printf format ambiguity

std c


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printf format ambiguity

Postby Chris Croughton » Tue, 11 Jan 2005 05:35:44 GMT

In ISO/IEC 9899-1999 (N843), section (the fprintf function) describes
the conversion specifications.

Paragraph 6 (flag characters) says:

  0     For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions,
        leading zeros (following any indication of sign or base) are 
        used to pad to the field width;
Paragraph 8 (conversion specifiers) says:

  e, E  A double argument ... is converted in the style [-]d.ddde+/-dd,
        where there is one digit (which is non-zero if the argument is
        non-zero) before the decimal point character...
(The descriptions of g,G and a,A in para 8 are similar.)

So if one has a format specifier of "<%014.4e>", with a parameter of 
-1.2345e2, should the result be: 


  <   -1.2345e+02>
(the former if para 6 is correct, the latter if para 8 is correct)?
glibc2 v2.2.5 does the latter, my implementation does the former (that's
how I noticed the contradiction in the spec.).

Has this been corrected later?  Neither of the Corrigenda on the ANSI website
mention it (ISO/IEC 9899/Cor1:2001 and ISO/IEC 9899/Cor2:2004).

Chris C

Re: printf format ambiguity

Postby Bjorn Reese » Wed, 12 Jan 2005 05:35:44 GMT

 > or
 >   <   -1.2345e+02>

Section says:

   - Zero or more flags (in any order) that modify the meaning of the
     conversion specification.

So the first result (<-0001.2345e+02>) ought to be correct.


Re: printf format ambiguity

Postby Chris Croughton » Wed, 12 Jan 2005 07:19:42 GMT

On Mon, 10 Jan 2005 21:35:44 +0100, Bjorn Reese 

OK, I've now tried several other libraries.  The one which fails is
glibc 2.2.5, and only on the a and A specifiers, all of the others
(well, the others I tried don't implement the a,A specifiers at all)
seem to think that you are correct (as does glibc 2.2.5 with f, e and
g).  Since the wording is the same for the a,A specifiers as for the
e,E one, I think it's a bug in glibc.

(The wording in the spec. is still ambiguous, though, because the other
paragraphs imply only one digit before the point...)

Chris C

Re: printf format ambiguity

Postby Thad Smith » Wed, 12 Jan 2005 14:01:09 GMT

I agree that it may be confusing on first reading, but "modifying the 
meaning", quoted above, makes it clear which interpretation takes 
precedence.  With that context it is not longer ambiguous, IMHO.


Re: printf format ambiguity

Postby Brian Inglis » Wed, 19 Jan 2005 16:32:23 GMT

On Mon, 10 Jan 2005 22:19:42 +0000 in comp.std.c, Chris Croughton ...

ISTM rephrasing as "one significant digit" makes the intent explicit
and resolves the issue. 

Thanks. Take care, Brian Inglis 	Calgary, Alberta, Canada

 XXXX@XXXXX.COM  	(Brian[dot]Inglis{at}SystematicSW[dot]ab[dot]ca)
    fake address		use address above to reply

Similar Threads:

1.printf warning: "format %08x expects format unsigned int ..."

Andreas Eibach wrote:
> .... but I have an unsigned long value in the printf.
> This warning came when I used gcc 4.x to compile.
> ....
> unsigned long offset = 0;
> ....
> Well OK, an "easy" way would be instead of
> printf ("eof found at offset %08x", offset);
> to do a type cast like


> printf ("eof found at offset %08x", (unsigned int) offset);
> Fine, but is this supposed to work for values like 0xFFFFFFFF, which is
> afaik the last possible long value. Unsigned int only goes to 0xFFFF.
"08lx" should do the job for unsigned long.

Ian Collins

2.C99 printf formats (2)

Hello, I have a new question about printf:
what is the correct output of printf("%#x", 0); ? (or with 0U for the
anal-retentive :)
According to my understanding of the standard, it is undefined
behaviour (the behaviour is defined only for nonzero values), so it
could output anything, including "pi=4".
All the libraries that I know will display "0" like for the %#o case
which is clearly specified.


3.printf(): giving format to an unsigned long questions


I would like to print on a win cmd console a register value,  the value
is an "unsigned long" and have some output like these:

Register: 0x00000000

Tryed with

printf("Register: %#010lx \n", register);
printf("Register: %#08lx \n", register);

but I get 0x000003, only the first 6 values ??? from LSB to MSB,
normally MSBs are cero, but I would like to see them all.

Is there a way to print, hexadecimal, the hole 32 bits and add a nice
0x at the beginning?
I have read some books, googled and couldn't give a solution, any help
or info would be kindly appreciated.

Best Regards

4.Inverse of printf %x format

I've seen several postings asking this, but no simple, clear answer. My
C language reference book is deficient in this area.

If I

     short i = 0;

I might resonably expect the following output:


Now, what format code (%?? below) will restore i's value? e.g.

     short i;
     char s[5];
     sprintf((char *)&i,"%??",s);
     if (i==0) printf(">Hexed again<");


     >Hexed again<

Thanks in advance

5.printf format for double

Is it possible to print a float or double in exponential form with a 0 
before the decimal point?

E.g. 0.1234567E+00 rather than 1.2345670E-01 (using %14.7E) ? As far as I 
understand the format specifies in n1256, this isn't possible, is it?

Bye, Jojo 

6. max size for printf() format conversion?

7. Problem with printf formats

8. printf() formatting - stripping zeroes, padding

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