github.com/hlts2/go@v0.0.0-20170904000733-812b34efaed8/src/runtime/cgo/gcc_util.c (about)

     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 */