github.com/razvanm/vanadium-go-1.3@v0.0.0-20160721203343-4a65068e5915/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  
    10  static void* threadentry(void*);
    11  static pthread_key_t k1;
    12  
    13  #define magic1 (0x23581321U)
    14  
    15  static void
    16  inittls(void)
    17  {
    18  	uint32 x;
    19  	pthread_key_t tofree[128], k;
    20  	int i, ntofree;
    21  
    22  	/*
    23  	 * Allocate thread-local storage slot for g.
    24  	 * The key numbers start at 0x100, and we expect to be
    25  	 * one of the early calls to pthread_key_create, so we
    26  	 * should be able to get a pretty low number.
    27  	 *
    28  	 * In Darwin/386 pthreads, %gs points at the thread
    29  	 * structure, and each key is an index into the thread-local
    30  	 * storage array that begins at offset 0x48 within in that structure.
    31  	 * It may happen that we are not quite the first function to try
    32  	 * to allocate thread-local storage keys, so instead of depending
    33  	 * on getting 0x100, we try for 0x108, allocating keys until
    34  	 * we get the one we want and then freeing the ones we didn't want.
    35  	 *
    36  	 * Thus the final offset to use in %gs references is
    37  	 * 0x48+4*0x108 = 0x468.
    38  	 *
    39  	 * The linker and runtime hard-code this constant offset
    40  	 * from %gs where we expect to find g.
    41  	 * Known to ../../../liblink/sym.c:/468
    42  	 * and to ../sys_darwin_386.s:/468
    43  	 *
    44  	 * This is truly disgusting and a bit fragile, but taking care
    45  	 * of it here protects the rest of the system from damage.
    46  	 * The alternative would be to use a global variable that
    47  	 * held the offset and refer to that variable each time we
    48  	 * need a %gs variable (g).  That approach would
    49  	 * require an extra instruction and memory reference in
    50  	 * every stack growth prolog and would also require
    51  	 * rewriting the code that 8c generates for extern registers.
    52  	 *
    53  	 * Things get more disgusting on OS X 10.7 Lion.
    54  	 * The 0x48 base mentioned above is the offset of the tsd
    55  	 * array within the per-thread structure on Leopard and Snow Leopard.
    56  	 * On Lion, the base moved a little, so while the math above
    57  	 * still applies, the base is different.  Thus, we cannot
    58  	 * look for specific key values if we want to build binaries
    59  	 * that run on both systems.  Instead, forget about the
    60  	 * specific key values and just allocate and initialize per-thread
    61  	 * storage until we find a key that writes to the memory location
    62  	 * we want.  Then keep that key.
    63  	 */
    64  	ntofree = 0;
    65  	for(;;) {
    66  		if(pthread_key_create(&k, nil) < 0) {
    67  			fprintf(stderr, "runtime/cgo: pthread_key_create failed\n");
    68  			abort();
    69  		}
    70  		pthread_setspecific(k, (void*)magic1);
    71  		asm volatile("movl %%gs:0x468, %0" : "=r"(x));
    72  		pthread_setspecific(k, 0);
    73  		if(x == magic1) {
    74  			k1 = k;
    75  			break;
    76  		}
    77  		if(ntofree >= nelem(tofree)) {
    78  			fprintf(stderr, "runtime/cgo: could not obtain pthread_keys\n");
    79  			fprintf(stderr, "\ttried");
    80  			for(i=0; i<ntofree; i++)
    81  				fprintf(stderr, " %#x", (unsigned)tofree[i]);
    82  			fprintf(stderr, "\n");
    83  			abort();
    84  		}
    85  		tofree[ntofree++] = k;
    86  	}
    87  
    88  	/*
    89  	 * We got the key we wanted.  Free the others.
    90  	 */
    91  	for(i=0; i<ntofree; i++)
    92  		pthread_key_delete(tofree[i]);
    93  }
    94  
    95  void
    96  x_cgo_init(G *g)
    97  {
    98  	pthread_attr_t attr;
    99  	size_t size;
   100  
   101  	pthread_attr_init(&attr);
   102  	pthread_attr_getstacksize(&attr, &size);
   103  	g->stacklo = (uintptr)&attr - size + 4096;
   104  	pthread_attr_destroy(&attr);
   105  
   106  	inittls();
   107  }
   108  
   109  
   110  void
   111  _cgo_sys_thread_start(ThreadStart *ts)
   112  {
   113  	pthread_attr_t attr;
   114  	sigset_t ign, oset;
   115  	pthread_t p;
   116  	size_t size;
   117  	int err;
   118  
   119  	sigfillset(&ign);
   120  	pthread_sigmask(SIG_SETMASK, &ign, &oset);
   121  
   122  	pthread_attr_init(&attr);
   123  	pthread_attr_getstacksize(&attr, &size);
   124  	// Leave stacklo=0 and set stackhi=size; mstack will do the rest.
   125  	ts->g->stackhi = size;
   126  	err = pthread_create(&p, &attr, threadentry, ts);
   127  
   128  	pthread_sigmask(SIG_SETMASK, &oset, nil);
   129  
   130  	if (err != 0) {
   131  		fprintf(stderr, "runtime/cgo: pthread_create failed: %s\n", strerror(err));
   132  		abort();
   133  	}
   134  }
   135  
   136  static void*
   137  threadentry(void *v)
   138  {
   139  	ThreadStart ts;
   140  
   141  	ts = *(ThreadStart*)v;
   142  	free(v);
   143  
   144  	pthread_setspecific(k1, (void*)ts.g);
   145  
   146  	crosscall_386(ts.fn);
   147  	return nil;
   148  }