onnxruntime/docs/C_API.md

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# C API
## Features
* Creating an InferenceSession from an on-disk model file and a set of SessionOptions.
* Registering customized loggers.
* Registering customized allocators.
* Registering predefined providers and set the priority order. ONNXRuntime has a set of predefined execution providers, like CUDA, MKLDNN. User can register providers to their InferenceSession. The order of registration indicates the preference order as well.
* Running a model with inputs. These inputs must be in CPU memory, not GPU. If the model has multiple outputs, user can specify which outputs they want.
* Converting an in-memory ONNX Tensor encoded in protobuf format to a pointer that can be used as model input.
* Setting the thread pool size for each session.
* Setting graph optimization level for each session.
* Dynamically loading custom ops. [Instructions](/docs/AddingCustomOp.md)
## Usage Overview
1. Include [onnxruntime_c_api.h](/include/onnxruntime/core/session/onnxruntime_c_api.h).
2. Call OrtCreateEnv
3. Create Session: OrtCreateSession(env, model_uri, nullptr,...)
- Optionally add more execution providers (e.g. for CUDA use OrtSessionOptionsAppendExecutionProvider_CUDA)
4. Create Tensor
1) OrtCreateAllocatorInfo
2) OrtCreateTensorWithDataAsOrtValue
5. OrtRun
## Sample code
The example below shows a sample run using the SqueezeNet model from ONNX model zoo, including dynamically reading model inputs, outputs, shape and type information, as well as running a sample vector and fetching the resulting class probabilities for inspection.
* [../csharp/test/Microsoft.ML.OnnxRuntime.EndToEndTests.Capi/C_Api_Sample.cpp](../csharp/test/Microsoft.ML.OnnxRuntime.EndToEndTests.Capi/C_Api_Sample.cpp)