CIMAG(3) Linux Programmer's Manual CIMAG(3)

cimag, cimagf, cimagl - get imaginary part of a complex number

#include <complex.h>
double cimag(double complex z);
float cimagf(float complex z);
long double cimagl(long double complex z);
Link with -lm.

These functions return the imaginary part of the complex number z.
One has:
    z = creal(z) + I * cimag(z)

These functions first appeared in glibc in version 2.1.

For an explanation of the terms used in this section, see attributes(7).
Interface Attribute Value
cimag (), cimagf (), cimagl () Thread safety MT-Safe

C99, POSIX.1-2001, POSIX.1-2008.

gcc also supports __imag__. That is a GNU extension.

cabs(3), creal(3), complex(7)