Text file src/pkg/runtime/cgo/gcc_util.c

     1	// Copyright 2009 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	#include "libcgo.h"
     6	
     7	/* Stub for creating a new thread */
     8	void
     9	x_cgo_thread_start(ThreadStart *arg)
    10	{
    11		ThreadStart *ts;
    12	
    13		/* Make our own copy that can persist after we return. */
    14		_cgo_tsan_acquire();
    15		ts = malloc(sizeof *ts);
    16		_cgo_tsan_release();
    17		if(ts == nil) {
    18			fprintf(stderr, "runtime/cgo: out of memory in thread_start\n");
    19			abort();
    20		}
    21		*ts = *arg;
    22	
    23		_cgo_sys_thread_start(ts);	/* OS-dependent half */
    24	}
    25	
    26	#ifndef CGO_TSAN
    27	void(* const _cgo_yield)() = NULL;
    28	#else
    29	
    30	#include <string.h>
    31	
    32	char x_cgo_yield_strncpy_src = 0;
    33	char x_cgo_yield_strncpy_dst = 0;
    34	size_t x_cgo_yield_strncpy_n = 0;
    35	
    36	/*
    37	Stub for allowing libc interceptors to execute.
    38	
    39	_cgo_yield is set to NULL if we do not expect libc interceptors to exist.
    40	*/
    41	static void
    42	x_cgo_yield()
    43	{
    44		/*
    45		The libc function(s) we call here must form a no-op and include at least one
    46		call that triggers TSAN to process pending asynchronous signals.
    47	
    48		sleep(0) would be fine, but it's not portable C (so it would need more header
    49		guards).
    50		free(NULL) has a fast-path special case in TSAN, so it doesn't
    51		trigger signal delivery.
    52		free(malloc(0)) would work (triggering the interceptors in malloc), but
    53		it also runs a bunch of user-supplied malloc hooks.
    54	
    55		So we choose strncpy(_, _, 0): it requires an extra header,
    56		but it's standard and should be very efficient.
    57	
    58		GCC 7 has an unfortunate habit of optimizing out strncpy calls (see
    59		https://golang.org/issue/21196), so the arguments here need to be global
    60		variables with external linkage in order to ensure that the call traps all the
    61		way down into libc.
    62		*/
    63		strncpy(&x_cgo_yield_strncpy_dst, &x_cgo_yield_strncpy_src,
    64		        x_cgo_yield_strncpy_n);
    65	}
    66	
    67	void(* const _cgo_yield)() = &x_cgo_yield;
    68	
    69	#endif  /* GO_TSAN */

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