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ref: d246077e78bb1aeafe8829927db23f281cd03c72
parent: 44b5291b48e77810a096933d0b6f8dae5a17300c
author: Ben Harris <[email protected]>
date: Mon Jan 2 11:48:20 EST 2023

Add a macro of an upper bound on the formatted length of an integer

There are lots of places where Puzzles formats integers into
fixed-length buffers using sprintf() with a "%d" format.  This isn't
very safe, since C doesn't guarantee any particular maximum size for an
"int".  However, the restrictions on representations of integers means
we can infer an upper bound using sizeof(), CHAR_BIT, and an
approximation to the binary log of 10.

--- a/devel.but
+++ b/devel.but
@@ -4819,6 +4819,17 @@
 These macros may evaluate their arguments multiple times. Avoid side
 effects.
 
+\S{utils-max-digits} \cw{MAX_DIGITS()}
+
+The \cw{MAX_DIGITS()} macro, defined in the main Puzzles header file,
+takes a type (or a variable of that type) and expands to an integer
+constant representing a reasonable upper bound on the number of
+characters that a number of that type could expand to when formatted
+as a decimal number using the \c{%u} or \c{%d} format of
+\cw{printf()}.  This is useful for allocating a fixed-size buffer
+that's guaranteed to be big enough to \cw{sprintf()} a value into.
+Don't forget to add one for the trailing \cw{'\\0'}!
+
 \S{utils-pi} \cw{PI}
 
 The main Puzzles header file defines a macro \cw{PI} which expands
--- a/puzzles.h
+++ b/puzzles.h
@@ -18,6 +18,9 @@
 #define STR_INT(x) #x
 #define STR(x) STR_INT(x)
 
+/* An upper bound on the length of sprintf'ed integers (signed or unsigned). */
+#define MAX_DIGITS(x) (sizeof(x) * CHAR_BIT / 3 + 2)
+
 /* NB not perfect because they evaluate arguments multiple times. */
 #ifndef max
 #define max(x,y) ( (x)>(y) ? (x) : (y) )