< c‎ | numeric‎ | complex
Defined in header <complex.h>
#define _Imaginary_I /* unspecified */
(since C99)

The _Imaginary_I macro expands to a value of type const float _Imaginary with the value of the imaginary unit.

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)


This macro allows for the precise way to assemble a complex number from its real and imaginary components, e.g. with (double complex)((double)x + _Imaginary_I * (double)y). This pattern was standardized in C11 as the macro CMPLX. Note that if _Complex_I is used instead, this expression is allowed to convert negative zero to positive zero in the imaginary position.


#include <stdio.h>
#include <complex.h>
#include <math.h>
int main(void)
    double complex z1 = 0.0 + INFINITY * _Imaginary_I;
    printf("z1 = %.1f%+.1fi\n", creal(z1), cimag(z1));
    double complex z2 = 0.0 + INFINITY * _Complex_I;
    printf("z2 = %.1f%+.1fi\n", creal(z2), cimag(z2));


z1 = 0.0+Infi 
z2 = NaN+Infi


  • C11 standard (ISO/IEC 9899:2011):
  • 7.3.1/5 _Imaginary_I (p: 188)
  • G.6/1 _Imaginary_I (p: 537)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.3.1/3 _Imaginary_I (p: 170)
  • G.6/1 _Imaginary_I (p: 472)

See also

the complex unit constant i
(macro constant)
the complex or imaginary unit constant i
(macro constant)