doc/articles/c_go_cgo.html

<!--{
"Title": "C? Go? Cgo!",
"Template": true
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<p>
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.
</p>

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

{{code "/doc/progs/cgo1.go" `/package rand/` `/END/`}}

<p>
Let’s look at what's happening here, starting with the import statement.
</p>

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

<p>
The rand package contains four references to the <code>C</code> package:
the calls to <code>C.random</code> and <code>C.srandom</code>, the
conversion <code>C.uint(i)</code>, and the import statement.
</p>

<p>
The <code>Random</code> function calls the libc random function and returns
the result.  In C, random returns a value of the C type <code>long</code>,
which cgo represents as the type <code>C.long</code>. 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:
</p>

{{code "/doc/progs/cgo1.go" `/func Random/` `/STOP/`}}

<p>
Here’s an equivalent function that uses a temporary variable to illustrate
the type conversion more explicitly:
</p>

{{code "/doc/progs/cgo2.go" `/func Random/` `/STOP/`}}

<p>
The <code>Seed</code> function does the reverse, in a way. It takes a
regular Go <code>int</code>, converts it to the C <code>unsigned int</code>
type, and passes it to the C function srandom.
</p>

{{code "/doc/progs/cgo1.go" `/func Seed/` `/END/`}}

<p>
Note that cgo knows the unsigned int type as C.uint; see the
<a href="/cmd/cgo">cgo documentation</a> for a complete list of these
numeric type names.
</p>

<p>
The one detail of this example we haven't examined yet is the comment
above the import statement.
</p>

{{code "/doc/progs/cgo1.go" `/INCLUDE/` `/STOP/`}}

<p>
Cgo recognizes this comment and uses it as a header when compiling the C
parts of the package. In this case it is just a simple include statement,
but it can be any valid C code. The comment must be immediately before the
line that imports "C", without any intervening blank lines, just like a
documentation comment.
</p>

<p>
<b>Strings and things</b>
</p>

<p>
Unlike Go, C doesn’t have an explicit string type. Strings in C are
represented by a zero-terminated array of chars.
</p>

<p>
Conversion between Go and C strings is done with the
<code>C.CString</code>, <code>C.GoString</code>, and
<code>C.GoStringN</code> functions. These conversions make a copy of the
string data.
</p>

<p>
This next example implements a <code>Print</code> function that writes a
string to standard output using C's <code>fputs</code> function from the
<code>stdio</code> library:
</p>

{{code "/doc/progs/cgo3.go" `/package print/` `/END/`}}

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

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

{{code "/doc/progs/cgo4.go" `/func Print/` `/END/`}}

<p>
<b>Building cgo packages</b>
</p>

<p>
To build cgo packages, just use <a href="/cmd/go/#Compile_packages_and_dependencies">"go build"</a> or
<a href="/cmd/go/#Compile_and_install_packages_and_dependencies">"go install"</a>
as usual. The go tool recognizes the special "C" import and automatically uses
cgo for those files.
</p>

<p>
<b>More cgo resources</b>
</p>

<p>
The <a href="/cmd/cgo/">cgo command</a> documentation has more detail about
the C pseudo-package and the build process. The cgo examples in the Go tree
demonstrate more advanced concepts.
</p>

<p>
For a simple, idiomatic example of a cgo-based package, see Russ Cox’s <a
href="http://code.google.com/p/gosqlite/source/browse/sqlite/sqlite.go">gosqlite</a>.
Also, the Go Project Dashboard lists <a
href="https://godashboard.appspot.com/project?tag=cgo">several other
cgo packages</a>.
</p>

<p>
Finally, if you’re curious as to how all this works internally, take a look
at the introductory comment of the runtime package’s <a href="/src/pkg/runtime/cgocall.c">cgocall.c</a>.
</p>