runtime/pprof: document SetCPUProfile with c-archive/c-shared

When using c-archive/c-shared, the signal handler for SIGPROF will not
be installed, which means that runtime/pprof.StartCPUProfile won't work.
There is no really good solution here, as the main program may want to
do its own profiling.  For now, just document that runtime/pprof doesn't
work as expected, but that it will work if you use Notify to install the
Go signal handler.

Fixes #14043.

Change-Id: I7ff7a01df6ef7f63a7f050aac3674d640a246fb4
Reviewed-on: https://go-review.googlesource.com/18911
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/pprof/pprof.go

@@ -567,6 +567,14 @@ var cpu struct {
 // StartCPUProfile enables CPU profiling for the current process.
 // While profiling, the profile will be buffered and written to w.
 // StartCPUProfile returns an error if profiling is already enabled.
+//
+// On Unix-like systems, StartCPUProfile does not work by default for
+// Go code built with -buildmode=c-archive or -buildmode=c-shared.
+// StartCPUProfile relies on the SIGPROF signal, but that signal will
+// be delivered to the main program's SIGPROF signal handler (if any)
+// not to the one used by Go.  To make it work, call os/signal.Notify
+// for syscall.SIGPROF, but note that doing so may break any profiling
+// being done by the main program.
 func StartCPUProfile(w io.Writer) error {
 	// The runtime routines allow a variable profiling rate,
 	// but in practice operating systems cannot trigger signals