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 <code>rand</code> package imports <code>"C"</code>, 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 <code>rand</code> 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 <code>import</code> statement.
</p>

<p>
The <code>Random</code> function calls the standard C library's <code>random</code>
function and returns the result.  In C, <code>random</code> 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 <code>srandom</code>.
</p>

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

<p>
Note that cgo knows the <code>unsigned int</code> type as <code>C.uint</code>;
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 <code>import</code> statement.
</p>

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

<p>
Cgo recognizes this comment.  Any lines starting
with <code>#cgo</code>
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 <code>#include</code>
statement, but they can be almost any C code.  The <code>#cgo</code>
directives are
used to provide flags for the compiler and linker when building the C
parts of the package.
</p>

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

<p>
The <code>#cgo</code> and <code>//export</code> directives are
documented in
the <a href="/cmd/cgo/">cgo documentation</a>.
</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"><code>unsafe.Pointer</code></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"><code>defer</code></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">"
<code>go build</code>"</a> or
<a href="/cmd/go/#Compile_and_install_packages_and_dependencies">"<code>go install</code>
"</a> as usual. The go tool recognizes the special <code>"C"</code> 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 <a href="/misc/cgo/">cgo examples</a>
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>