile();
    }
    else
    {
    shmctl( iShmId, IPC_RMID, NULL );
    printf( "dvrcollector not running.\n" );
    }
    }
    else
    {
    DisplayHelp();
    }
    return 0;
    }