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      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 }
    150