[RFC] Collecting strings at tracepoints

[Stan Shebs]

Collection of strings is a problem for tracepoint users, because the 
literal interpretation of "collect str", where str is a char *, is to 
collect the address of the string, but not any of its contents.  It is 
possible to use the '@' syntax to get some contents, for instance 
"collect str@40" acquires the first 40 characters, but it is a poor 
approximation; if the string is shorter than that, you collect more than 
necessary, and possibly run into access trouble if str+40 is outside the 
program's address space, or else the string is longer, in which case you 
may miss the part you really wanted.

For normal printing of strings GDB has a couple tricks it does.  First, 
it explicitly recognizes types that are pointers to chars, and 
automatically dereferences and prints the bytes it finds.  Second, the 
print elements limit prevents excessive output in case the string is long.

For tracepoint collection, I think the automatic heuristic is probably 
not a good idea.  In interactive use, if you print too much string, or 
just wanted to see the address, there's no harm in displaying extra 
data.  But for tracing, the user needs a little more control, so that 
the buffer doesn't inadvertantly fill up too soon.  So I think that 
means that we should have the user explicitly request collection of 
string contents.

Looking at how '@' syntax works, we can extend it without disrupting 
expression parsing much.  For instance, "str@@" could mean to deference 
str, and collect bytes until a 0 is seen, or the print elements limit is 
reached (implication is that we would have to tell the target that 
number).  The user could exercise even finer control by supplying the 
limit explicitly, for instance "str@/80" to collect at most 80 chars of 
the string.  ("str@@80" seems like it would cause ambiguity problems vs 
"str@@").

This extended syntax could work for the print command too, in lieu of 
tweaking the print element limit, and for types that GDB does not 
recognize as a string type.

Under the hood, it's not yet clear if we will need additional bytecodes, 
but probably so.

Stan