Linux Processes from Python HOW-TO?

Benyang Tang btang at pacific.jpl.nasa.gov
Mon May 1 19:15:45 CEST 2000


Thomas Wouters wrote:
> 
> On Fri, Apr 28, 2000 at 07:39:00PM +0000, chris_barker at my-deja.com wrote:
> 
> > Has anyone written a HOW-TO or tutorial that explains how to start and
> > manipulate other processes with Python. I imagine it's trivial if you
> > are familiar with the Linux/Unix process model, but I get very confused
> > about when to use:
> 
> > system
> > popen and it's variants
> > exec and it's variants
> > fork
> > etc.
> 
> Well, it's detailed in most books about UNIX, and in most books about
> operating systems in general. (At least on the programming level.) I dont
> know any HOWTO-level explanation of it, but I'm happy to supply you with
> some short descriptions. I'll do it in the reverse order of what you listed,
> though, because it's easier that way ;)
> 
> First, the basics: under UNIX, and most current operating systems, a running
> program consists of one or more processes. A single program, or a single
> task, is usually a single process, but it's possible for several processes
> to communicate with eachother, to share workload. Each process has its own
> memoryspace, invisible to other processes, and it can only access its own
> memoryspace.
> 
> To create a new process, you use the system call fork(). It duplicates the
> current process, copies all of its memory, instruction pointers, function
> stack, everything. The only difference between the two processes is the
> process-id (pid), and the return code of fork(): In the new process, the
> 'child', fork returns 0. In the original process, the 'parent', fork()
> returns the pid of the newly created child process. (If fork() fails, it
> wont create a child process, of course, and the return code will signify an
> error.)
> 
> Because fork() creates an exact duplicate of the old process, you can't use
> it to start a new program. You use one of the 'exec*()' calls for that. This
> system call *replaces* the calling process by a new process, running a new
> program as indicated by the first argument to the exec() call. If exec()
> returns anything, it's always an error, because if it succeeds, the process
> that called it will be gone. The new process isn't really a new process,
> though, because it will have the pid of the old one. It'll just be running a
> different program ;)
> 
> The difference between the various exec*() calls is fairly subtle, and a bit
> strange from a Python point of view: they are mostly about how to pass the
> arguments: the execl*() calls take a variable number of arguments,
> whereas the execv*() calls take a pointer to a list of arguments. The
> exec*p() calls search the environment variable PATH for the name of the
> executable, whereas the other exec*() functions expect the name of the
> executable to be a full path to the program. And lastly, the exec*e()
> calls also take an argument that list the environment variables that
> should be passed to the new program, whereas the other calls pass the same
> environment as the current process has.
> 
> These two system calls are all that is necessary to create as many and as
> varying processes as you wish. None the less, the Standard C Library
> provides a few funky and helpful functions that save you a lot of time in
> the common cases:
> 
> system() is a library call that takes a single string describing the program it
> should start, and the arguments it should get. It forks() and exec()s for
> you, and then waits for the child to exit (using one of the wait() calls)
> and returns the exit code. You dont have particularly much control over how
> the string is executed, other than via the standard 'shell' commands: system
> always starts /bin/sh with your string as argument.
> 
> popen() and it's friends are similar functions, but instead of running the
> string you pass and returning the exitcode, it starts the new process with
> its standard input or standard output (or both, in the case of popen2)
> connected to the calling process (using a UNIX 'pipe', hence the first p in
> popen()). This way, the calling process can pass data to the program just as
> a user would type it on a computer, or read data that the new program writes
> to the terminal.
> 
> So, basically, if you simply want to run a command and dont care much about
> runtime influence, you usually use system(). If you need to pass it some
> data for it to work on, for example 'gzip' or some other program that works
> on the input stream, you use popen(). And if you want more control over
> things, or if you simply want your python process to do two tasks at once,
> or you wish to start a lot of programs in a specific order, or other
> complicated things, you use fork() and exec() manually.
> 
> > I'm really hoping someopne has written this up!
> 
> Well, I just did, didn't I ? ;) There are a lot more details I didn't
> mention, by the way: threads (processes that share their memory space);
> redirecting stdin, stdout and stderr somewhere specific, or to another
> process; creating pipes and sockets yourself; how to avoid deadlocks in
> popen2; shared memory; controling ttys, how to see the difference between a
> simple pipe and a real user, and how to fool programs that try to see that
> difference; UNIX session ids; setuid() and other ways to change the current
> process's permissions; And probably a lot more ;P Nevertheless, I hope this
> helps a bit :-)
> 
> If you need more info than this, either read a UNIX manualpage for one of
> those functions, or if you prefer readable material, buy a good book on UNIX
> programming. They usually start with a good explanation of processes, the
> memory layout, pipes, sockets and networking. Some even cover shared memory.
> 
> Drained-ly y'rs,
> --
> Thomas Wouters <thomas at xs4all.net>
> 
> Hi! I'm a .signature virus! copy me into your .signature file to help me spread!


Can this process manipulation be done for Windows 95 or 98?



More information about the Python-list mailing list