code review 4815063: cgo: add GoBytes, fix gmp example

src/cmd/cgo/doc.go

 // Copyright 2009 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 /*
 
 Cgo enables the creation of Go packages that call C code.
 
 Usage: cgo [compiler options] file.go
 
@@ skipping 34 matching lines @@
 
 Within the Go file, C identifiers or field names that are keywords in Go
 can be accessed by prefixing them with an underscore: if x points at a C
 struct with a field named "type", x._type accesses the field.
 
 The standard C numeric types are available under the names
 C.char, C.schar (signed char), C.uchar (unsigned char),
 C.short, C.ushort (unsigned short), C.int, C.uint (unsigned int),
 C.long, C.ulong (unsigned long), C.longlong (long long),
 C.ulonglong (unsigned long long), C.float, C.double.
+The C type void* is represented by Go's unsafe.Pointer.
 
 To access a struct, union, or enum type directly, prefix it with
 struct_, union_, or enum_, as in C.struct_stat.
 
 Any C function that returns a value may be called in a multiple
 assignment context to retrieve both the return value and the
 C errno variable as an os.Error.  For example:
 
         n, err := C.atoi("abc")
 
 In C, a function argument written as a fixed size array
 actually requires a pointer to the first element of the array.
 C compilers are aware of this calling convention and adjust
 the call accordingly, but Go cannot.  In Go, you must pass
 the pointer to the first element explicitly: C.f(&x[0]).
 
+A few special functions convert between Go and C types
+by making copies of the data.  In pseudo-Go definitions:
+
+        // Go string to C string
+        func C.CString(string) *C.char
+
+        // C string to Go string
+        func C.GoString(*C.char) string
+
+        // C string, length to Go string
+        func C.GoStringN(*C.char, C.int) string
+
+        // C pointer, length to Go []byte
+        func C.GoBytes(unsafe.Pointer, C.int) []byte
+
 Cgo transforms the input file into four output files: two Go source
 files, a C file for 6c (or 8c or 5c), and a C file for gcc.
 
 The standard package makefile rules in Make.pkg automate the
 process of using cgo.  See $GOROOT/misc/cgo/stdio and
 $GOROOT/misc/cgo/gmp for examples.
 
 Cgo does not yet work with gccgo.
 */
 package documentation