log2 not found ?

kjordan · 09-29-2005, 03:10 PM

Seems I was even outdated, it's up to 2.08 and the definition is fixed:

Quote:

SYNOPSIS
#include <math.h>

double log2(double x);
float log2f(float x);
long double log2l(long double x);

Compile with -std=c99; link with -lm.

DESCRIPTION
The log2() function returns the base 2 logarithm of x.

ERRORS
The log2() function can return the following error:

EDOM The argument x is negative.

ERANGE The argument x is zero. The log of zero is not defined (minus
infinity).

CONFORMING TO
SVID 3, POSIX, 4.3BSD, ISO 9899. The float and the long double vari-
ants are C99 requirements.

PTrenholme · 09-29-2005, 05:08 PM

Quote:

Originally posted by Dark_Helmet
Interesting point. I'm guessing that may be a copy-paste error in the man page.
[snip]

Yes, I, too, think it's an error in the manual. I get

Code:

$ c99 -lm -o test test.c

$ ./test 1 2 3 4 5 6 7 8
Log base 2 of 1.000000 is 0.000000
Log base 2 of 2.000000 is 1.000000
Log base 2 of 3.000000 is 1.584963
Log base 2 of 4.000000 is 2.000000
Log base 2 of 5.000000 is 2.321928
Log base 2 of 6.000000 is 2.584963
Log base 2 of 7.000000 is 2.807355
Log base 2 of 8.000000 is 3.000000

on my system.

PS: I slightly modified your [Dark_Helmet's] code to eliminate an error message (no return value for "main") and to process all the arguments internally. Like this at the end:

Code:

  for (i=1; i < argc; ++i) {
    given_value = strtod( argv[i], NULL );

    log_base_2 = log2( given_value );
    printf( "Log base 2 of %f is %f\n",
          given_value,
          log_base_2 );
  }

  return 0;

PPS: As you see, FC4 defines a c99 script, which just calls gcc with the -std=c99 argument.

PTrenholme · 09-29-2005, 07:24 PM

I've reported the manual error to the glibc maintainers.