diff --git a/src/cmd/cgo/doc.go b/src/cmd/cgo/doc.go
index e6f9173aaf8..e5d288167ec 100644
--- a/src/cmd/cgo/doc.go
+++ b/src/cmd/cgo/doc.go
@@ -216,6 +216,13 @@ by making copies of the data.  In pseudo-Go definitions:
 	// C data with explicit length to Go []byte
 	func C.GoBytes(unsafe.Pointer, C.int) []byte
 
+As a special case, C.malloc does not call the C library malloc directly
+but instead calls a Go helper function that wraps the C library malloc
+but guarantees never to return nil. If C's malloc indicates out of memory,
+the helper function crashes the program, like when Go itself runs out
+of memory. Because C.malloc cannot fail, it has no two-result form
+that returns errno.
+
 C references to Go
 
 Go functions can be exported for use by C code in the following way: