On Oct 8, 2005, at 6:55 AM, Damien Bobillot wrote: Daniel Jalkut wrote :
I noticed in the documentation that for vfork in particular, because it allows the child process to take over the parent's address space until exec'ing, the follow-fork may not be very helpful.
You're wrong : the child process did really not use the parent adress space. And even if it were true, gdb cannot follow both process at the same time, it must choose to follow the parent OR the child. Try to put a breakpoint just after a vfork, you'll see that gdb only break in the parent process (in the default configuration).
I'm sure I'm technically wrong in some ways, but I am going on what I've observed, and trying to make sense of it in terms of what I have read, for instance:
"When a child process is spawned by vfork, you cannot debug the child or parent until an exec call completes."
What I experience is that when I change the mode to "follow child," the behavior is no different. My breakpoint that should be triggered in the child process yields a Crash Reporter dialog (EXC_BREAKPOINT), while debugging carries on in the parent. If you use fork, the whole parent memory is copied. If you use vfork, the memory is lazily copied : the system wait until it really need to make the copy, ie if both processes don't write anything in a given memory page, the system won't make the copy, but if one do write something, the system make the copy just before effectively writing things. This use the copy-on-write flag of the paged memory manager.
The man page for vfork (10.4 Tiger) doesn't imply such a polite interaction. It describes a scenario in which the child process literally takes over the parent's "memory and thread of control" until execve or exit is called. If things really do behave as you describe, then a documentation bug report is in order, I think. The only difference between fork and vfork is that vfork is more efficient : for the developer, even when debugging, both functions seem to work the same way.
Again I haven't observed thread following to work in this way. If you're skeptical, you could try on your own machine by attaching to SystemUIServer, setting follow-fork-mode child, and try to set a breakpoint on execve. If you get the luxury of hitting that breakpoint (and not a crash report) then I'd be very interested to determine the differences between our configurations.
Daniel
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