75 lines
3.2 KiB
Plaintext
75 lines
3.2 KiB
Plaintext
.\"
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.\" ----------------------------------------------------------------------------
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.\" "THE BEER-WARE LICENSE" (Revision 42):
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.\" <phk@login.dknet.dk> wrote this file. As long as you retain this notice you
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.\" can do whatever you want with this stuff. If we meet some day, and you think
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.\" this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
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.\" ----------------------------------------------------------------------------
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.\"
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.\" $FreeBSD$
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.\"
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.ds RH Introduction
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.NH
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Introduction
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.PP
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Most programs need to allocate storage dynamically in addition
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to whatever static storage the compiler reserved at compile-time.
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To C programmers this fact is rather obvious, but for many years
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this was not an accepted and recognized fact, and many languages
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still used today don't support this notion adequately.
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.PP
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The classic UNIX kernel provides two very simple and powerful
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mechanisms for obtaining dynamic storage, the execution stack
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and the heap.
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The stack is usually put at the far upper end of the address-space,
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from where it grows down as far as needed, though this may depend on
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the CPU design.
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The heap starts at the end of the
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.B bss
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segment and grows upwards as needed.
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.PP
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There isn't really a kernel-interface to the stack as such.
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The kernel will allocate some amount of memory for it,
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not even telling the process the exact size.
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If the process needs more space than that, it will simply try to access
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it, hoping that the kernel will detect that access have been
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attempted outside the allocated memory, and try to extend it.
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If the kernel fails to extend the stack, this could be because of lack
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of resources or permissions or because it may just be impossible
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to do in the first place, the process will usually be shot down by the
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kernel.
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.PP
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In the C language, there exists a little used interface to the stack,
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.B alloca(3) ,
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which will explicitly allocate space on the stack.
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This is not a interface to the kernel, but merely an adjustment
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done to the stack-pointer such that space will be available and
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unharmed by any subroutine calls yet to be made while the context
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of the current subroutine is intact.
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.PP
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Due to the nature of normal use of the stack, there is no corresponding
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"free" operator, but instead the space is returned when the current
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function returns to its caller and the stack frame is dismantled.
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This is the cause of much grief, and probably the single most important
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reason that alloca(3) is not, and should not be, used widely.
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.PP
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The heap on the other hand has an explicit kernel-interface in the
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system call
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.B brk(2) .
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The argument to brk(2) is a pointer to where the process wants the
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heap to end.
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There is also a interface called
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.B sbrk(2)
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taking an increment to the current end of the heap, but this is merely a
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.B libc
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front for brk(2).
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.PP
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In addition to these two memory resources, modern virtual memory kernels
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provide the mmap(2)/mmunmap(2) interface which allows almost complete
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control over any bit of virtual memory in the process address space.
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.PP
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Because of the generality of the mmap(2) interface and the way the
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data structures representing the regions are laid out, sbrk(2) is actually
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faster in use than the equivalent mmap(2) call, simply because
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mmap(2) has to search for information that is implicit in the sbrk(2) call.
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