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Text file src/runtime/cgo/gcc_darwin_386.c

Documentation: runtime/cgo

     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; mstart 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	}

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