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4.1 C Type Interoperability

Basic types map directly: an int in Go is an int in C, etc. Go byte is equivalent to C unsigned char. Pointers in Go are pointers in C. A Go struct is the same as C struct with the same field names and types.

The Go string type is currently defined as a two-element structure:

struct __go_string {
  const unsigned char *__data;
  int __length;

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 *[10]int is equivalent to C int*, assuming that the C pointer does point to 10 elements.

A slice in Go is a structure. The current definition is:

struct __go_slice {
  void *__values;
  int __count;
  int __capacity;

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:

func GoFunction(int) (int, float)
struct { int i; float f; } CFunction(int)

A pointer to a Go function is equivalent to a pointer to a C function when the functions have equivalent types.

Go interface, channel, and map types have no corresponding C type (interface is a two-element struct and channel and map are pointers to structs in C, but the structs are deliberately undocumented). C enum types correspond to some integer type, but precisely which one is difficult to predict in general; use a cast. C union types have no corresponding Go type. C struct types containing bitfields have no corresponding Go type. C++ class types have no corresponding Go type.

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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