---
title: Qiskit SDK C API documentation (v2.1)
description: Index of all the modules in qiskit-c v2.1.
source: https://eu-de.quantum.cloud.ibm.com/docs/en/api/qiskit-c/2.1
---

# Qiskit C API (`qiskit.h`)

The Qiskit C API is a low level interface to the core data model of Qiskit. It is designed to provide a high performance interface to Qiskit for compiled languages and provides a defined ABI to the internal Rust data model that is used to create the Python API. There are two expected modes of operation for the C API:

- A standalone shared library for creating and working with Qiskit objects from compiled languages without a runtime dependency on Python, and
- For building Python extensions that are using Qiskit but interface directly with the Rust objects from the extension code without using Python for better performance.

To get started, see [Install the Qiskit C API](/docs/guides/install-c-api). To combine the C API with custom Python extensions, see [Extend Qiskit in Python with C](/docs/guides/c-extension-for-python).

As this interface is still new in Qiskit it should be considered experimental and the interface might change between minor version releases.

## Quantum Circuit

- [QkCircuit](/docs/api/qiskit-c/2.1/qk-circuit)
- [QkQuantumRegister](/docs/api/qiskit-c/2.1/qk-quantum-register)
- [QkClassicalRegister](/docs/api/qiskit-c/2.1/qk-classical-register)

## Quantum information

- [QkObs](/docs/api/qiskit-c/2.1/qk-obs)
- [QkObsTerm](/docs/api/qiskit-c/2.1/qk-obs-term)
- [QkBitTerm](/docs/api/qiskit-c/2.1/qk-bit-term)
- [QkComplex64](/docs/api/qiskit-c/2.1/qk-complex-64)
- [QkExitCode](/docs/api/qiskit-c/2.1/qk-exit-code)
- [QkTarget](/docs/api/qiskit-c/2.1/qk-target)
- [QkTargetEntry](/docs/api/qiskit-c/2.1/qk-target-entry)


## Contents

- [API index](/docs/api/qiskit-c/2.1)
- [Release notes](/docs/api/qiskit/release-notes)
- [QkBitTerm](/docs/api/qiskit-c/2.1/qk-bit-term)
- [QkCircuit](/docs/api/qiskit-c/2.1/qk-circuit)
- [QkClassicalRegister](/docs/api/qiskit-c/2.1/qk-classical-register)
- [QkComplex64](/docs/api/qiskit-c/2.1/qk-complex-64)
- [QkExitCode](/docs/api/qiskit-c/2.1/qk-exit-code)
- [QkObs](/docs/api/qiskit-c/2.1/qk-obs)
- [QkObsTerm](/docs/api/qiskit-c/2.1/qk-obs-term)
- [QkQuantumRegister](/docs/api/qiskit-c/2.1/qk-quantum-register)
- [QkTarget](/docs/api/qiskit-c/2.1/qk-target)
- [QkTargetEntry](/docs/api/qiskit-c/2.1/qk-target-entry)