A question on passing descriptors between processes (in C)

Praise

18 Apr 2003 18 Apr '03

15:40

Hi all! I am writing a little irc-like server to learn unix programming in C. As a cool excercise, I would like to do a way to make my program upgrade without any downtime. So here it is my idea: First process: loader This is not the part of the server which must be upgraded. The only thing to do is getting a deamon, then forking and execing the real server. Then sleeping until a signal awakes it to fork and exec the next (upgraded) server. Second process: the server It must be upgraded. It does the real job, handles connections and so on. It handles also the downloading of the upgraded server. It works until it receives a signal that put it in the "greeting mode" which I describe below. Third process: the upgraded server It does the real job, but first he must have all data and socket descriptors from the "Second process", so it starts in "coming mode". The greeting and coming modes are those when the data (and socket descriptors) exchange occur. I am thinking about unix domain sockets to make socket descriptors and data pass (as described in Stevens' book, the first one on network programming). So, this is my question: there is a simpler way to do that? Is my idea good? Praise

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alan＠ibgames.com

18 Apr 18 Apr

17:33

New subject: [suse-programming-e] A question on passing descriptors between processes (in C)

On 18 Apr 2003 at 17:40, Praise wrote: From: Praise To: suse-programming-e@suse.com Date sent: Fri, 18 Apr 2003 17:40:37 +0200 Subject: [suse-programming-e] A question on passing descriptors between processes (in C)

...

Hi all!

I am writing a little irc-like server to learn unix programming in C. As a cool excercise, I would like to do a way to make my program upgrade without any downtime. So here it is my idea:

First process: loader This is not the part of the server which must be upgraded. The only thing to do is getting a deamon, then forking and execing the real server. Then sleeping until a signal awakes it to fork and exec the next (upgraded) server.

Second process: the server It must be upgraded. It does the real job, handles connections and so on. It handles also the downloading of the upgraded server. It works until it receives a signal that put it in the "greeting mode" which I describe below.

Third process: the upgraded server It does the real job, but first he must have all data and socket descriptors from the "Second process", so it starts in "coming mode".

The greeting and coming modes are those when the data (and socket descriptors) exchange occur. I am thinking about unix domain sockets to make socket descriptors and data pass (as described in Stevens' book, the first one on network programming). So, this is my question: there is a simpler way to do that? Is my idea good?

Praise

As far as I know the process described in Stevens book is the standard way of transferring socket descriptors in cases where you have unrelated processes. However with if you had the old server fork() the new one, then the new one would inherit all the descriptors from the old server :) alan -- http://www.ibgames.net/alan Registered Linux user #6822 http://counter.li.org Winding Down - Weekly Tech Newsletter - subscribe at http://www.ibgames.net/alan/winding/mailing.html

Praise

18:01

New subject: [suse-programming-e] A question on passing descriptors between processes (in C)

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As far as I know the process described in Stevens book is the standard way of transferring socket descriptors in cases where you have unrelated processes. However with if you had the old server fork() the new one, then the new one would inherit all the descriptors from the old server :)

Right, but I should exec* anyway, and I think exec clear all descriptors, or am I wrong? Praise

Jerry Feldman

19:13

New subject: [suse-programming-e] A question on passing descriptors between processes (in C)

On Fri, 18 Apr 2003 20:01:05 +0200 Praise wrote:

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As far as I know the process described in Stevens book is the standard way of transferring socket descriptors in cases where you have unrelated processes. However with if you had the old server fork() the new one, then the new one would inherit all the descriptors from the old server :)

Right, but I should exec* anyway, and I think exec clear all descriptors, or am I wrong?

File descriptors are inherited over exec. One of the things init does is to close all file descriptors. -- Jerry Feldman Boston Linux and Unix user group http://www.blu.org PGP key id:C5061EA9 PGP Key fingerprint:053C 73EC 3AC1 5C44 3E14 9245 FB00 3ED5 C506 1EA9

Praise

20:46

New subject: [suse-programming-e] A question on passing descriptors between processes (in C)

Alle 21:13, venerdì 18 aprile 2003, Jerry Feldman ha scritto:

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On Fri, 18 Apr 2003 20:01:05 +0200

Praise wrote:

...
...
As far as I know the process described in Stevens book is the standard way of transferring socket descriptors in cases where you have unrelated processes. However with if you had the old server fork() the new one, then the new one would inherit all the descriptors from the old server :)

Right, but I should exec* anyway, and I think exec clear all descriptors, or am I wrong?

File descriptors are inherited over exec. One of the things init does is to close all file descriptors.

And how could I access them? If I exec a new program it never knows about the former descriptors, even if they remain opened... Praise

Graham Murray

21:20

New subject: [suse-programming-e] A question on passing descriptors between processes (in C)

Praise writes:

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And how could I access them? If I exec a new program it never knows about the former descriptors, even if they remain opened...

In some instances it does. How do think the shell sets STDIN and STDOUT (think pipes and redirection as well as controlling terminal)for programs it launches? Or the consequences (for the subprocess) of calling popen(3).

Praise

21:25

New subject: [suse-programming-e] A question on passing descriptors between processes (in C)

Alle 23:20, venerdì 18 aprile 2003, Graham Murray ha scritto:

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Praise writes:

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And how could I access them? If I exec a new program it never knows about the former descriptors, even if they remain opened...

In some instances it does. How do think the shell sets STDIN and STDOUT (think pipes and redirection as well as controlling terminal)for programs it launches? Or the consequences (for the subprocess) of calling popen(3).

Well, I had not clear that:) I supposed that before, but I did not know perfectly. From the manual page: "The exec family of functions replaces the current process image with a new process image. " I thought descriptors (at least, not the standard descriptors) were part of a process image. Praise

Graham Murray

21:37

New subject: [suse-programming-e] A question on passing descriptors between processes (in C)

Praise writes:

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Well, I had not clear that:) I supposed that before, but I did not know perfectly. From the manual page:

"The exec family of functions replaces the current process image with a new process image. "

I usually use the info pages rather than man for libc functions, and the info page for the exec family includes File descriptors open in the existing process image remain open in the new process image, unless they have the `FD_CLOEXEC' (close-on-exec) flag set. The files that remain open inherit all attributes of the open file description from the existing process image, including file locks. File descriptors are discussed in *Note Low-Level I/O::.

alan＠ibgames.com

19 Apr 19 Apr

06:24

New subject: [suse-programming-e] A question on passing descriptors between processes (in C)

On 18 Apr 2003 at 22:46, Praise wrote: From: Praise To: suse-programming-e@suse.com Date sent: Fri, 18 Apr 2003 22:46:52 +0200 Subject: Re: [suse-programming-e] A question on passing descriptors between processes (in C)

...

Alle 21:13, venerdì 18 aprile 2003, Jerry Feldman ha scritto:

...
On Fri, 18 Apr 2003 20:01:05 +0200

Praise wrote:

...
...
As far as I know the process described in Stevens book is the standard way of transferring socket descriptors in cases where you have unrelated processes. However with if you had the old server fork() the new one, then the new one would inherit all the descriptors from the old server :)

Right, but I should exec* anyway, and I think exec clear all descriptors, or am I wrong?

File descriptors are inherited over exec. One of the things init does is to close all file descriptors.

And how could I access them? If I exec a new program it never knows about the former descriptors, even if they remain opened...

Praise

You could use poll (man 2 poll). Call it over the the max possible range of file descriptors and see which ones return POLLNVAL... alan -- http://www.ibgames.net/alan Registered Linux user #6822 http://counter.li.org Winding Down - Weekly Tech Newsletter - subscribe at http://www.ibgames.net/alan/winding/mailing.html

Anders Johansson

18 Apr 18 Apr

21:29

New subject: [suse-programming-e] A question on passing descriptors between processes (in C)

On Fri, 2003-04-18 at 21:13, Jerry Feldman wrote:

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File descriptors are inherited over exec. One of the things init does is to close all file descriptors.

That depends on the value of FD_CLOEXEC. See "man fcntl", specifically the section on F_GETFD and F_SETFD

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alan＠ibgames.com
Anders Johansson
Graham Murray
Jerry Feldman
Praise