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skrzyp |
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<HTML
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><HEAD
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>System Calls</TITLE
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"><LINK
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REL="HOME"
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TITLE="eCos Synthetic Target"
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HREF="hal-synth-arch.html"><LINK
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><TH
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COLSPAN="3"
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ALIGN="center"
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>eCos Synthetic Target</TH
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VALIGN="bottom"
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><HR
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="SYNTH-SYSCALLS">System Calls</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN483"
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></A
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><H2
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>Name</H2
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>cyg_hal_sys_xyz -- Access Linux system facilities</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN486"><H2
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>Synopsis</H2
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CLASS="FUNCSYNOPSIS"
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><TR
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><TD
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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>#include <cyg/hal/hal_io.h
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</PRE
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></TD
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></TABLE
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>int cyg_hal_sys_xyzzy</CODE
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>(...);</CODE
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></P
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><P
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="SYNTH-SYSCALLS-DESCRIPTION"
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></A
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><H2
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>Description</H2
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><P
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>On a real embedded target eCos interacts with the hardware by peeking
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and poking various registers, manipulating special regions of memory,
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and so on. The synthetic target does not access hardware directly.
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Instead I/O and other operations are emulated by making appropriate
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Linux system calls. The HAL package exports a number of functions
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which allow other packages, or even application code, to make these
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same system calls. However this facility must be used with care: any
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code which calls, for example, <TT
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CLASS="FUNCTION"
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>cyg_hal_sys_write</TT
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>
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will only ever run on the synthetic target; that functionality is
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obviously not provided on any real hardware because there is no
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underlying Linux kernel to implement it.
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</P
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><P
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>The synthetic target only provides a subset of the available system
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calls, specifically those calls which have proved useful to implement
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I/O emulation. This subset can be extended fairly easily if necessary.
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All of the available calls, plus associated data structures and
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macros, are defined in the header file <TT
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CLASS="FILENAME"
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>cyg/hal/hal_io.h</TT
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>. There is a simple
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convention: given a Linux system call such as
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<TT
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CLASS="FUNCTION"
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>open</TT
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>, the synthetic target will prefix
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<TT
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CLASS="LITERAL"
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>cyg_hal_sys</TT
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> and provide a function with that name.
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The second argument to the <TT
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CLASS="FUNCTION"
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>open</TT
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> system call is
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a set of flags such as <TT
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CLASS="CONSTANT"
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>O_RDONLY</TT
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>, and the header
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file will define a matching constant
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<TT
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CLASS="CONSTANT"
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>CYG_HAL_SYS_O_RDONLY</TT
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>. There are also data
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structures such as <SPAN
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CLASS="STRUCTNAME"
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>cyg_hal_sys_sigset_t</SPAN
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>,
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matching the Linux data structure <SPAN
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CLASS="STRUCTNAME"
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>sigset_t</SPAN
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>.
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</P
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><P
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>In most cases the functions provided by the synthetic target behave as
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per the documentation for the Linux system calls, and section 2 of the
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Linux man pages can be consulted for more information. There is one
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important difference: typically the documentation will say that a
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function returns <TT
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CLASS="LITERAL"
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>-1</TT
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> to indicate an error, with the
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actual error code held in <TT
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CLASS="VARNAME"
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>errno</TT
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>; the actual
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underlying system call and hence the
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<TT
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CLASS="FUNCTION"
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>cyg_hal_sys_xyz</TT
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> provided by eCos instead returns
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a negative number to indicate an error, with the absolute value of
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that number corresponding to the error code; usually it is the C
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library which handles this and manipulates errno, but of course
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synthetic target applications are not linked with that Linux library.
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</P
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><P
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>However, there are some exceptions. The Linux kernel has evolved over
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the years, and some of the original system call interfaces are no
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longer appropriate. For example the original
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<TT
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CLASS="FUNCTION"
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>select</TT
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> system call has been superseded by
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<TT
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CLASS="FUNCTION"
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>_newselect</TT
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>, and that is what the
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<TT
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CLASS="FUNCTION"
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>select</TT
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> function in the C library actually uses.
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The old call is still available to preserve binary compatibility but,
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like the C library, eCos makes use of the new one because it provides
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the appropriate functionality. In an attempt to reduce confusion the
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eCos function is called <TT
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CLASS="FUNCTION"
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>cyg_hal_sys__newselect</TT
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>,
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in other words it matches the official system call naming scheme. The
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authoritive source of information on such matters is the Linux kernel
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sources themselves, and especially its header files.
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</P
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><P
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>eCos packages and applications should never
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<TT
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CLASS="LITERAL"
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>#include</TT
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> Linux header files directly. For example,
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doing a <TT
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CLASS="LITERAL"
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>#include </usr/include/fcntl.h></TT
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>
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to access additional macros or structure definitions, or alternatively
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manipulating the header file search path, will lead to problems
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because the Linux header files are likely to duplicate and clash with
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definitions in the eCos headers. Instead the appropriate functionality
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should be extracted from the Linux headers and moved into either
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<TT
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CLASS="FILENAME"
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>cyg/hal/hal_io.h</TT
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> or into
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application code, with suitable renaming to avoid clashes with eCos
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names. Users should be aware that large-scale copying may involve
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licensing complications.
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</P
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><P
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>Adding more system calls is usually straightforward and involves
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adding one or more lines to the platform-specific file in the
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appropriate platform HAL, for example
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<TT
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CLASS="FILENAME"
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>syscall-i386-linux-1.0.S</TT
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>. However it is necessary
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to do some research first about the exact interface implemented by the
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system call, because of issues such as old system calls that have been
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superseded. The required information can usually be found fairly
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easily by searching through the Linux kernel sources and possibly the
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GNU C library sources.
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</P
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>The Console Device</TD
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