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    SubjectRe: A good reason to use vfork()
    Theodore Y. Ts'o writes:
    > The problem with the default is at least in theory, every single text
    > page could get modified if a debugger attached to the process and set
    > breakpoints everywhere.

    Excellent point.

    > Therefore, Digital Unix doesn't allow a process to fork() unless it can
    > commit swap space for every single text and data page for the process.

    As I understand it, HP-UX has an interesting way of dealing with this. If
    an executable's or shared library's file permissions are read-only, it is
    assumed you won't debug it. If an executable or shared library has file
    permissions that allow writing by *anyone*, it is assumed you might debug
    it. I believe this is checked once at the time of exec() and never again.
    Not infallible, but not a bad guess either.

    Tradeoff: Once a page is read-only, you don't have the option of setting a
    breakpoint on that page. At least this algorithm gives you the *option* of
    either behavior (memory saving vs. debugging) via toggling file
    permissions.

    Removing all write permissions from executables and shared libraries on
    HP-UX is a nice, but little known, optimization. I don't know if this is
    specific to HP-UX, but so far have only heard of this in reference to
    HP-UX.

    OT: You can wreak havoc by setting breakpoints in libc.
    Impress your friends. :-o

    Regards,
    Chuck

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