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8.9 KiB
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<TITLE>The POSTGRES95 User Manual - Appendix A:</TITLE>
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<BODY>
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<font size=-1>
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<A HREF="pg95user.html">[ TOC ]</A>
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<A HREF="refs.html">[ Previous ]</A>
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<HR>
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<H1>Appendix A: Linking Dynamically-Loaded Functions</H1>
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<HR>
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After you have created and registered a user-defined
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function, your work is essentially done. POSTGRES,
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however, must load the object code (e.g., a .o file, or
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a shared library) that implements your function. As
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previously mentioned, POSTGRES loads your code at
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runtime, as required. In order to allow your code to be
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dynamically loaded, you may have to compile and
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linkedit it in a special way. This section briefly
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describes how to perform the compilation and
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linkediting required before you can load your user-defined
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functions into a running POSTGRES server. Note that
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this process has changed as of Version 4.2.<A HREF="#11">11</A> You
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should expect to read (and reread, and re-reread) the
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manual pages for the C compiler, cc(1), and the link
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editor, ld(1), if you have specific questions. In
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addition, the regression test suites in the directory
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/usr/local/postgres95/src/regress contain several
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working examples of this process. If you copy what these
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tests do, you should not have any problems.
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The following terminology will be used below:
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<DL>
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<DT>Dynamic loading
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<DD>is what POSTGRES does to an object file. The
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object file is copied into the running POSTGRES
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server and the functions and variables within the
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file are made available to the functions within
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the POSTGRES process. POSTGRES does this using
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the dynamic loading mechanism provided by the
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operating system.
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<DT>Loading and link editing
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<DD>is what you do to an object file in order to produce
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another kind of object file (e.g., an executable
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program or a shared library). You perform
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this using the link editing program, ld(1).
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</DL>
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<p>
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The following general restrictions and notes also apply
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to the discussion below.
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<UL>
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<LI>Paths given to the create function command must be
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absolute paths (i.e., start with "/") that refer to
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directories visible on the machine on which the
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POSTGRES server is running.<A HREF="#12">12</A>
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<LI>The POSTGRES user must be able to traverse the path
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given to the create function command and be able to
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read the object file. This is because the POSTGRES
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server runs as the POSTGRES user, not as the user
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who starts up the frontend process. (Making the
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file or a higher-level directory unreadable and/or
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unexecutable by the "postgres" user is an extremely
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common mistake.)
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<LI>Symbol names defined within object files must not
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conflict with each other or with symbols defined in
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POSTGRES.
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<LI>The GNU C compiler usually does not provide the special
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options that are required to use the operating
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system's dynamic loader interface. In such cases,
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the C compiler that comes with the operating system
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must be used.
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</UL>
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<p>
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<B>ULTRIX</B><br>
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It is very easy to build dynamically-loaded object
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files under ULTRIX. ULTRIX does not have any sharedlibrary
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mechanism and hence does not place any restrictions on
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the dynamic loader interface. On the other
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hand, we had to (re)write a non-portable dynamic loader
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ourselves and could not use true shared libraries.
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Under ULTRIX, the only restriction is that you must
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produce each object file with the option -G 0. (Notice
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that that's the numeral ``0'' and not the letter
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``O''). For example,
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<pre> # simple ULTRIX example
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% cc -G 0 -c foo.c
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</pre>
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produces an object file called foo.o that can then be
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dynamically loaded into POSTGRES. No additional loading or link-editing must be performed.
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<p>
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<B>DEC OSF/1</B><br>
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Under DEC OSF/1, you can take any simple object file
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and produce a shared object file by running the ld command over it with the correct options. The commands to
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do this look like:
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<pre> # simple DEC OSF/1 example
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% cc -c foo.c
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% ld -shared -expect_unresolved '*' -o foo.so foo.o
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</pre>
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The resulting shared object file can then be loaded
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into POSTGRES. When specifying the object file name to
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the create function command, one must give it the name
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of the shared object file (ending in .so) rather than
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the simple object file.<A HREF="#13">13</A> If the file you specify is
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not a shared object, the backend will hang!
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<p>
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<B>SunOS 4.x, Solaris 2.x and HP-UX</B><br>
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Under both SunOS 4.x, Solaris 2.x and HP-UX, the simple
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object file must be created by compiling the source
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file with special compiler flags and a shared library
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must be produced.
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The necessary steps with HP-UX are as follows. The +z
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flag to the HP-UX C compiler produces so-called
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"Position Independent Code" (PIC) and the +u flag
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removes
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some alignment restrictions that the PA-RISC architecture
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normally enforces. The object file must be turned
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into a shared library using the HP-UX link editor with
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the -b option. This sounds complicated but is actually
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very simple, since the commands to do it are just:
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<pre> # simple HP-UX example
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% cc +z +u -c foo.c
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% ld -b -o foo.sl foo.o
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</pre>
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As with the .so files mentioned in the last subsection,
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the create function command must be told which file is
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the correct file to load (i.e., you must give it the
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location of the shared library, or .sl file).
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Under SunOS 4.x, the commands look like:
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<pre> # simple SunOS 4.x example
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% cc -PIC -c foo.c
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% ld -dc -dp -Bdynamic -o foo.so foo.o
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</pre>
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and the equivalent lines under Solaris 2.x are:
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<pre> # simple Solaris 2.x example
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% cc -K PIC -c foo.c
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or
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% gcc -fPIC -c foo.c
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% ld -G -Bdynamic -o foo.so foo.o
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</pre>
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When linking shared libraries, you may have to specify
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some additional shared libraries (typically system
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libraries, such as the C and math libraries) on your ld
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command line.
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<HR>
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<A NAME="11"><B>11.</B></A> The old POSTGRES dynamic
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loading mechanism required
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in-depth knowledge in terms of executable format, placement
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and alignment of executable instructions within memory, etc.
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on the part of the person writing the dynamic loader. Such
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loaders tended to be slow and buggy. As of Version 4.2, the
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POSTGRES dynamic loading mechanism has been rewritten to use
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the dynamic loading mechanism provided by the operating
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system. This approach is generally faster, more reliable and
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more portable than our previous dynamic loading mechanism.
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The reason for this is that nearly all modern versions of
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UNIX use a dynamic loading mechanism to implement shared
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libraries and must therefore provide a fast and reliable
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mechanism. On the other hand, the object file must be
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postprocessed a bit before it can be loaded into POSTGRES. We
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hope that the large increase in speed and reliability will
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make up for the slight decrease in convenience.
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<hr width=50 align=left>
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<A NAME="12"><B>12.</B></A> Relative paths do in fact work,
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but are relative to
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the directory where the database resides (which is generally
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invisible to the frontend application). Obviously, it makes
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no sense to make the path relative to the directory in which
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the user started the frontend application, since the server
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could be running on a completely different machine!<br>
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<hr width=50 align=left>
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<A NAME="13"><B>13.</B></A> Actually, POSTGRES does not care
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what you name the
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file as long as it is a shared object file. If you prefer
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to name your shared object files with the extension .o, this
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is fine with POSTGRES so long as you make sure that the correct
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file name is given to the create function command. In
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other words, you must simply be consistent. However, from a
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pragmatic point of view, we discourage this practice because
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you will undoubtedly confuse yourself with regards to which
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files have been made into shared object files and which have
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not. For example, it's very hard to write Makefiles to do
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the link-editing automatically if both the object file and
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the shared object file end in .o!<br>
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<HR>
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<font size=-1>
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<A HREF="pg95user.html">[ TOC ]</A>
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<A HREF="refs.html">[ Previous ]</A>
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[ Next ]
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