github.com/FenixAra/go@v0.0.0-20170127160404-96ea0918e670/src/runtime/cgo/gcc_darwin_386.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 <string.h> /* for strerror */
     6  #include <pthread.h>
     7  #include <signal.h>
     8  #include "libcgo.h"
     9  #include "libcgo_unix.h"
    10  
    11  static void* threadentry(void*);
    12  static pthread_key_t k1;
    13  
    14  #define magic1 (0x23581321U)
    15  
    16  static void
    17  inittls(void)
    18  {
    19  	uint32 x;
    20  	pthread_key_t tofree[128], k;
    21  	int i, ntofree;
    22  
    23  	/*
    24  	 * Allocate thread-local storage slot for g.
    25  	 * The key numbers start at 0x100, and we expect to be
    26  	 * one of the early calls to pthread_key_create, so we
    27  	 * should be able to get a pretty low number.
    28  	 *
    29  	 * In Darwin/386 pthreads, %gs points at the thread
    30  	 * structure, and each key is an index into the thread-local
    31  	 * storage array that begins at offset 0x48 within in that structure.
    32  	 * It may happen that we are not quite the first function to try
    33  	 * to allocate thread-local storage keys, so instead of depending
    34  	 * on getting 0x100, we try for 0x108, allocating keys until
    35  	 * we get the one we want and then freeing the ones we didn't want.
    36  	 *
    37  	 * Thus the final offset to use in %gs references is
    38  	 * 0x48+4*0x108 = 0x468.
    39  	 *
    40  	 * The linker and runtime hard-code this constant offset
    41  	 * from %gs where we expect to find g.
    42  	 * Known to ../../../liblink/sym.c:/468
    43  	 * and to ../sys_darwin_386.s:/468
    44  	 *
    45  	 * This is truly disgusting and a bit fragile, but taking care
    46  	 * of it here protects the rest of the system from damage.
    47  	 * The alternative would be to use a global variable that
    48  	 * held the offset and refer to that variable each time we
    49  	 * need a %gs variable (g).  That approach would
    50  	 * require an extra instruction and memory reference in
    51  	 * every stack growth prolog and would also require
    52  	 * rewriting the code that 8c generates for extern registers.
    53  	 *
    54  	 * Things get more disgusting on OS X 10.7 Lion.
    55  	 * The 0x48 base mentioned above is the offset of the tsd
    56  	 * array within the per-thread structure on Leopard and Snow Leopard.
    57  	 * On Lion, the base moved a little, so while the math above
    58  	 * still applies, the base is different.  Thus, we cannot
    59  	 * look for specific key values if we want to build binaries
    60  	 * that run on both systems.  Instead, forget about the
    61  	 * specific key values and just allocate and initialize per-thread
    62  	 * storage until we find a key that writes to the memory location
    63  	 * we want.  Then keep that key.
    64  	 */
    65  	ntofree = 0;
    66  	for(;;) {
    67  		if(pthread_key_create(&k, nil) < 0) {
    68  			fprintf(stderr, "runtime/cgo: pthread_key_create failed\n");
    69  			abort();
    70  		}
    71  		pthread_setspecific(k, (void*)magic1);
    72  		asm volatile("movl %%gs:0x468, %0" : "=r"(x));
    73  		pthread_setspecific(k, 0);
    74  		if(x == magic1) {
    75  			k1 = k;
    76  			break;
    77  		}
    78  		if(ntofree >= nelem(tofree)) {
    79  			fprintf(stderr, "runtime/cgo: could not obtain pthread_keys\n");
    80  			fprintf(stderr, "\ttried");
    81  			for(i=0; i<ntofree; i++)
    82  				fprintf(stderr, " %#x", (unsigned)tofree[i]);
    83  			fprintf(stderr, "\n");
    84  			abort();
    85  		}
    86  		tofree[ntofree++] = k;
    87  	}
    88  
    89  	/*
    90  	 * We got the key we wanted.  Free the others.
    91  	 */
    92  	for(i=0; i<ntofree; i++)
    93  		pthread_key_delete(tofree[i]);
    94  }
    95  
    96  void
    97  x_cgo_init(G *g)
    98  {
    99  	pthread_attr_t attr;
   100  	size_t size;
   101  
   102  	pthread_attr_init(&attr);
   103  	pthread_attr_getstacksize(&attr, &size);
   104  	g->stacklo = (uintptr)&attr - size + 4096;
   105  	pthread_attr_destroy(&attr);
   106  
   107  	inittls();
   108  }
   109  
   110  
   111  void
   112  _cgo_sys_thread_start(ThreadStart *ts)
   113  {
   114  	pthread_attr_t attr;
   115  	sigset_t ign, oset;
   116  	pthread_t p;
   117  	size_t size;
   118  	int err;
   119  
   120  	sigfillset(&ign);
   121  	pthread_sigmask(SIG_SETMASK, &ign, &oset);
   122  
   123  	pthread_attr_init(&attr);
   124  	pthread_attr_getstacksize(&attr, &size);
   125  	// Leave stacklo=0 and set stackhi=size; mstack will do the rest.
   126  	ts->g->stackhi = size;
   127  	err = _cgo_try_pthread_create(&p, &attr, threadentry, ts);
   128  
   129  	pthread_sigmask(SIG_SETMASK, &oset, nil);
   130  
   131  	if (err != 0) {
   132  		fprintf(stderr, "runtime/cgo: pthread_create failed: %s\n", strerror(err));
   133  		abort();
   134  	}
   135  }
   136  
   137  static void*
   138  threadentry(void *v)
   139  {
   140  	ThreadStart ts;
   141  
   142  	ts = *(ThreadStart*)v;
   143  	free(v);
   144  
   145  	pthread_setspecific(k1, (void*)ts.g);
   146  
   147  	crosscall_386(ts.fn);
   148  	return nil;
   149  }