clog(), clogf(), clogl()

Updated: October 26, 2022

Compute the complex natural (base-e) logarithm of a complex number


#include <complex.h>

double complex clog(double complex z );

float complex clogf(float complex z );

long double complex clogl(long double complex z );


The complex value that you want to compute the complex natural logarithm of.


The general-purpose math library.
(QNX Neutrino 7.1 or later) 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:

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


These functions compute the complex natural (base-e) logarithm of the complex number specified by z with a branch cut along the negative real axis. The natural logarithm of a complex number z with polar coordinate components (r,θ) equals ln r + i(θ+2nπ), with the principal value ln r + iθ.

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


If no errors occur, the complex natural logarithm of z, in the range of a strip in the interval [−iπ, +iπ] along the imaginary axis and mathematically unbounded along the real axis.

If z is: These functions return: Errors:
-0 + 0i -Inf + πi FE_DIVBYZERO
0 + 0i -Inf + 0i FE_DIVBYZERO
x + Infi, for any finite x Inf + (π/2)i
x + NaNi, for any finite x NaN + NaNi
-Inf + yi, for any finite positive y -Inf + πi
Inf + yi, for any finite positive y -Inf + 0i
-Inf + Infi Inf + (3π/4)i
Inf + Infi Inf + (π/4)i
±Inf + NaNi Inf + NaNi
NaN + yi, for any finite y NaN + NaNi
NaN + Infi NaN + NaNi
NaN + NaNi NaN + NaNi

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


C11, POSIX 1003.1

Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes