cmd/cgo: doc updates

1. note that to use C.free <stdlib.h> must be included
2. can also extract errno from a void C function

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/6935045

diff --git a/src/cmd/cgo/doc.go b/src/cmd/cgo/doc.go
index 1bb48f44e9735c8c10abae27aa060b3ad997fc68..6f3343de54ddab28216b8a2994abb8a859aabf4b 100644 (file)
--- a/src/cmd/cgo/doc.go
+++ b/src/cmd/cgo/doc.go
@@ -65,11 +65,13 @@ struct_, union_, or enum_, as in C.struct_stat.
 
 Go structs cannot embed fields with C types.
 
-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 error.  For example:
+Any C function (even void functions) may be called in a multiple
+assignment context to retrieve both the return value (if any) and the
+C errno variable as an error (use _ to skip the result value if the
+function returns void).  For example:
 
        n, err := C.atoi("abc")
+       _, err := C.voidFunc()
 
 In C, a function argument written as a fixed size array
 actually requires a pointer to the first element of the array.
@@ -83,7 +85,8 @@ by making copies of the data.  In pseudo-Go definitions:
        // Go string to C string
        // The C string is allocated in the C heap using malloc.
        // It is the caller's responsibility to arrange for it to be
-       // freed, such as by calling C.free.
+       // freed, such as by calling C.free (be sure to include stdlib.h
+       // if C.free is needed).
        func C.CString(string) *C.char
 
        // C string to Go string