re2c output is not atomic

[terpstra]

When I run this command and then interrupt it:
re2c -8 --no-generation-date src/symbol.re -o src/symbol.cpp

It will create an empty symbol.cpp file. This is problematic for make because the output now exists and so the job does not get rerun. To work around it, I must do this:

%.cpp: %.re
re2c -8 --no-generation-date $< > $@.tmp
mv $@.tmp $@

However, it would be nicer if '-o' did this internally.

[skvadrik]

Yes, this seems useful. The question is, how to create a temporary file portably in C++98?

One possible way is to open filename.tmp in the output directory (the name can be made a bit more random, e.g. to include timestamp) and fail if the file already exists. For that I think we can use open with O_EXCL on POSIX systems (which should cover Unixes, BSDs and macOS) and _open with _O_CREAT|_O_EXCL on Windows.

Out of curiosity, is your output file large? It seems that in most cases re2c does not take that long to generate output.

[terpstra]

My re2c output is 944kB. More relevant, however, is it takes 13 seconds to run... and re2c seems to open the file fairly early in the process.

[terpstra]

FYI, we will be open sourcing our tool which used re2c (wake) in the next few months, so you can see what all my complaining was driving towards. ;)

[skvadrik]

That's really good --- 13s sounds way too long, and I hope it can be reduced.

[skvadrik]

Pushed a fix: 2d30bc1, let's see how it works. Thanks to tmpfileplus for inspiration.