lgamma(), lgamma_r(), lgammaf(), lgammaf_r(), lgammal(), lgammal_r()

QNX SDP8.0C Library ReferenceAPIDeveloper

Log gamma function

Synopsis:

#include <math.h>

double lgamma( double x );

double lgamma_r( double x,
                 int* signgamp );

float lgammaf( float x );

float lgammaf_r( float x,
                 int* signgamp );

long double lgammal( long double x );

long double lgammal_r( long double x,
                       int* signgamp );

extern int signgam;

Arguments:

x: An arbitrary number.
signgamp: (lgamma_r(), lgammaf_r(), and lgammal_r() only) A pointer to a location where the function can store the sign of Γ(x).

Library:

libm: The general-purpose math library.
libm-sve: A library that optimizes the code for ARMv8.2 chips that have Scalable Vector Extension hardware.

Your system requirements will determine how you should work with these libraries:

If you want only selected processes to run with the SVE version, you can include both libraries in your OS image and use the -l m or -l m-sve option to qcc to link explicitly against the appropriate one.
If you want all processes to use the SVE version, include libm-sve.so in your OS image and set up a symbolic link from libm.so to libm-sve.so. Use the -l m option to qcc to link against the library.

Note:

Compile your program with the -fno-builtin option to prevent the compiler from using a built-in version of the function.

Description:

The lgamma*() and lgamma*_r() functions return the natural log (ln) of the Γ function. These functions return ln|Γ(x)|, where Γ(x) is defined as follows:

For x > 0.0:
For x < 1.0:: n / (Γ( 1-x ) * sin( nx ))

The results converge when x is between 0.0 and 1.0. The Γ function has the property:

Γ(N) = Γ(N-1)×N

The lgamma* functions compute the log because the Γ function grows very quickly.

The lgamma() and lgammaf() functions use the external integer signgam to return the sign of Γ(x), while lgamma_r() and lgammaf_r() use the user-allocated space addressed by signgamp.

Note:

The global signgam variable isn't set until lgamma() or lgammaf() returns. For example, don't use the expression:

g = signgam * exp( lgamma( x ));

to compute g = Γ(x)'. Instead, compute lgamma() first:

lg = lgamma(x);
g = signgam * exp( lg );

Note that Γ(x) must overflow when x is large enough, underflow when -x is large enough, and generate a division by 0 exception at the singularities x a nonpositive integer.

To check for error situations, use feclearexcept() and fetestexcept(). For example:

Call feclearexcept(FE_ALL_EXCEPT) before calling lgamma(), lgamma_r(), lgammaf(), lgammaf_r(), lgammal(), or lgammal_r().
On return, if fetestexcept(FE_ALL_EXCEPT) is nonzero, then an error has occurred.

Returns:

ln|Γ(x)|


If:	These functions return:	Errors:
`x` is 1.0 or 2.0	0.0	—
`x` is a non-positive integer	Inf	FE_DIVBYZERO
`x` is ±Inf	Inf	—
`x` is NaN	NaN	—
The correct value would cause overflow	Inf	FE_OVERFLOW

These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.

Examples:

#include <stdio.h>
#include <fenv.h>
#include <inttypes.h>
#include <math.h>
#include <stdlib.h>

int main(int argc, char** argv) 
{
    double a, b;
    int except_flags;

    a = 0.5;
    feclearexcept(FE_ALL_EXCEPT);
    b = lgamma(a);
    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    printf("lgamma(%f) = %f \n", a, b);

    return EXIT_SUCCESS;
}

The above program produces the following output:

lgamma(0.500000) = 0.572365 0

Classification:

lgamma(), lgammaf(), and lgammal() are C11, POSIX 1003.1; lgamma_r(), lgammaf_r(), and lgammal_r() are QNX OS


Safety:
Cancellation point	No
Signal handler	Yes
Thread	Yes

Page updated: August 11, 2025