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#if 0
#define _REENTRANT 1 /* Should be defined before including math.h for Mac OS X. */
/* Do not use with iOS. Disabled by default and unnecessary. */
#endif
#include <math.h>
/*
* General factorial function in C for double precision floating point.
* Uses the threadsafe lgamma_r(3) function, if _REENTRANT is defined.
* Works for any floating point value.
* Recommend using tgamma(3) (true gamma) function instead, if available.
*
* Link with -lm
*
* Returns (arg!) (same as gamma(arg+1)).
* Sets "errno" external variable on overflow or error.
*/
double
factorial(double arg)
{
double d;
#if USE_TGAMMA
d = tgamma(arg + 1.0);
return d;
#else
#if _REENTRANT
int sign;
d = exp(lgamma_r(arg + 1.0, &sign));
return(d * sign);
#else
d = exp(lgamma(arg + 1.0)) * signgam;
return d;
#endif
#endif
}
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