%o and %x can now format signed integers

Optionally treat integers as signed numbers with %o
and %x format specifiers, instead of treating them as
a machine-dependent two’s complement representation.
This option is more machine-independent, allows formats
like "#x%x" to be useful for reading later, and is
better-insulated for future changes involving bignums.
Setting the new variable ‘binary-as-unsigned’ to nil
enables the new behavior (Bug#32252).
This is a simplified version of the change proposed in:
https://lists.gnu.org/r/emacs-devel/2018-07/msg00763.html
I simplified that proposal by omitting bitwidth modifiers, as
I could not find an any example uses in the Emacs source code
that needed them and doing them correctly would have been
quite a bit more work for apparently little benefit.
* doc/lispref/strings.texi (Formatting Strings):
Document that %x and %o format negative integers in a
platform-dependent way.  Also, document how to format
numbers so that the same values can be read back in.
* etc/NEWS: Document the change.
* src/editfns.c (styled_format): Treat integers as signed
numbers even with %o and %x, if binary-as-unsigned is nil.
Support the + and space flags with %o and %x, since they’re
about signs.
(syms_of_editfns): New variable binary-as-unsigned.
* test/src/editfns-tests.el (read-large-integer):
Test that maximal integers can be read after printing
with all integer formats, if binary-as-unsigned is nil.

1 files changed, 15 insertions, 2 deletions

diff --git a/doc/lispref/strings.texi b/doc/lispref/strings.texi
index 2fff3c7c75c..3558f17301d 100644
--- a/doc/lispref/strings.texi
+++ b/doc/lispref/strings.texi
@@ -922,7 +922,8 @@ Functions}).  Thus, strings are enclosed in @samp{"} characters, and
 @item %o
 @cindex integer to octal
 Replace the specification with the base-eight representation of an
-unsigned integer.  The object can also be a nonnegative floating-point
+integer.  Negative integers are formatted in a platform-dependent
+way.  The object can also be a nonnegative floating-point
 number that is formatted as an integer, dropping any fraction, if the
 integer does not exceed machine limits.
 
@@ -935,7 +936,8 @@ formatted as an integer, dropping any fraction.
 @itemx %X
 @cindex integer to hexadecimal
 Replace the specification with the base-sixteen representation of an
-unsigned integer.  @samp{%x} uses lower case and @samp{%X} uses upper
+integer.  Negative integers are formatted in a platform-dependent
+way.  @samp{%x} uses lower case and @samp{%X} uses upper
 case.  The object can also be a nonnegative floating-point number that
 is formatted as an integer, dropping any fraction, if the integer does
 not exceed machine limits.
@@ -1108,6 +1110,17 @@ shows only the first three characters of the representation for
 precision is what the local library functions of the @code{printf}
 family produce.
 
+@cindex formatting numbers for rereading later
+  If you plan to use @code{read} later on the formatted string to
+retrieve a copy of the formatted value, use a specification that lets
+@code{read} reconstruct the value.  To format numbers in this
+reversible way you can use @samp{%s} and @samp{%S}, to format just
+integers you can also use @samp{%d}, and to format just nonnegative
+integers you can also use @samp{#x%x} and @samp{#o%o}.  Other formats
+may be problematic; for example, @samp{%d} and @samp{%g} can mishandle
+NaNs and can lose precision and type, and @samp{#x%x} and @samp{#o%o}
+can mishandle negative integers.  @xref{Input Functions}.
+
 @node Case Conversion
 @section Case Conversion in Lisp
 @cindex upper case