以下程序(内联解释)显示了我的问题的本质：

#include <stdio.h>
#include <sys/mman.h>
#include <assert.h>
#include <signal.h>

#define HEAP_SIZE 4096

unsigned long int *heap;

void gc(int n) {
  signal(SIGBUS,SIG_DFL); // just for debugging
  printf("GC TIME\n");
}

int main () {

  // Allocate twice the required heap size (two semi-spaces)
  heap = mmap(NULL,HEAP_SIZE * 2,PROT_READ | PROT_WRITE,MAP_ANON | MAP_SHARED,-1,0);
  assert (heap != MAP_Failed);
  // 2nd semi-space is unreadable. Using "bump-pointer allocation",a SIGBUS
  // tells us we are out of space and need to GC.
  void *guard = mmap(heap + HEAP_SIZE,HEAP_SIZE,PROT_NONE,MAP_ANON |
                     MAP_SHARED | MAP_FIXED,0);
  assert (guard != MAP_Failed);
  signal(SIGBUS,gc);
  heap[HEAP_SIZE] = 90; // pretend we are out of heap space
  return 0;
}

我在Mac OS X 10.6上编译并运行程序,并获得我期望的输出：

$gcc debug.c
$./a.out
GC TIME
Bus error

我想使用GDB运行和调试这个程序.特别是,我想在gc函数中设置一个断点(真的是gc信号处理程序).当然,我需要告诉GDB也不要停止SIGBUS：

$gdb ./a.out 
GNU gdb 6.3.50-20050815 (Apple version gdb-1346) (Fri Sep 18 20:40:51 UTC 2009)
... snip ...
(gdb) handle SIGSEGV SIGBUS nostop noprint
Signal        Stop  Print   Pass to program Description
SIGBUS        No    No  Yes     Bus error
SIGSEGV       No    No  Yes     Segmentation fault
(gdb) break gc
Breakpoint 1 at 0x100000d6f

但是,我们从来没有达到断点：

(gdb) run
Starting program: /snip/a.out 
Reading symbols for shared libraries +. done

Program received signal EXC_BAD_ACCESS,Could not access memory.
Reason: KERN_PROTECTION_FAILURE at address: 0x0000000100029000
0x0000000100000e83 in main ()
(gdb)

显然,不调用信号处理程序(GC TIME未打印).此外,我们仍然在main(),在错误的mov：

0x0000000100000e83 <main+247>:  movq   $0x5a,(%rax)

有任何想法吗？

谢谢.