gdb: set a breakpoint for a SIGBUS handler
c, garbage-collection, gdb
Solution
The same code (modified to handle SIGSEGV too) works as expected in GDB on Linux; it may be a bug in OS X or GDB's port to that platform.
Googling finds broken OS X behavior just like yours all the way back on 10.1, with a sort-of workaround (`set inferior-bind-exception-port off` before running the program).
(There's a similar bug on Windows.)
Problem
I'm trying to debug a simple stop-and-copy garbage collector (written in C) using GDB. The GC works by handling SIGBUS. I've set a breakpoint at the top of my SIGBUS signal handler. I've told GDB to pass SIGBUS to my program. However, it doesn't appear to work. The following program (explained inline) shows the essence of my problem: ``` #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, -1, 0); assert (guard != MAP_FAILED); signal(SIGBUS, gc); heap[HEAP_SIZE] = 90; // pretend we are out of heap space return 0; } ``` I compile and run the program on Mac OS X 10.6 and get the output I expect: ``` $ gcc debug.c $ ./a.out GC TIME Bus error ``` I want to run and debug this program using GDB. In particular, I want to set a breakpoint at the gc function (really, the gc signal handler). Naturally, I need to tell GDB to not halt on SIGBUS as well: ``` $ 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 ``` However, we never reach the breakpoint: ``` (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) ``` Apparently, the signal handler is not invoked (GC TIME is not printed). Furthermore, we're still in main(), at the faulting mov: ``` 0x0000000100000e83 <main+247>: movq $0x5a,(%rax) ``` Any ideas? Thanks.