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<h3 class="section">3.1 C Type Interoperability</h3>
<p>Basic types map directly: an <code>int</code> in Go is an <code>int</code> in C,
etc. Go <code>byte</code> is equivalent to C <code>unsigned char</code>.
Pointers in Go are pointers in C. A Go <code>struct</code> is the same as C
<code>struct</code> with the same field names and types.
<p><a name="index-g_t_0040code_007bstring_007d-in-C-8"></a>The Go <code>string</code> type is currently defined as a two-element
structure:
<pre class="smallexample"> struct __go_string {
const unsigned char *__data;
int __length;
};
</pre>
<p>You can't pass arrays between C and Go. However, a pointer to an
array in Go is equivalent to a C pointer to the equivalent of the
element type. For example, Go <code>*[10]int</code> is equivalent to C
<code>int*</code>, assuming that the C pointer does point to 10 elements.
<p><a name="index-g_t_0040code_007bslice_007d-in-C-9"></a>A slice in Go is a structure. The current definition is:
<pre class="smallexample"> struct __go_slice {
void *__values;
int __count;
int __capacity;
};
</pre>
<p>The type of a Go function with no receiver is equivalent to a C
function whose parameter types are equivalent. When a Go function
returns more than one value, the C function returns a struct. For
example, these functions have equivalent types:
<pre class="smallexample"> func GoFunction(int) (int, float)
struct { int i; float f; } CFunction(int)
</pre>
<p>A pointer to a Go function is equivalent to a pointer to a C function
when the functions have equivalent types.
<p>Go <code>interface</code>, <code>channel</code>, and <code>map</code> types have no
corresponding C type (<code>interface</code> is a two-element struct and
<code>channel</code> and <code>map</code> are pointers to structs in C, but the
structs are deliberately undocumented). C <code>enum</code> types
correspond to some integer type, but precisely which one is difficult
to predict in general; use a cast. C <code>union</code> types have no
corresponding Go type. C <code>struct</code> types containing bitfields
have no corresponding Go type. C++ <code>class</code> types have no
corresponding Go type.
<p>Memory allocation is completely different between C and Go, as Go uses
garbage collection. The exact guidelines in this area are
undetermined, but it is likely that it will be permitted to pass a
pointer to allocated memory from C to Go. The responsibility of
eventually freeing the pointer will remain with C side, and of course
if the C side frees the pointer while the Go side still has a copy the
program will fail. When passing a pointer from Go to C, the Go
function must retain a visible copy of it in some Go variable.
Otherwise the Go garbage collector may delete the pointer while the C
function is still using it.
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