SIGTRAP during thr_new syscall

David Xu davidxu at
Thu Sep 25 03:43:38 UTC 2008

Amol Dixit wrote:
> Hi,
> I am seeing an unexpected SIGTRAP being reported to gdbserver when the 
> debugged process creates a new thread via the _pthread_create() call of 
> libthr library.  [libthr/thread/thr_create.c,v, Freebsd 6.0]
> Gdbserver has internally set a breakpoint on address of 
> _thread_bp_create() so that it gets notified on thread creation and is 
> expecting a SIGTRAP at address (stop pc) of _thread_bp_create(). But 
> instead SIGTRAP happens as a side-effect of thr_new() system call and 
> the stop pc at that point is that of routine thread_start() which is the 
> starting function of new thread. So gdbserver cannot match expected 
> breakpoint (ie. _thread_bp_create) and is confused.
> For testing purpose, if I call _thread_bp_create() before thr_new() in 
> _pthread_create(), I get the _expected_ SIGTRAP with address of 
> _thread_bp_create. But that is not the fix.
> Does anyone have any idea about this SIGTRAP being reported to tracing 
> process gdbserver as part of thr_new? Where is it originating from and why?
> Thanks,
> Amol

I found kernel clears trap flag for new process but not for new thread
in cpu_fork(), you may try following patch:

Index: i386/i386/vm_machdep.c
--- i386/i386/vm_machdep.c	(revision 183337)
+++ i386/i386/vm_machdep.c	(working copy)
@@ -413,6 +413,15 @@
  	bcopy(td0->td_frame, td->td_frame, sizeof(struct trapframe));

+	 * If the current thread has the trap bit set (i.e. a debugger had
+	 * single stepped the process to the system call), we need to clear
+	 * the trap flag from the new frame. Otherwise, the new thread will
+	 * receive a (likely unexpected) SIGTRAP when it executes the first
+	 * instruction after returning to userland.
+	 */
+	td->td_frame->tf_eflags &= ~PSL_T;
+	/*
  	 * Set registers for trampoline to user mode.  Leave space for the
  	 * return address on stack.  These are the kernel mode register values.

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