Type-generic math

The header <tgmath.h> includes the headers <math.h> and <complex.h> and defines several type-generic macros that determine which real or, when applicable, complex function to call based on the types of the arguments.

For each macro, the parameters whose corresponding real type in the unsuffixed math.h function is double are known as generic parameters (for example, both parameters of pow are generic parameters, but only the first parameter of scalbn is a generic parameter).

When a <tgmath.h> macro is used the types of the arguments passed to the generic parameters determine which function is selected by the macro as described below. If the types of the arguments are not compatible with the parameter types of the selected function, the behavior is undefined (e.g. if a complex argument is passed into a real-only tgmath macro: float complex fc; ceil(fc) or double complex dc; double d; fmax(dc, d) are examples of undefined behavior).

Note: type-generic macros were implemented in implementation-defined manner in C99, but C11 keyword _Generic makes it possible to implement these macros in portable manner.

Complex/real type-generic macros

For all functions that have both real and complex counterparts, a type-generic macro XXX exists, which calls either of:

• real function:
  • float variant XXXf
  • double variant XXX
  • long double variant XXXl
• complex function:
  • float variant cXXXf
  • double variant cXXX
  • long double variant cXXXl

An exception to the above rule is the fabs macro (see the table below).

The function to call is determined as follows:

• If any of the arguments for the generic parameters is imaginary, the behavior is specified on each function reference page individually (in particular, sin, cos, tag, cosh, sinh, tanh, asin, atan, asinh, and atanh call real functions, the return types of sin, tan, sinh, tanh, asin, atan, asinh, and atanh are imaginary, and the return types of cos and cosh are real)
• If any of the arguments for the generic parameters is complex, then the complex function is called, otherwise the real function is called.
• If any of the arguments for the generic parameters is long double, then the long double variant is called. Otherwise, if any of the parameters is double or integer, then the double variant is called. Otherwise, float variant is called.

The type-generic macros are as follows:

Real-only functions

For all functions that do not have complex counterparts, with the exception of modf, a type-generic macro XXX exists, which calls either of the variants of a real function:

• float variant XXXf
• double variant XXX
• long double variant XXXl

The function to call is determined as follows:

• If any of the arguments for the generic parameters is long double, then the long double variant is called. Otherwise, if any of the arguments for the generic parameters is double, then the double variant is called. Otherwise, float variant is called.

Complex-only functions

For all complex number functions that do not have real counterparts, a type-generic macro cXXX exists, which calls either of the variants of a complex function:

• float complex variant cXXXf
• double complex variant cXXX
• long double complex variant cXXXl

The function to call is determined as follows:

• If any of the arguments for the generic parameters is real, complex, or imaginary, then the appropriate complex function is called.

Example

#include <stdio.h>
#include <tgmath.h>
 
int main(void)
{
    int i = 2;
    printf("sqrt(2) = %f\n", sqrt(i)); // argument type is int, calls sqrt
 
    float f = 0.5;
    printf("sin(0.5f) = %f\n", sin(f));   // argument type is float, calls sinf
 
    float complex dc = 1 + 0.5*I;
    float complex z = sqrt(dc);      // argument type is float complex, calls csqrtf
    printf("sqrt(1 + 0.5i) = %f+%fi\n",
           creal(z),  // argument type is float complex, calls crealf
           cimag(z)); // argument type is float complex, calls cimagf
}

sqrt(2) = 1.414214
sin(0.5f) = 0.479426
sqrt(1 + 0.5i) = 1.029086+0.242934i

References

• C11 standard (ISO/IEC 9899:2011):
  • 7.25 Type-generic math <tgmath.h> (p: 373-375)
• C99 standard (ISO/IEC 9899:1999):
  • 7.22 Type-generic math <tgmath.h> (p: 335-337)