initial readme

README.md

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+# sbffi
+
+A super-quick [FFI](https://en.wikipedia.org/wiki/Foreign_function_interface) for Node.js.
+
+## Usage
+
+**`sbffi.getNativeFunction(pathToSharedLibrary, functionName, returnType, [argType1, argType2, ...])`**
+
+All the arguments are strings. The types must be standard C types. When functions take 64-bit types, the parameters must be passed as BigInts. 64-bit return values will also be BigInts.
+
+```
+// some C library compiled to libadder.so
+
+uint32_t add(uint32_t a, uint32_t b) {
+  return a + b;
+}
+```
+
+```
+// index.js
+
+const { getNativeFunction } = require('sbffi');
+
+const libPath = '/path/to/libadder.so';
+const add = getNativeFunction(libPath, 'add', 'uint32_t', ['uint32_t', 'uint32_t']);
+
+const result = add(23, 34);
+// 57
+```
+
+### Pointers
+
+Pointers are currently assumed to be 64-bit, and can be passed to native functions by specifying the type as `uint64_t`. (Note: this will change in the future!)
+
+You can put raw data into a Buffer, and then get a pointer to the start of that buffer with:
+
+**`const bufferPointer = sbffi.getBufferPointer(buffer);`**
+
+Arrays and strings must be passed as pointers.
+
+## Installation
+
+Installation requires that [`cmake`](https://cmake.org/) is installed. Other than that, this should install as an npm module without issue.