Hcreate(3)

https://linux.die.net/man/3/hcreate

by luu • 2 years ago

53 35 2 years ago

linux.die.net

ropejumper• 2 years ago

Does someone know why these are not reentrant without the GNU extension? It seems like a really weird choice to make a library for hash tables and then make the state global like that.

kragen3• 2 years ago

probably they were added in 2.9bsd or something when programs couldn't be more than about 5000 lines of code before you ran out of the 16-bit address space

hmm, no, it doesn't seem to be in https://www.tuhs.org/Archive/Distributions/UCB/2.9BSD/ or even in https://www.tuhs.org/Archive/Distributions/UCB/4.2BSD/ or https://www.tuhs.org/Archive/Distributions/UCB/4.3BSD/. and https://man.freebsd.org/cgi/man.cgi?query=hsearch&apropos=0&... says explicitly:

The hcreate(), hdestroy(), and hsearch() functions first appeared in AT&T System V UNIX.

which... does seem a bit late for thinking it was a good idea to only support one hash table per process? that was in 01983, 6 years after the vax was introduced and 4 years after the 68000. i mean awk supported arbitrarily many hash tables in a single process in 7th edition unix, even before it had subroutines

Someone• 2 years ago

i mean awk supported arbitrarily many hash tables in a single process

This API supports that, too, as long as you don’t call one of the functions while another call is in progress. You get that for free if your process is single threaded.

https://en.wikipedia.org/wiki/Thread_(computing)#Threading_m... says

“History of threading models in Unix systems

SunOS 4.x implemented light-weight processes or LWPs. NetBSD 2.x+, and DragonFly BSD implement LWPs as kernel threads (1:1 model)”

According to Wikipedia, SunOS 4.0 is from 1988, NetBSD 2.0 from 2004, so I guess chances are good that AT&T System V UNIX didn’t support threads in user space programs.

clbrmbr• 2 years ago

Don’t key prefixes take care of the need for an arbitrary number of tables?

tedunangst• 2 years ago

Probably started as a function in one program. Somebody copied it into a second program. Somebody wanted it in a third program and copied it into a library.

loeg• 2 years ago

They're really, really old. Dating to System V Unix. At the time it was pretty common to have stateful libc functions.

layer82• 2 years ago

You can look them up here in System V Interface Definition Issue 2 from 1986: http://www.bitsavers.org/pdf/att/unix/SVID/System_V_Interfac.... It has a note “Future Directions: The restriction on only having one hash search table active at any given time will be removed.” Which of course couldn’t be done without breaking compatibility.

Most programs at the time probably didn’t need more than one hash table, and it made for a simpler interface. (A program could in principle also work around the limitation by storing multiple values per hash table entry.) Most Unix libraries worked with global state at the time, since multi-threading also didn’t exist yet. I remember running into the limitation of only being able to have one lexer/parser grammar (via lex/yacc) per program.

kelnos• 2 years ago

Back when these functions were created, it was normal and expected for libc to store global state for a variety of things. Reentrancy wasn't really an issue that was considered then.

(Though this particular API has an even weirder limitation: you can only create a single hash table at a time in your entire program!)

metadat2• 2 years ago

Is there a command-line interface for this, or is it strictly library-only?

Would be cool if there were a way to point to a memmapped region under /dev/shm or equivalent.

sph• 2 years ago

Type `man redis` or `man sqlite3` for memmapped hash tables with a lot of bells and whistles.

I can't see what would you do with a command line hash table CLI. That's basically a poor man's database.

metadat2• 2 years ago

Redis is way too much, but you've got a really good point with SQLite. Thanks!

pdw• 2 years ago

Don't use die.net! The manpages there were last updated 10+ years ago. It's very unfortunate that that site does so well on Google.

nvy3• 2 years ago

Is there a better man page site to use instead? I abhor reading in the terminal.

ivanjermakov• 2 years ago

The only other one I'm aware of is Ubuntu manuals: https://manpages.ubuntu.com/manpages/bionic/en/

o11c• 2 years ago

man7.org is "official" (linked from kernel.org)

This page there: https://man7.org/linux/man-pages/man3/hsearch.3.html

layer82• 2 years ago

Linux: https://man7.org/linux/man-pages/

Debian: https://manpages.debian.org/

Ubuntu: https://manpages.ubuntu.com/

FreeBSD: https://man.freebsd.org/

OpenBSD: https://man.openbsd.org/

alan-crowe• 2 years ago

The documentation pages for FreeBSD include manual pages, for example

https://man.freebsd.org/cgi/man.cgi?query=hcreate&manpath=Fr...

self_awareness3• 2 years ago

What's the difference between reading letters and digits on a webpage, and reading them in a terminal?

PhilipRoman2• 2 years ago

Mankier is nice if you like fancy formatting

https://www.mankier.com/1/qemu

chrsw• 2 years ago

Not only that, but I've been having lots of problems with Cloudflare lately. How much of the internet do they provide service for? Good grief...

alurm• 2 years ago

In this case it doesn't matter much I think. I suppose the api existed for ages.

But I prefer openbsd or man7 pages, sure.

Neywiny• 2 years ago

I usually turn to C++ when I need such things. Hopefully I remember this exists next time.

nextaccountic• 2 years ago

Don't use this, it's worse than useless

self_awareness3• 2 years ago

I did a quick search on GitHub for today's code that uses it, and actually found out that people do use it today, but generally I agree, this is a legacy API, legacy implementation, and the best thing we can do for it is to forget about it.

The `hcreate(3)` API is interesting only because it has a very limited use, yet it still exists in libc. So it's kind of interesting in a circus way; we want to see it, but then we want to get out of it, and we want to continue our rational lives without clowns and bearded ladies.

There are probably many thirdparty hashtables for C which work much better than this.

fanf2• 2 years ago

This API is laughably bad, almost completely useless. But it is in POSIX https://pubs.opengroup.org/onlinepubs/9699919799/functions/h... so it is ubiquitous as well as useless.

Hcreate(3)

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