Docs: Emphasize SvPVbyte and SvPVutf8 over SvPV. This updates

perlguts, perlxs, perlxstut, and perlapi.

Issue #18600

dist/ExtUtils-ParseXS/lib/perlxs.pod

@@ -603,7 +603,7 @@ and C<$type> can be used as in typemaps.
 
      bool_t
      rpcb_gettime(host,timep)
-          char *host = (char *)SvPV_nolen($arg);
+          char *host = (char *)SvPVbyte_nolen($arg);
           time_t &timep = 0;
         OUTPUT:
           timep
@@ -630,7 +630,7 @@ Here's a truly obscure example:
      bool_t
      rpcb_gettime(host,timep)
           time_t &timep; /* \$v{timep}=@{[$v{timep}=$arg]} */
-          char *host + SvOK($v{timep}) ? SvPV_nolen($arg) : NULL;
+          char *host + SvOK($v{timep}) ? SvPVbyte_nolen($arg) : NULL;
         OUTPUT:
           timep
 
@@ -993,7 +993,7 @@ The XS code, with ellipsis, follows.
           char *host = "localhost";
         CODE:
 	  if( items > 1 )
-	       host = (char *)SvPV_nolen(ST(1));
+	       host = (char *)SvPVbyte_nolen(ST(1));
 	  RETVAL = rpcb_gettime( host, &timep );
         OUTPUT:
           timep
@@ -1294,7 +1294,7 @@ prototypes.
           char *host = "localhost";
         CODE:
 		  if( items > 1 )
-		       host = (char *)SvPV_nolen(ST(1));
+		       host = (char *)SvPVbyte_nolen(ST(1));
 		  RETVAL = rpcb_gettime( host, &timep );
         OUTPUT:
           timep

dist/ExtUtils-ParseXS/lib/perlxstut.pod

@@ -1143,7 +1143,8 @@ Mytest.xs:
 	    for (n = 0; n <= numpaths; n++) {
 		HV * rh;
 		STRLEN l;
-		char * fn = SvPV(*av_fetch((AV *)SvRV(paths), n, 0), l);
+		SV * path = *av_fetch((AV *)SvRV(paths), n, 0);
+		char * fn = SvPVbyte(path, l);
 
 		i = statfs(fn, &buf);
 		if (i != 0) {

pod/perldelta.pod

@@ -143,8 +143,13 @@ XXX
 
 =head1 Documentation
 
-XXX Changes to files in F<pod/> go here.  Consider grouping entries by
-file and be sure to link to the appropriate page, e.g. L<perlfunc>.
+L<perlguts> now explains in greater detail the need to consult SvUTF8
+when calling SvPV (or variants). A new "How do I pass a Perl string to a C
+library?" section in the same document discusses when to use which style of
+macro to read an SV's string value.
+
+L<perlapi>, L<perlguts>, L<perlxs>, and L<perlxstut> now prefer SvPVbyte
+over SvPV.
 
 =head2 New Documentation
 

pod/perlguts.pod

@@ -153,37 +153,86 @@ Perl's own functions typically add a trailing C<NUL> for this reason.
 Nevertheless, you should be very careful when you pass a string stored
 in an SV to a C function or system call.
 
-To access the actual value that an SV points to, you can use the macros:
-
-    SvIV(SV*)
-    SvUV(SV*)
-    SvNV(SV*)
-    SvPV(SV*, STRLEN len)
-    SvPV_nolen(SV*)
-
-which will automatically coerce the actual scalar type into an IV, UV, double,
-or string.
-
-In the C<SvPV> macro, the length of the string returned is placed into the
-variable C<len> (this is a macro, so you do I<not> use C<&len>).  If you do
-not care what the length of the data is, use the C<SvPV_nolen> macro.
-Historically the C<SvPV> macro with the global variable C<PL_na> has been
-used in this case.  But that can be quite inefficient because C<PL_na> must
+To access the actual value that an SV points to, Perl's API exposes
+several macros that coerce the actual scalar type into an IV, UV, double,
+or string:
+
+=over
+
+=item * C<SvIV(SV*)> (C<IV>) and C<SvUV(SV*)> (C<UV>)
+
+=item * C<SvNV(SV*)> (C<double>)
+
+=item * Strings are a bit complicated:
+
+=over
+
+=item * Byte string: C<SvPVbyte(SV*, STRLEN len)> or C<SvPVbyte_nolen(SV*)>
+
+If the Perl string is C<"\xff\xff">, then this returns a 2-byte C<char*>.
+
+This is suitable for Perl strings that represent bytes.
+
+=item * UTF-8 string: C<SvPVutf8(SV*, STRLEN len)> or C<SvPVutf8_nolen(SV*)>
+
+If the Perl string is C<"\xff\xff">, then this returns a 4-byte C<char*>.
+
+This is suitable for Perl strings that represent characters.
+
+B<CAVEAT>: That C<char*> will be encoded via Perl's internal UTF-8 variant,
+which means that if the SV contains non-Unicode code points (e.g.,
+0x110000), then the result may contain extensions over valid UTF-8.
+See L<perlapi/is_strict_utf8_string> for some methods Perl gives
+you to check the UTF-8 validity of these macros' returns.
+
+=item * You can also use C<SvPV(SV*, STRLEN len)> or C<SvPV_nolen(SV*)>
+to fetch the SV's raw internal buffer. This is tricky, though; if your Perl
+string
+is C<"\xff\xff">, then depending on the SV's internal encoding you might get
+back a 2-byte B<OR> a 4-byte C<char*>.
+Moreover, if it's the 4-byte string, that could come from either Perl
+C<"\xff\xff"> stored UTF-8 encoded, or Perl C<"\xc3\xbf\xc3\xbf"> stored
+as raw octets. To differentiate between these you B<MUST> look up the
+SV's UTF8 bit (cf. C<SvUTF8>) to know whether the source Perl string
+is 2 characters (C<SvUTF8> would be on) or 4 characters (C<SvUTF8> would be
+off).
+
+B<IMPORTANT:> Use of C<SvPV>, C<SvPV_nolen>, or
+similarly-named macros I<without> looking up the SV's UTF8 bit is
+almost certainly a bug if non-ASCII input is allowed.
+
+When the UTF8 bit is on, the same B<CAVEAT> about UTF-8 validity applies
+here as for C<SvPVutf8>.
+
+=back
+
+(See L</How do I pass a Perl string to a C library?> for more details.)
+
+In C<SvPVbyte>, C<SvPVutf8>, and C<SvPV>, the length of the C<char*> returned
+is placed into the
+variable C<len> (these are macros, so you do I<not> use C<&len>). If you do
+not care what the length of the data is, use C<SvPVbyte_nolen>,
+C<SvPVutf8_nolen>, or C<SvPV_nolen> instead.
+The global variable C<PL_na> can also be given to
+C<SvPVbyte>/C<SvPVutf8>/C<SvPV>
+in this case.  But that can be quite inefficient because C<PL_na> must
 be accessed in thread-local storage in threaded Perl.  In any case, remember
 that Perl allows arbitrary strings of data that may both contain NULs and
 might not be terminated by a C<NUL>.
 
-Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
+Also remember that C doesn't allow you to safely say C<foo(SvPVbyte(s, len),
 len);>.  It might work with your
 compiler, but it won't work for everyone.
 Break this sort of statement up into separate assignments:
 
     SV *s;
     STRLEN len;
     char *ptr;
-    ptr = SvPV(s, len);
+    ptr = SvPVbyte(s, len);
     foo(ptr, len);
 
+=back
+
 If you want to know if the scalar value is TRUE, you can use:
 
     SvTRUE(SV*)
@@ -200,7 +249,7 @@ add space for the trailing C<NUL> byte (perl's own string functions typically do
 C<SvGROW(sv, len + 1)>).
 
 If you want to write to an existing SV's buffer and set its value to a
-string, use SvPV_force() or one of its variants to force the SV to be
+string, use SvPVbyte_force() or one of its variants to force the SV to be
 a PV.  This will remove any of various types of non-stringness from
 the SV while preserving the content of the SV in the PV.  This can be
 used, for example, to append data from an API function to a buffer
@@ -3243,6 +3292,66 @@ There is no published API for dealing with this, as it is subject to
 change, but you can look at the code for C<pp_lc> in F<pp.c> for an
 example as to how it's currently done.
 
+=head2 How do I pass a Perl string to a C library?
+
+A Perl string, conceptually, is an opaque sequence of code points.
+Many C libraries expect their inputs to be "classical" C strings, which are
+arrays of octets 1-255, terminated with a NUL byte. Your job when writing
+an interface between Perl and a C library is to define the mapping between
+Perl and that library.
+
+Generally speaking, C<SvPVbyte> and related macros suit this task well.
+These assume that your Perl string is a "byte string", i.e., is either
+raw, undecoded input into Perl or is pre-encoded to, e.g., UTF-8.
+
+Alternatively, if your C library expects UTF-8 text, you can use
+C<SvPVutf8> and related macros. This has the same effect as encoding
+to UTF-8 then calling the corresponding C<SvPVbyte>-related macro.
+
+Some C libraries may expect other encodings (e.g., UTF-16LE). To give
+Perl strings to such libraries
+you must either do that encoding in Perl then use C<SvPVbyte>, or
+use an intermediary C library to convert from however Perl stores the
+string to the desired encoding.
+
+Take care also that NULs in your Perl string don't confuse the C
+library. If possible, give the string's length to the C library; if that's
+not possible, consider rejecting strings that contain NUL bytes.
+
+=head3 What about C<SvPV>, C<SvPV_nolen>, etc.?
+
+Consider a 3-character Perl string C<$foo = "\x64\x78\x8c">.
+Perl can store these 3 characters either of two ways:
+
+=over
+
+=item * bytes: 0x64 0x78 0x8c
+
+=item * UTF-8: 0x64 0x78 0xc2 0x8c
+
+=back
+
+Now let's say you convert C<$foo> to a C string thus:
+
+    STRLEN strlen;
+    char *str = SvPV(foo_sv, strlen);
+
+At this point C<str> could point to a 3-byte C string or a 4-byte one.
+
+Generally speaking, we want C<str> to be the same regardless of how
+Perl stores C<$foo>, so the ambiguity here is undesirable. C<SvPVbyte>
+and C<SvPVutf8> solve that by giving predictable output: use
+C<SvPVbyte> if your C library expects byte strings, or C<SvPVutf8>
+if it expects UTF-8.
+
+If your C library happens to support both encodings, then C<SvPV>--always
+in tandem with lookups to C<SvUTF8>!--may be safe and (slightly) more
+efficient.
+
+B<TESTING> B<TIP:> Use L<utf8>'s C<upgrade> and C<downgrade> functions
+in your tests to ensure consistent handling regardless of Perl's
+internal encoding.
+
 =head2 How do I convert a string to UTF-8?
 
 If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade

sv.h

@@ -801,7 +801,9 @@ compiler will complain if you were to try to modify the contents of the string,
 (unless you cast away const yourself).
 
 =for apidoc Am|STRLEN|SvCUR|SV* sv
-Returns the length of the string which is in the SV.  See C<L</SvLEN>>.
+Returns the length, in bytes, of the PV inside the SV.
+Note that this may not match Perl's C<length>; for that, use
+C<sv_len_utf8(sv)>. See C<L</SvLEN>> also.
 
 =for apidoc Am|STRLEN|SvLEN|SV* sv
 Returns the size of the string buffer in the SV, not including any part
@@ -855,8 +857,8 @@ Set the value of the MAGIC pointer in C<sv> to val.  See C<L</SvIV_set>>.
 Set the value of the STASH pointer in C<sv> to val.  See C<L</SvIV_set>>.
 
 =for apidoc Am|void|SvCUR_set|SV* sv|STRLEN len
-Set the current length of the string which is in the SV.  See C<L</SvCUR>>
-and C<SvIV_set>>.
+Sets the current length, in bytes, of the C string which is in the SV.
+See C<L</SvCUR>> and C<SvIV_set>>.
 
 =for apidoc Am|void|SvLEN_set|SV* sv|STRLEN len
 Set the size of the string buffer for the SV. See C<L</SvLEN>>.
@@ -1657,6 +1659,14 @@ see C<L</SvPV_force>>.
 
 The differences between the forms are:
 
+The forms with neither C<byte> nor C<utf8> in their names (e.g., C<SvPV> or
+C<SvPV_nolen>) can expose the SV's internal string buffer. If
+that buffer consists entirely of bytes 0-255 and includes any bytes above
+127, then you B<MUST> consult C<SvUTF8> to determine the actual code points
+the string is meant to contain. Generally speaking, it is probably safer to
+prefer C<SvPVbyte>, C<SvPVutf8>, and the like. See
+L<perlguts/How do I pass a Perl string to a C library?> for more details.
+
 The forms with C<flags> in their names allow you to use the C<flags> parameter
 to specify to process 'get' magic (by setting the C<SV_GMAGIC> flag) or to skip
 'get' magic (by clearing it).  The other forms process 'get' magic, except for