Defined in header <complex.h>
#define imaginary _Imaginary
(since C99)

This macro expands to the keyword _Imaginary.

This is a convenience macro that makes it possible to use float imaginary, double imaginary, and long double imaginary as an alternative way to write the three pure imaginary C types float _Imaginary, double _Imaginary, and long double _Imaginary.

As with any pure imaginary number support in C, this macro is only defined if the imaginary numbers are supported.

A compiler that defines __STDC_IEC_559_COMPLEX__ is not required to support imaginary numbers. POSIX recommends checking if the macro _Imaginary_I is defined to identify imaginary number support.

(since C99)
(until C11)

Imaginary numbers are supported if __STDC_IEC_559_COMPLEX__ is defined.

(since C11)


Programs are allowed to undefine and perhaps redefine the imaginary macro.


#include <stdio.h>
#include <complex.h>
int main(void)
    double imaginary i = -2.0*I; // pure imaginary
    double f = 1.0; // pure real
    double complex z = f + i; // complex number
    printf("z = %.1f%+.1fi\n", creal(z), cimag(z));


z = 1.0-2.0i


  • C17 standard (ISO/IEC 9899:2018):
    • 7.3.1/5 imaginary (p: 136)
    • G.6/1 imaginary (p: 391-392)
  • C11 standard (ISO/IEC 9899:2011):
    • 7.3.1/5 imaginary (p: 188)
    • G.6/1 imaginary (p: 537)
  • C99 standard (ISO/IEC 9899:1999):
    • 7.3.1/3 imaginary (p: 170)
    • G.6/1 imaginary (p: 472)

See also

complex type macro
(keyword macro)

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