Fix some printf bugs and document other

src/fmt/gen_fmt.h

@@ -773,8 +773,8 @@ static inline int pfx(_dtoa)(
     // Print decimal point
     if(decimal_point_n) out('.');
     // Print fractional part that's made out of digits
-    int prec_digs = prec - mant_digits_n;
+    int prec_digs = prec < mant_digits_n ? prec : mant_digits_n;
-    while(mant_digits_n) {
+    while(mant_digits_n < prec_digs) {
         if(-exp >= mant_digits_n) {
             out('0');
         }

test/math.c

@@ -106,15 +106,15 @@ int main() {
     if(fetestexcept(FE_INVALID)) printf("    FE_INVALID was raised\n");
 
     printf("\n\n=== exp === \n");
-    printf("exp(1) = %f\n", exp(1));
+    printf("exp(1) = %e\n", exp(1));
-    printf("FV of $100, continuously compounded at 3%% for 1 year = %f\n",
+    printf("FV of $100, continuously compounded at 3%% for 1 year = %e\n",
             100*exp(0.03));
     // special values
-    printf("exp(-0) = %f\n", exp(-0.0));
+    printf("exp(-0) = %e\n", exp(-0.0));
-    printf("exp(-Inf) = %f\n", exp(-INFINITY));
+    printf("exp(-Inf) = %e\n", exp(-INFINITY));
     //error handling
     errno = 0; feclearexcept(FE_ALL_EXCEPT);
-    printf("exp(710) = %f\n", exp(710));
+    printf("exp(710) = %e\n", exp(710));
     if(errno == ERANGE) printf("    errno == ERANGE\n");
     if(fetestexcept(FE_OVERFLOW)) printf("    FE_OVERFLOW raised\n");
 

todo

@@ -48,6 +48,8 @@ stdio.h:
 *   tmpnam
 *   tmpfile_s
 *   tmpnam_s
+*   Printing 8.98846567431157854073e+307
+*   Printing negative zero
 
 stdlib.h:
 *   Strtod base 16 must be correctly rounded