Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!wuarchive!udel!brahms.udel.edu!apperson
From: apperson@brahms.udel.edu (Apperson H Johnson)
Newsgroups: comp.unix.aix
Subject: ? write and read double as text reliably
Keywords: double precision, scanf, atof, printf, significant digits
Message-ID: <17815@brahms.udel.edu>
Date: 17 Jan 91 21:17:04 GMT
Followup-To: apperson@brahms.udel.edu
Distribution: usa
Organization: Dis
Lines: 48

What is the best way to write double precision numbers if
1. they are to be read by people
2. they are to bre read by a C program, and MUST preserve their exact values.

I need a general format that will preserve all the significant bits of any 
double I write.

I have tried "%.30g" with the following results

2 of 4 systems tried do NOT preserve the values of the floats
across sprintf/sscanf (I tried Xenix 386, SCO Unix, AIX 3.1, Sun OS)

NOTE that NO TWO systems printed EXACTLY the SAME values !!! 

Is there a portable, general solution to this problem? 

It would be nice if system A could read what it wrote (exactly)
it would be REAL nice if for any two systems (A,B) A could read
what B wrote (exactly) - don't any standards cover this ? 

---------------------  tear on dotted line ------------------------------------
#include <stdio.h>

char *s1 = "0.12345678901234547890123456789012345678901234567890";
/* note that i dont REALLY expect all of these digits to affect the value,
   i just want to guarantee excess significant digits */


char s2[128];

main()
{
	double d1, d2, atof();
	
	d1 = atof(s1);
	sprintf(s2,"%.60g",d1);
	sscanf(s2,"%lf",&d2);
	
	printf("d1 and d2 are %sequal!\n", d1 == d2 ? "" : "NOT ");
	printf("s1 = %s\nd1 = %.60g\ns2 = %s\nd2 = %.60g\n",
	   s1,d1,s2,d2);
		
}

----------------------- cry on dotted line ------------------------------------

thanks for any help you can offer,
apperson@brahms.udel.edu