Older blog entries for redi (starting at number 263)

The GCC checkin policy requires a ChangeLog entry for each change, and that entry is usually also used as the commit message. I used to use a fairly crappy, homemade vim plugin to extract the first entry from a ChangeLog file into the editor buffer to use as the svn commit message. I decided to replace that vim plugin with a Git prepate-commit-msg hook that automatically prepares the commit message for me by extracting the first entry from each ChangeLog that is part of the commit.

As a cursory web search didn't find anything similar, I'm posting it here in case it's useful to anyone else.

$ cat .git/hooks/prepare-commit-msg
# Hook script to prepare the commit log message from the first
# entry in each modified ChangeLog being committed.

set -e

[[ "$2,$3" == 'merge,' ]] && exit

awk -f $0.awk $(git status -s | awk '/^M .*ChangeLog/{print $NF}') > $tmpf
sed 1d $1 >> $tmpf
mv $tmpf $1

$ cat .git/hooks/prepare-commit-msg.awk
# When there is more than one ChangeLog in the commit, precede
# the entry by the directory name.
FNR == 1 && ARGC > 2 {
printf "%s:\n\n", gensub("/ChangeLog$", "", "", FILENAME)

# Save the first line until we know if we want to print it.
FNR == 1 {

# If second line is blank there is only one patch author, omit it.
FNR == 2 && /^$/ {

# If second line is not blank there are multiple authors, print them.
FNR == 2 && /^./ {
print line1

# Stop processing the file at the next entry.
FNR > 3 && /^20[1-9][0-9]-[0-1][1-9]-[0-3][0-9] / {

# Otherwise, just print the line.
FNR > 2 {
12 Apr 2012 (updated 12 Apr 2012 at 20:37 UTC) »

In response to repeated claims that Clang has far better diagnostics than GCC I decided to re-run some of the tests on the Clang Expressive Diagnostics page (which uses the ancient GCC 4.2 release that Apple still use) to show how much GCC has improved. The results are at Clang Diagnostics Comparison in the GCC wiki. Enjoy.

8 Feb 2012 (updated 8 Feb 2012 at 16:50 UTC) »
fzort and cdfrey, the conditional (a.k.a ternary) operator doesn't strictly require the second and third operands to have the same type, but it must be possible to convert one operand to the type of the other. In fzort's example both needed to be converted to foo*, neither can be converted to the other type, so it's ill-formed.

(There is another form allowed which doesn't require conversion, where one of the operands is a throw-expression: which ? new foo : throw "catch me". This isn't entirely useless syntactic sugar, it can be used to throw exceptions from constexpr functions, which don't allow if statements.)
12 Jan 2012 (updated 8 Feb 2012 at 22:26 UTC) »

I've finally drunk the git kool aid. I'd tried it before, but didn't inhale. Maybe I'm just feeling the effects of that cocktail of mixed metaphors but I really like it.

No more manually copying files between my home machine and my netbook (which isn't big enough or strong enough to handle a whole gcc subversion checkout, let alone the whole git repo) so I can work on the train. And I've already created about one new local branch a day in my gcc clone, so I can juggle fixes for several different PRs at once without constantly applying and reverting patches to my svn working dir.

I signed up to github so I can easily fork other people's repositories and send pull requests via the web interface, but decided to use gitorious for hosting some of my own projects. I like that gitorious itself is open source, but unfortunately ... it just doesn't work as well! I don't mind occasional weird rendering bugs on the web pages, but now I can't push/pull anything, due to an ssh error that didn't happen previously. Ho hum, I'll stick with it for a while yet. Github, on the other hand, is pretty damn slick.

6 Jan 2012 (updated 8 Jan 2012 at 17:09 UTC) »

Vim syntax highlighting for C++11 raw strings

A little over a year ago I mentioned a vim syntax file for C++0x which I was using for a while, but it only adds support for some new keywords, not new syntax such as lambda expressions and list-initialization. Last month I found cpp11.vim in the Vim online script database, which intends to support the new C++11 syntax, by replacing your existing cpp.vim entirely.

Not quite satisfied with either of those, I've started using my own cpp.vim to extend the default cpp.vim file (which is done by putting it in ~/.vim/after/syntax/cpp.vim) with syntax highlighting for the new keywords and also C++11 raw strings.

With my colour settings they look like this:
char raw[] = R"#(s/"\([^"]*\)"/'\1'/g)#";
which is equivalent to:
char yuk[] = "s/\"\\([^\"]*\\\"/'\\1'/g";
only without suffering from leaning toothpick syndrome.

The # in the example can be (almost) any sequence of zero or more characters acting as a delimiter. The example above requires a non-empty delimiter because the string contains )", which would be taken as the end of the raw string otherwise:
char raw[] = R"(s/"\([^"]*\)"/'\1'/g)"; // ERROR!

You could choose to use "" as the delimiter sequence, as a nod to Python
char raw[] = R"""(triple-quoted for great justice)""";
20 Dec 2011 (updated 20 Dec 2011 at 13:56 UTC) »

broonie asked what's wrong with switch statements?

The default behaviour of switch statements is broken, it's too easy to forget a break and introduce a fall-through bug. That's harder to do with the if-else-if-else form.

Breaking after each case should have been the default and continue should have been required to explicitly fall through. It wouldn't even have needed a new keyword.

20 Dec 2011 (updated 20 Dec 2011 at 10:54 UTC) »
hjclub, I repeat this from the Advogato FAQ (emphasis mine):

the purpose of the trust metric is to certify that a given user account on Advogato is known by the Advogato community to actually belong to the individual who claims it and is known to be a member of the free software and open source community.

Certifying random spammers because you feel like it doesn't meet the purpose. You are of course free to use the trust metric for something different to its stated purpose, but don't be surprised if you get "exiled" as a spammer when you insist on certifying spammers. If you happen to know someone is who they claim to be, and they're a real member of the open source community, write a diary entry to say so, telling other users they're not an annoying spammer, even though they have an obviously computer-generated name!

FWIW, a spam rating of 9 does not mean 9 people rated you, users with a Master cert add 3 to a spam rating, Journeyers add 2, and Apprentices add 1, reflecting the trust the community places in people with a higher rating. So it only takes three Masters to take a spam rating to 9. I haven't rated you as a spammer because I recognised your username and think you should get a second chance to participate in the community, but if you continue to certify obvious spammers I will use my +3 spamhammer :)

Welcome back, hjclub.

Why did you certify fredericpete821 when that's clearly a spammer account? You got kicked out once before for doing that, after being advised not to.

Certifying spammers is not how to interact with this community

2 Nov 2011 (updated 24 Mar 2013 at 13:52 UTC) »

C++11 brainteaser

Here's a little program showing something that std::bind can do that lambdas can't.

#include <functional>
#include <string>
#include <iostream>

struct Polly
template<typename T, typename U>
auto operator()(T t, U u) const -> decltype(t + u)
{ return t + u; }

int main()
auto polly = std::bind(Polly(), std::placeholders::_1, "confusing");

std::cout << polly(4) << polly(std::string(" this is ")) << std::endl;

What will it print?
1 Nov 2011 (updated 1 Nov 2011 at 11:45 UTC) »

I wrote previously about needing to hack the clang sources to build it with gcc in a non-standard location. That patch is no longer necessary, to enable it to find the GCC headers and runtime files I now configure it as:

../llvm/configure --prefix=$PREFIX --enable-targets=host \
--enable-optimized --disable-jit \
LDFLAGS=-Wl,-R,$GCC_DIR/lib64 \
--with-cxx-include-root=$GCC_DIR/include/c++/$GCC_VER \

Due to some GNUisms in older versions of glibc (e.g 2.5 in my case) it's necessary to build LLVM with
make CFLAGS=-std=gnu89

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