Re: [Haskell-cafe] createProcess running non-existent programs

On 14 August 2012 17:22, Niklas Larsson wrote:

2012/8/14 Alexander Kjeldaas :

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On 13 August 2012 23:49, Richard O'Keefe wrote:

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On 13/08/2012, at 11:26 PM, Alexander Kjeldaas wrote:

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This isn't that hard - a pipe shouldn't be needed anymore. Just

require

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a post-2003 glibc.

fexecve is a system call in most BSDs. It is also implemented in glibc using a /proc hack.

fexecve is now in the Single Unix Specification, based on POSIX as of 2008, I believe. However, http://www.gnu.org/software/gnulib/manual/html_node/fexecve.html says Portability problems not fixed by Gnulib: * This function is missing on many non-glibc platforms: MacOS X 10.5, FreeBSD 6.0, NetBSD 5.0, OpenBSD 3.8, Minix 3.1.8, AIX 5.1, HP-UX 11, IRIX 6.5, OSF/1 5.1, Solaris 11 2010-11, Cygwin 1.5.x, mingw, MSVC 9, Interix 3.5, BeOS.

That warning doesn't seem to be fully up to date. I'm using MacOS X 10.6.8 and fexecve() isn't in the manuals or in .

FreeBSD 8.0 is covered. OpenBSD not covered OS X not covered

http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPa...

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Solaris probably not covered.

So support is pretty good, I'd say. For non-modern systems, checking the existence of the file first is possible. The race isn't important, and one can always upgrade to a modern operating system.

The check would be unreliable, the file's existence doesn't imply that it's executable.

See access(2)

Furthermore it would add unnecessary overhead, createProcess can be run thousands of times in a program and should be lean and mean.

Just to keep the bikeshedding doing, I'm going to state as a fact that running performance sensitive *server* workload on any unix other than Linux is purely of theoretical interest. No sane person would do it. Therefore, from a performance overhead, Linux performance is the only important performance measure. But even given the above, the overhead we're talking about is minuscule. A program like '/bin/echo -n ''' which does exactly *nothing*, requires 35(!) system calls to do its job :-). A more complex program like 'id' requires 250 system calls! Also, to see just how minuscule this is, the dynamic linker, ld-linux.so does a few extra access(2) system calls *to the same file*, /etc/ld.so.hwcaps, on startup of every dynamically linked executable. 2 in the 'echo' case, and 8 in the 'id' case above. Even the glibc folks haven't bothered to optimize those syscalls away. Alexander