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NAME
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needstack – check for execution stack overflow
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SYNOPSIS
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#include <u.h>
#include <libc.h>
int needstack(int n)
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DESCRIPTION
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Stack overflow in the thread library leads to bugs that are difficult
to diagnose. The Plan 9 libraries are careful about not allocating
large structures on the stack, so typically four or eight kilobytes
is plenty of stack for a thread. Other libraries are not always
as careful. Calling needstack indicates to the thread library
that an external routine is about to be
called that will require n bytes of stack space. If there is not
enough space left on the stack, the thread library prints an error
and terminates the program. The call needstack(0) can be used
to check whether the stack is currently overflowed.
Needstack is defined in libc.h so that library functions used
in threaded and non-threaded contexts can call it. The implementation
of needstack in lib9 is a no-op.
Needstack should be thought of as a comment checked at run time,
like assert(3).
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EXAMPLE
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The X Window library implementation of XLookupString allocates
some very large buffers on the stack, so /usr/local/plan9/src/cmd/devdraw/x11−itrans.c
calls needstack(64*1024) before making calls to XLookupString.
If a thread (in this case, the keyboard-reading thread used inside
the draw(3) library) does not allocate a large
enough stack, the problem is diagnosed immediately rather than
left to corrupt memory.
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SOURCE
SEE ALSO
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