access off end of array [LWN.net]

access off end of array

Posted Mar 26, 2013 9:00 UTC (Tue) by khim (subscriber, #9252)
In reply to: access off end of array by tjc
Parent article: Regehr: GCC 4.8 Breaks Broken SPEC 2006 Benchmarks

Actually, the ++/PDP-11 connection is urban legend -- see "More History", paragraph 2 at this link:

The Development of the C Language

Well, your own link shows that it's not an "urban legend" but more like oversimplification: This is historically impossible, since there was no PDP-11 when B was developed. The PDP-7, however, did have a few `auto-increment' memory cells, with the property that an indirect memory reference through them incremented the cell. This feature probably suggested such operators to Thompson; the generalization to make them both prefix and postfix was his own.

While factually incorrect (C design predates PDP-11) both "++" in C and "(RX)+" in PDP-11's assembler come from the same source.

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access off end of array

Posted Mar 26, 2013 16:04 UTC (Tue) by hummassa (subscriber, #307) [Link]

I know for a fact the 6809 microprocessors had some instructions "load/store from pointer with post/pre-auto-increment/decrement" so that one of:

a = *b++
a = *++b
a = *b--
a = *--b
*b++ = a
*++b = a
*b-- = a
*--b = a

was a single instruction; they made easy to implement real fast stacks and queues, and zero-terminated strings (because "a = *b++" &c set the Z flag if the char was zero).

access off end of array

Posted Mar 26, 2013 16:52 UTC (Tue) by brouhaha (subscriber, #1698) [Link]

Yes, but the 6809 came along much later than the PDP-11, so it's not relevant to discussion of where the C pre/post-increment/decrement operators came from.