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split_into_argv.c: tidy up large comment.

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I just happened to notice that just below my huge comment explaining
the two command-line splitting policies, there's a smaller one that
refers to it as '(see large comment below)'. It's not below - it's
above!

That was because the older parts of that comment had previously been
inside split_into_argv(), until I moved the explanation further up the
file to the top level. Another consequence of that was that the older
section of the comment was wrapped to a strangely narrow line width,
because it had previously been indented further right.

Folded the two comments together, and rewrapped the narrow paragraphs.
This commit is contained in:
Simon Tatham 2021-09-13 11:57:21 +01:00
parent 8df3ad6316
commit 18cac59b43
1 changed files with 40 additions and 35 deletions
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75
windows/utils/split_into_argv.c
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@ -51,6 +51,9 @@
* or more backslashes precedes two or more double quotes, starting
* inside a double-quoted string.
*
* Modern Visual Studio (as of 2021)
* ---------------------------------
*
* I investigated this in an ordinary CLI program, using the
* toolchain's crt0 to split a command line of the form
*
@ -87,6 +90,9 @@
* either opens or closes a quoted string, and if it closes one, it
* generates a literal " as a side effect.
*
* Older Visual Studio
* -------------------
*
* But here's the corresponding table from the older Visual Studio 7:
*
* backslashes
@ -107,48 +113,47 @@
* 10 0,3,n | 0,4,y 1,3,n 1,4,y 2,3,n
* 11 0,4,n | 0,4,n 1,4,n 1,4,n 2,4,n
*
* There is very weird mod-3 behaviour going on here in the
* number of quotes, and it even applies when there aren't any
* backslashes! How ghastly.
* There is very weird mod-3 behaviour going on here in the number of
* quotes, and it even applies when there aren't any backslashes! How
* ghastly.
*
* With a bit of thought, this extremely odd diagram suddenly
* coalesced itself into a coherent, if still ghastly, model of
* how things work:
* coalesced itself into a coherent, if still ghastly, model of how
* things work:
*
* - As before, backslashes are only special when one or more
* of them appear contiguously before at least one double
* quote. In this situation the backslashes do exactly what
* you'd expect: each one quotes the next thing in front of
* it, so you end up with n/2 literal backslashes (if n is
* even) or (n-1)/2 literal backslashes and a literal quote
* (if n is odd). In the latter case the double quote
* character right after the backslashes is used up.
* - As before, backslashes are only special when one or more of them
* appear contiguously before at least one double quote. In this
* situation the backslashes do exactly what you'd expect: each one
* quotes the next thing in front of it, so you end up with n/2
* literal backslashes (if n is even) or (n-1)/2 literal
* backslashes and a literal quote (if n is odd). In the latter
* case the double quote character right after the backslashes is
* used up.
*
* - After that, any remaining double quotes are processed. A
* string of contiguous unescaped double quotes has a mod-3
* behaviour:
* - After that, any remaining double quotes are processed. A string
* of contiguous unescaped double quotes has a mod-3 behaviour:
*
* * inside a quoted segment, a quote ends the segment.
* * _immediately_ after ending a quoted segment, a quote
* simply produces a literal quote.
* * otherwise, outside a quoted segment, a quote begins a
* quoted segment.
* * _immediately_ after ending a quoted segment, a quote simply
* produces a literal quote.
* * otherwise, outside a quoted segment, a quote begins a quoted
* segment.
*
* So, for example, if we started inside a quoted segment
* then two contiguous quotes would close the segment and
* produce a literal quote; three would close the segment,
* produce a literal quote, and open a new segment. If we
* started outside a quoted segment, then two contiguous
* quotes would open and then close a segment, producing no
* output (but potentially creating a zero-length argument);
* but three quotes would open and close a segment and then
* produce a literal quote.
*/
/*
* We select between two behaviours depending on the version of Visual
* Studio (see large comment below). I don't know exactly when the bug
* fix happened, but I know that VS7 had the odd mod-3 behaviour.
* So, for example, if we started inside a quoted segment then two
* contiguous quotes would close the segment and produce a literal
* quote; three would close the segment, produce a literal quote,
* and open a new segment. If we started outside a quoted segment,
* then two contiguous quotes would open and then close a segment,
* producing no output (but potentially creating a zero-length
* argument); but three quotes would open and close a segment and
* then produce a literal quote.
*
* I don't know exactly when the bug fix happened, but I know that VS7
* had the odd mod-3 behaviour. So the #if below will ensure that
* modern (2015 onwards) versions of VS use the new more sensible
* behaviour, and VS7 uses the old one. Things in between may be
* wrong; if anyone cares, patches to change the cutoff version in
* this #if are welcome.
*/
#if _MSC_VER < 1400
#define MOD3 1