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195 lines
6.9 KiB
Plaintext
195 lines
6.9 KiB
Plaintext
INTRODUCTION
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======================
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This directory contains a port of eXternal Data Representation
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(XDR) code from SunRPC (derived from the relicensed -- to
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3-clause BSD -- implementation in Fedora 11's libtirpc package
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version 0.1.10-7). It has been adapted for newlib in the
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following ways:
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1) xdr_* functions for fixed-width integral types have been
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added, such as xdr_int32_t() and similar. The implementation
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of stream-specific x_putlong() and x_getlong() functions
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has been modified to work properly whenever possible, even
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if sizeof(long) > 32bits -- and to correctly report failure
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when that is not possible.
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2) Use of DEFUN(), EXFUN(), and various other portability
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macros.
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3) Uses of 64bit types, such as xdr_hyper, xdr_u_longlong_t,
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and xdr_int64_t, as well as the xdr-specific typedefs
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quad_t and u_quad_t, are guarded by ___int64_t_defined.
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4) Out-of-memory conditions are indicated by returning FALSE
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and setting errno = ENOMEM, rather than by printing error
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messages to stderr. (See #8, below).
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5) Only xdrstdio.c requires stdio support, and it is only
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compiled if the target supports stdio (see stdio_dir in
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configure.host)
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6) Uses a local implementation of ntohl/htonl, rather than
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one provided elsewhere. No dependency on any networking
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functions.
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7) Floating point support refactored. Currently supports
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IEEE single and double precision, and VAX single and
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double precision.
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a) Those platforms which use float to represent double
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do not provide xdr_double().
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8) Error reporting can be customized using a private hook.
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This is described below.
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xdr is compiled and supported only for those platforms which
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set xdr_dir nonempty in configure.host. At present, the list
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of platforms which do this is:
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cygwin
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PORTING
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======================
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To port XDR to a new newlib target, first enable building it
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by modifying configure.host. Search for the 'case' statement
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where various *_dir= variables are set, and look for your
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target's entry (or add one if not present). Set xdr_dir:
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*-*-myplatform*)
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xdr_dir=xdr
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;;
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If your platform does not use IEEE754 standard formats for
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floating point values (floats, doubles) you may need to add
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a new xdr_float_*.c implementation, and modify the bottom of
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xdr_float.c:
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...
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#elif defined(__vax__)
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#include "xdr_float_vax.c"
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+#elif defined(__my_platform__)
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+#include "xdr_float_my_platform.c"
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#endif
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You may want to customize your platform's startup objects to set
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the error reporting callback for xdr (not likely, but see ERROR
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MESSAGES section).
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You may also want to customize the memory allocation semantics
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employed by the xdr routines. As stated in the xdr.h header:
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XDR_DECODE may allocate space if the pointer [to the location
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at which the decoded data is to be stored] is NULL. This
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data can be freed with the XDR_FREE operation.
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The default implementation defines the following macros in
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rpc/types.h, used throughout xdr/ to deal with memory
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allocation:
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#ifndef mem_alloc
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#define mem_alloc(bsize) calloc(1, bsize)
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#endif
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#ifndef mem_free
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#define mem_free(ptr, bsize) free(ptr)
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#endif
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By arranging that these symbols are #defined to some other
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memory allocation functions, different memory semantics can be
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imposed. To disallow memory allocation entirely, use the
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following:
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-D'mem_alloc(a)'=NULL -D'mem_free(a,b)'='do { ; } while(0)'
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In this case, any operations which would otherwise require
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memory to be allocated, will instead fail (return FALSE),
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and set errno=ENOMEM.
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ERROR MESSAGES
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======================
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This implementation of xdr provides a special hook, so that
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error messages generated by xdr may be captured by a user-
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defined facility. For certain error conditions, the internal
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printf-like function
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xdr_warnx (fmt, ...)
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is called. However, that function simply delegates to an
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internal function pointer to a callback function if set;
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otherwise, xdr_warnx does nothing.
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By setting this function pointer to a user-defined callback,
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the user can enable these messages to go to a syslog, stderr,
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or some other facility. The function should match the
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following typedef (see xdr_private.h):
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typedef void (* xdr_vprintf_t) (const char *, va_list);
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The desired callback can be registered by calling:
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xdr_vprintf_t xdr_set_vprintf (xdr_vprintf_t fnptr);
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The return value is the "old" function pointer, which may
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be NULL.
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However, neither the typedef nor the registration function
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are declared in the public headers. Clients wishing to use
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them must either declare the necessary symbols manually,
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or #include "xdr_private.h". More on this point, below.
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For instance:
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#include <stdarg.h>
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#include <stdio.h>
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typedef void (* xdr_vprintf_t) (const char *, va_list);
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xdr_vprintf_t xdr_set_vprintf (xdr_vprintf_t fnptr);
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void my_vwarnx (const char * fmt, va_list ap)
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{
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(void) fprintf (stderr, fmt, ap);
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}
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main()
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{
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(void) xdr_set_vprintf (&my_vwarnx);
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...
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}
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will cause xdr-generated error messages to go to stderr.
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It is not expected that end-user applications will make use
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of this facility. Rather, it is expected that IF certain
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*platforms* desire that these error messages be recorded,
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instead of expecting client apps to print error messages as
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necessary (*), then those platforms will, in their startup
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objects or static initialization, direct these messages to
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a logging facility, strace debug facility, etc.
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Therefore, the platform startup code, if part of newlib, can
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#include "xdr_private.h", or simply copy the two declarations
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from that file.
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However, most newlib targets will probably be satisfied with
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the default (silent) behavior. Note that the original Sun RPC
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implementation of XDR, as well as the glibc implementation,
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print these error messages to stderr. Cygwin, for greater
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similarity to glibc, registers an error message handler similar
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to the example above, within its startup code.
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(*) Client apps should already check for FALSE return values.
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In this case when xdr function return FALSE, the client
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app would then check errno and act appropriately.
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LICENSING AND PEDIGREE
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======================
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For years, the Sun RPC code, and the XDR implementation, was in
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legal license limbo
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http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=181493
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as its license terms, while open, were of debatable compatibility
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with the GPL. In February of 2009, that changed:
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http://blogs.sun.com/webmink/entry/old_code_and_old_licenses
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http://lwn.net/Articles/319648/
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As documented in the libtirpc rpm.spec file from Fedora 11:
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* Tue May 19 2009 Tom "spot" Callaway <xxxx@redhat.com> 0.1.10-7
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- Replace the Sun RPC license with the BSD license, with the
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explicit permission of Sun Microsystems
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So, in the XDR implementation from Fedora 11's libtirpc package,
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after the modification above by Tom Callaway, each file carries
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the 3-clause BSD license and not the so-called "SunRPC" license.
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It is from this version that the newlib implementation here was
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derived, with the modifications described in the introduction,
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above.
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