-Cgo lets Go packages call C code. Given a Go source file written with some special features, cgo outputs Go and C files that can be combined into a single Go package.

-To lead with an example, here's a Go package that provides two functions - `Random` and `Seed` - that wrap C's `random` and `srandom` functions.

-The `rand` package imports `"C"`, but you'll find there's no such package in the standard Go library. That's because `C` is a "pseudo-package", a special name interpreted by cgo as a reference to C's name space.

-The `rand` package contains four references to the `C` package: the calls to `C.random` and `C.srandom`, the conversion `C.uint(i)`, and the `import` statement.

-The `Random` function calls the standard C library's `random` function and returns the result. In C, `random` returns a value of the C type `long`, which cgo represents as the type `C.long`. It must be converted to a Go type before it can be used by Go code outside this package, using an ordinary Go type conversion:

-The `Seed` function does the reverse, in a way. It takes a regular Go `int`, converts it to the C `unsigned`int` type, and passes it to the C function `srandom`.

-Note that cgo knows the `unsigned`int` type as `C.uint`; see the [[https://golang.org/cmd/cgo][cgo documentation]] for a complete list of these numeric type names.

-Cgo recognizes this comment. Any lines starting with `#cgo` followed by a space character are removed; these become directives for cgo. The remaining lines are used as a header when compiling the C parts of the package. In this case those lines are just a single `#include` statement, but they can be almost any C code. The `#cgo` directives are used to provide flags for the compiler and linker when building the C parts of the package.

-There is a limitation: if your program uses any `//export` directives, then the C code in the comment may only include declarations (`extern`int`f();`), not definitions (`int`f()`{`return`1;`}`). You can use `//export` directives to make Go functions accessible to C code.

-Conversion between Go and C strings is done with the `C.CString`, `C.GoString`, and `C.GoStringN` functions. These conversions make a copy of the string data.

-This next example implements a `Print` function that writes a string to standard output using C's `fputs` function from the `stdio` library:

-Memory allocations made by C code are not known to Go's memory manager. When you create a C string with `C.CString` (or any C memory allocation) you must remember to free the memory when you're done with it by calling `C.free`.

-The call to `C.CString` returns a pointer to the start of the char array, so before the function exits we convert it to an [[https://golang.org/pkg/unsafe/#Pointer][`unsafe.Pointer`]] and release the memory allocation with `C.free`. A common idiom in cgo programs is to [[https://golang.org/doc/articles/defer_panic_recover.html][`defer`]] the free immediately after allocating (especially when the code that follows is more complex than a single function call), as in this rewrite of `Print`:

-To build cgo packages, just use [[https://golang.org/cmd/go/#Compile_packages_and_dependencies][`go`build`]] or [[https://golang.org/cmd/go/#Compile_and_install_packages_and_dependencies][`go`install`]] as usual. The go tool recognizes the special `"C"` import and automatically uses cgo for those files.

-The [[https://golang.org/cmd/cgo/][cgo command]] documentation has more detail about the C pseudo-package and the build process. The [[https://golang.org/misc/cgo/][cgo examples]] in the Go tree demonstrate more advanced concepts.

-Finally, if you're curious as to how all this works internally, take a look at the introductory comment of the runtime package's [[https://golang.org/src/runtime/cgocall.go][cgocall.go]].