So it seems like due to the low-level APIs that readlnImpl calls, it
flies under the radar of Steve's detection.
I think using an Appender with the useExistingBuffer primitive would
work, but would be suboptimal - each read can only use as many
characters as read the last time, which would trigger quite a few
allocation (whenever the line size increases). Please let me know
whether this assessment is correct.
A possible solution would be to cache the last buffer and its maximum
length in static variables inside readlnImpl. I think it's reasonable to
say that readln is the owner of the char[] and you shouldn't take
portions of it and expect they won't be changed. However, it will not
trump over existing arrays.
Andrei
David Simcha wrote:
On Thu, Feb 4, 2010 at 12:10 AM, Andrei Alexandrescu <[email protected]
<mailto:[email protected]>> wrote:
David Simcha wrote:
I recently filed bug 3673
(http://d.puremagic.com/issues/show_bug.cgi?id=3763) and started
looking at possible fixes. This is a bug in
std.stdio.readlnImpl(). However, the more I think about it the
more I think the function needs to be completely rethought, not
just patched. I'm not sure if this is a high priority given
that it has nothing to do with the language spec and TDPL but
it's a pretty embarrassing quality of implementation issue.
Anyhow, readlnImpl() takes a ref char[] and tries to recycle the
memory to read in another line. In doing so, it queries
GC.capacity for the relevant memory block and resizes the array
to the size of the memory block. This is arguably unsafe in the
general case because, if someone passes in a slice that starts
at the beginning of a GC block to use as the buffer, everything
else in the same GC block can get overwritten. This massively
violates the principle of least surprise. On the other hand,
when encapsulated in the higher level API of byLine(), it's both
safe and efficient.
Could you please give an example?
import std.stdio;
void main() {
auto writer = File("foo.txt", "wb");
foreach(i; 0..1000) {
writer.write('a');
}
writer.writeln();
writer.close();
auto chars = new char[500];
chars[] = 'b';
auto buf = chars[0..100];
auto reader = File("foo.txt", "rb");
reader.readln(buf);
writeln(chars[$ - 1]); // a
assert(chars[$ - 1] == 'b'); // FAILS.
}
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