implement fp_toradix_n()

It is not implemened yet, just added to the headerfile. Therefore I don't
think it is a ABI breakage if I change maxlen from int to unsigned int.

The function releases fp_toradix() for the work which in turn now calls
fp_toradix_n() with a largest possible maxlen parameter.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

src/bin/fp_toradix.c

@@ -1,59 +1,103 @@
 /* TomsFastMath, a fast ISO C bignum library.
- * 
+ *
  * This project is meant to fill in where LibTomMath
  * falls short.  That is speed ;-)
  *
  * This project is public domain and free for all purposes.
- * 
+ *
  * Tom St Denis, tomstdenis@gmail.com
  */
 #include <tfm.h>
 
-int fp_toradix(fp_int *a, char *str, int radix)
+/**
+ * a:		pointer to fp_int representing the input number
+ * str:		output buffer
+ * radix:	number of character to use for encoding of the number
+ * maxlen:	maximum number of the buffer that can be used
+ *
+ * The radix value can be in the range 2 to 64. This function converts number
+ * a into a string str. This function writes at most size bytes (including the
+ * terminating null byte to str. It behaves like snprintf(3) in regard to this.
+ *
+ * Return: If invalid parameter are detected a negative value is returned. On
+ * success the function returns the number of bytes that would be written if
+ * the function had enough space. Thus a return value of maxlen or more means
+ * that the function was not able store all characters and the output is
+ * incomplete.
+ */
+int fp_toradix_n(fp_int *a, char *str, int radix, unsigned int maxlen)
 {
   int     digs;
   fp_int  t;
   fp_digit d;
   char   *_s = str;
+  unsigned int wrote;
 
   /* check range of the radix */
-  if (radix < 2 || radix > 64) {
-    return FP_VAL;
+  if (radix < 2 || radix > 64)
+    return -1;
+
+  /* quick check for zero */
+  if (fp_iszero(a) == 1) {
+    if (maxlen >= 2)
+       *str++ = '0';
+    if (maxlen >= 1)
+      *str = '\0';
+    return 1;
   }
 
-  /* quick out if its zero */
-  if (fp_iszero(a) == 1) {
-     *str++ = '0';
-     *str = '\0';
-     return FP_OKAY;
-  }
+  wrote = 0;
 
   fp_init_copy(&t, a);
 
   /* if it is negative output a - */
   if (t.sign == FP_NEG) {
     ++_s;
-    *str++ = '-';
+    wrote++;
+    if (wrote < maxlen)
+      *str++ = '-';
     t.sign = FP_ZPOS;
   }
 
   digs = 0;
   while (fp_iszero (&t) == FP_NO) {
     fp_div_d (&t, (fp_digit) radix, &t, &d);
-    *str++ = fp_s_rmap[d];
+    wrote++;
+    if (wrote < maxlen)
+      *str++ = fp_s_rmap[d];
     ++digs;
   }
 
   /* reverse the digits of the string.  In this case _s points
    * to the first digit [exluding the sign] of the number]
    */
-  fp_reverse ((unsigned char *)_s, digs);
+  if (wrote < maxlen)
+    fp_reverse ((unsigned char *)_s, digs);
 
-  /* append a NULL so the string is properly terminated */
-  *str = '\0';
-  return FP_OKAY;
+    /* append a NULL so the string is properly terminated */
+  if (maxlen >= 1)
+    *str = '\0';
+  return wrote;
 }
 
-/* $Source$ */
-/* $Revision$ */
-/* $Date$ */
+/**
+ * a:		pointer to fp_int representing the input number
+ * str:		output buffer
+ * radix:	number of character to use for encoding of the number
+ *
+ * The radix value can be in the range 2 to 64. This function converts number
+ * a into a string str. Please don't use this function because a too small
+ * chosen str buffer would lead to an overflow which can not be detected.
+ * Please use fp_toradix_n() instead.
+ *
+ * Return: FP_VAL on error, FP_OKAY on success.
+ */
+int fp_toradix(fp_int *a, char *str, int radix)
+{
+  int ret;
+
+  ret = fp_toradix_n(a, str, radix, UINT_MAX);
+  if (ret < 0)
+    return FP_VAL;
+  return FP_OKAY;
+}

src/headers/tfm.h

@@ -465,7 +465,7 @@ int fp_read_radix(fp_int *a, char *str, int radix);
 
 int fp_radix_size(fp_int *a, int radix, int *size);
 int fp_toradix(fp_int *a, char *str, int radix);
-int fp_toradix_n(fp_int * a, char *str, int radix, int maxlen);
+int fp_toradix_n(fp_int * a, char *str, int radix, unsigned int maxlen);
 
 
 /* VARIOUS LOW LEVEL STUFFS */