GNU C supports complex data types. You can declare both complex integer types and complex floating types, using the keyword __complex__.
For example, “__complex__ double x;” declares x as a variable whose real part and imaginary part are both of type double. “__complex__ short int y;” declares y to have real and imaginary parts of type short int; this is not likely to be useful, but it shows that the set of complex types is complete.
To write a constant with a complex data type, use the suffix “i” or “j” (either one; they are equivalent). For example, 2.5fi has type __complex__ float and 3i has type __complex__ int. Such a constant always has a pure imaginary value, but you can form any complex value you like by adding one to a real constant.
To extract the real part of a complex-valued expression exp, write __real__ exp. Likewise, use __imag__ to extract the imaginary part.
The operator “~” performs complex conjugation when used on a value with a complex type.
The compiler can allocate complex automatic variables in a noncontiguous fashion; it's even possible for the real part to be in a register while the imaginary part is on the stack (or vice-versa). None of the supported debugging info formats has a way to represent noncontiguous allocation like this, so the compiler describes a noncontiguous complex variable as if it were two separate variables of noncomplex type. If the variable's actual name is foo, the two fictitious variables are named foo$real and foo$imag. You can examine and set these two fictitious variables with your debugger.
A future version of the debugger will know how to recognize such pairs and treat them as a single variable with a complex type.