In POSIX, there is the fork() function to create a sub-process. How can I achieve fork()'s functionality in Windows?
There is no direct equivalent of fork() on Windows.
CreateProcess() is the native function that can be used to create a new process (but, again, the semantics are rather different to fork()'s).
To put this another way, on Unix it is possible for a process to cheaply create a clone of itself. There is no inexpensive way to do this on Windows.
If you don't care about the cloning aspect of fork(), then CreateProcess() should do just fine.
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I have searched the web for long time without success. How can I add a new command to the kernel command line? e.g. a command like ver without any arguments, that simply prints Hello World message when executed.
The Linux kernel does not handle any commands (but the GRUB bootloader can pass some arguments to the booted kernel). It is just handling system calls. You could add some new one, but learn about advanced linux programming & syscalls(2) before hacking the kernel.
Perhaps you want to add a new command available to your shell. This is quite simple, put a new executable file (either an ELF executable, e.g. compiled from C code, or some script) in a directory known to your PATH. You might also make a new shell function.
The system call to create a new process is fork(2) (sometimes emulated thru some appropriate call to clone(2)...). fork is difficult to understand, so take several hours to read about it. Almost every process (with a few exceptions, notably /sbin/init) is created by fork. The system call to execute a new program is execve(2). Your shell will often use fork & execve to run commands (except builtin ones, like cd). Use strace(1) to understand what syscalls are done (by some program).
PS. Don't dare hacking your kernel before understanding quite well user-land Linux system programming....
I'm new to Windows system programming and I'm trying to learn the CreateProcess() function.
I know that it's possible to run a new process, for instance, notepad.exe or cmd.exe by the calling program by giving the name (notepad or cmd.exe) as parameter to the CreateProcess() function in the calling program.
What is the use of doing that, and could you explain any real world application for that?
Can I use this create process function to clone itself and do something in parallel?
What is the use of doing that, and could you explain any real world application for that?
CreateProcess is the way to create new processes on Windows. Obvious examples of its use would be for the shell to start new applications. Or for the command line interpreter to execute external commands.
Can I use this create process function to clone itself and do something in parallel?
No. Windows processes don't use the *nix fork idiom. There is no analogue in Windows to forking.
Can I use this create process function to clone itself and do something in parallel?
Not so much a clone, no. But the calling app can spawn a separate instance of itself, by specify its own filename, possibly with command-line parameter(s) to tell the spawned process what to do. So in that regard, yes, you can have multiple instances of your app running in parallel.
I wish to know what tools my IDE runs and what command line arguments it uses.
Is there such tool that will provide such information?
I can make an app that will save all processes every second or even faster but I'm still not sure that it will hook everything. There must be some tool already that will do that work much better. Like hooking OS calls for WinExec or CreateProcess(Ex).
Sysinternals Process Monitor can log process events, among other things. It works by monitoring Native API calls, so it'll work regardless of which library function programs use.
My problem is not the best scenario for fork(). However, this is the best func I can get.
I am working on a Firefox plugin on Mac OSX. To make it robust, I need to create a new process to run my plugin. The problem is, when I forked a new process, much like this:
if (fork() == 0) exit(other_main());
However, since the state is not cleaned, I cannot properly initialized my new process (call NSApplicationLoad etc.). Any ideas? BTW, I certainly don't want create a new binary and exec it.
In general, you need to exec() after fork() on Mac OS X.
From the fork(2) man page:
There are limits to what you can do in the child process. To be totally safe you should restrict your-self to only executing async-signal safe operations until such time as one of the exec functions is called. All APIs, including global data symbols, in any framework or library should be assumed to be unsafe after a fork() unless explicitly documented to be safe or async-signal safe. If you need to use these frameworks in the child process, you must exec. In this situation it is reasonable to exec yourself.
TN2083 also comments on this subject:
Many Mac OS X frameworks do not work reliably if you call fork but do not call exec. The only exception is the System framework and, even there, the POSIX standard places severe constraints on what you can do between a fork and an exec.
IMPORTANT: In fact, in Mac OS X 10.5 and later, Core Foundation will detect this situation and print the warning message shown in Listing 13.
Listing 13: Core Foundation complaining about fork-without-exec
The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec().
Break on __THE_PROCESS_HAS_FORKED_AND_YOU_CANNOT_USE_THIS_COREFOUNDATION_FUNCTIONALITY___YOU_MUST_EXEC__() to debug.
fork without exec is basically entirely unsafe on OSX. You will end up with stale mach ports for example.
I'm writing the FreeWRL plugin for Firefox (Linux at the moment, Mac & Windows soon).
http://freewrl.sourceforge.net/
It's based on fork+exec to launch FreeWRL and swallow its window into Firefox.
You'll have to use a pipe to correctly handle the possible failure of fork+exec or the failure of your child process :
How to handle execvp(...) errors after fork()?
Cheers,
C
In Windows -- and probably Unix for that matter -- using the chdir() function in a (32-bit) program doesn't change the directory when the program exits. (It does in a 16-bit Windows program.)
Does anybody know how to do that in a Windows 32-bit program?
Uhm... IMHO it's exactly one of the things that the OS must guarantee not to happen. The current dir is a per-process property, a child process usually inherits it from the parent process, but the reverse should not happen (and it doesn't).
To obtain what you want, the parent could actively watch some information (message, file, shared memory...) in which the child process stores the new directory, and then call chdir() with the new value.
As far as I know, Windows' cmd.exe doesn't have any mechanism like that.
Actually, by using code injection techniques (e.g. CreateRemoteThread) on the parent process it could be possible to force it to do something unexpected, but it's a very dirty trick, not at all good neither general.
Win16 was different: there was a single "msdos" state for all the programs, but it was a limitation, not a feature.
It sounds like you're asking one process (your Win32 program) to change the CWD of another process (your shell). As far as I know, this is impossible without the latter process providing an API for such a purpose. The nearest I can come to any sort of reference for this assertion, however, is the following quote from MSDN:
A parent process can directly alter the environment variables of a child process during process creation. This is the only situation when a process can directly change the environment settings of another process.
Well yeah it's true the popular API calls to change directory change it for the process. ... BUT ...
(1.) 16-bit windows programs can change the global directory; probably because they run in the same process as the command.com thing. That's what I've been happily using for years; I assume XP somehow emulates this? ... But now Windows 7 64-bit won't run 16-bit programs anymore! (?)
(2.) Both Windows and Unix "cd" commands can of course change directories for the calling process -- presumably because they are built-in commands of the command shell. But successor Windows shells manage to accomplish this, or at least I hope PowerShell can do that. All built-ins?
(3.) The way I've wound-up doing it is modifying my programs that used to call the API to simply emit "cd \dst\directory" to stdout, then in a procedure do
chdirprogram >t~.bat
call T~.bat
Which works great. And of course the usual point of a change-directory program is to provide the functionality in a batch procedure with a computed destination. Which of course you can do in Unix with Bash etc. variables, but not in Windows batch files, although maybe (?) in the numerous successor Windows procedure things, which I don't want to use. ... Since this functionality is obviously useful, I was hoping someone knew of a sneaky Windows call what'd do it. The explanation that it's somehow wrong for a process to change the directory for a calling process is one of those bogus, "you're not supposed to do that and I won't tell you why" excuses. ... But I guess I'll just stick to my pitiful little batch files.
Are you talking about the SetCurrentDirectory function of Windows API? The article says that the function "changes the current directory for the current process". In for instance Delphi, there is a function ChDir that actually calls this API function.