This paper describes a fixed-point arithmetic implementation of linear time-invariant digital filters in the C programming language, targeted for the use in real-time applications. FIR and IIR filters are presented in different configurations and representations, such as direct and transposed forms, as well as wave, state-space and normal forms. All these forms provide a first, second and $N^{th}$ order tick or update function, and they may also be arranged in cascades. Scaling of parameters and input values is adressed as well as quantization of filtered output values with optional noise shaping. The implementation is independent of filter types (low pass, high pass, etc.) and filter families (Butterworth, Bessel, etc.). All filters are implemented in a dynamic linkable library, both real-time safe for the GNU/Linux user space and the Linux kernel.

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