src/runtime/cgo/gcc_util.c

// 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.

#include "libcgo.h"

/* Stub for creating a new thread */
void
x_cgo_thread_start(ThreadStart *arg)
{
	ThreadStart *ts;

	/* Make our own copy that can persist after we return. */
	_cgo_tsan_acquire();
	ts = malloc(sizeof *ts);
	_cgo_tsan_release();
	if(ts == nil) {
		fprintf(stderr, "runtime/cgo: out of memory in thread_start\n");
		abort();
	}
	*ts = *arg;

	_cgo_sys_thread_start(ts);	/* OS-dependent half */
}

#ifndef CGO_TSAN
void(* const _cgo_yield)() = NULL;
#else

#include <string.h>

char x_cgo_yield_strncpy_src = 0;
char x_cgo_yield_strncpy_dst = 0;
size_t x_cgo_yield_strncpy_n = 0;

/*
Stub for allowing libc interceptors to execute.

_cgo_yield is set to NULL if we do not expect libc interceptors to exist.
*/
static void
x_cgo_yield()
{
	/*
	The libc function(s) we call here must form a no-op and include at least one
	call that triggers TSAN to process pending asynchronous signals.

	sleep(0) would be fine, but it's not portable C (so it would need more header
	guards).
	free(NULL) has a fast-path special case in TSAN, so it doesn't
	trigger signal delivery.
	free(malloc(0)) would work (triggering the interceptors in malloc), but
	it also runs a bunch of user-supplied malloc hooks.

	So we choose strncpy(_, _, 0): it requires an extra header,
	but it's standard and should be very efficient.

	GCC 7 has an unfortunate habit of optimizing out strncpy calls (see
	https://golang.org/issue/21196), so the arguments here need to be global
	variables with external linkage in order to ensure that the call traps all the
	way down into libc.
	*/
	strncpy(&x_cgo_yield_strncpy_dst, &x_cgo_yield_strncpy_src,
	        x_cgo_yield_strncpy_n);
}

void(* const _cgo_yield)() = &x_cgo_yield;

#endif  /* GO_TSAN */