I have this in my program:
execv (programname, (char **)argv);
I'm not sure if the command is actually being executed correctly. How can I find out? Is this being run in the background?
I highly recommend getting a book that relates to the task you're trying to do. It's going to be a really long road if you ask a new question on SO on every step of the way. We love to help, but sometimes books are better.
Advanced UNIX Programming is an excellent one that contains a full sample of a shell, including pipelines. In fact, the example programs are available for download for free (but I recommend picking up a copy of the book anyway).
Since execv replace the current process, the command will be run on the same state as the parent process.
One way to know if your command is executed is to make the command print something on the console, if it is possible.
I believe execv() is supposed to overlay the current process with "programname". If you want to run a program in a separate process, you want fork() or system() -- I don't believe the latter is "standard" but it seems to be fairly ubiquitous.
From man page of execv.
RETURN VALUE
If any of the exec() functions returns, an error will have occurred. The return value is -1, and errno will be set to indicate the error.
So, if you get a return value, something went wrong.
Related
i want to do a simple program that will run another program (easy task in C++), hook the process and, when the program is closed, run another program (needed to sync a file modified by the first program).
The second program prompt out a popout asking a simple Yes/No buttons.
There is any way?
I am writing an article about hooking : http://ntvalk.blogspot.nl/2013/11/hooking-explained-detouring-library.html
It describes the probably most common methods for hooking under Windows. (jmp/vtable/etc)
gldraphael already described a method of running another program.
Check this out: http://faq.cprogramming.com/cgi-bin/smartfaq.cgi?answer=1044654269&id=1043284392
It lists the various ways you can run another program.
Recently, i've got interested in making a front-end for command-line program.
I guess there's two way to do it.
First one is just including source code and calling main proc with arguments
(Of course, there should be some changes in source code).
Second one, which is there's no source code and just program, is just executing program internally then reading the command line with APIs.
Though I well know about the first solution, i don't know what APIs is needed to do the second solution.
I'm talking about the APIs that get a command-line string or something like that.
See this question for information on how to run an external application; basically, you need to call CreateProcess function. I'm not sure what you mean by "reading the command line", I suppose you mean reading the output of an executed program? As for capturing an external application's output, there's already another question asking for that, you will probably find this answer most helpful.
here is a codeProject project that I have used and can handle command line arguments for you (in the setup you describe). If you are not happy with it, you can use the direct WinApi calls using CommandLineToArgvW.
Enjoy!
I am new in Linux and system programming .
I Want to write a c program which finds processes whose cpu% usage are more than a specific given value and sends them to background.
anybody can help me !
I really appreciate it
I'm fairly sure that what you're asking is that you want to detect if a process is using X amount of CPU and if so, take it off the CPU for a while. There's a piece of software already that does this: It's called the kernel. I'm not aware of any way to programatically take another process off CPU unless that other program supports an external interface to reduce its load.
Most likely what you really want to do is configure the nice and other scheduler parameters of the running process so the kernel is more like to to take it off CPU when another program needs to do work.
But what underlying problem are you really trying to solve here? Maybe if you tell us that we can offer an alternate solution.
Please look at source code of process managament utilities like:
htop
top (standard unix command)
ps (standard unix command)
IMHO, You can't.
Background management ensures the shell. So, the & is interpreted for example by /bin/bash command. When pressed CTRL-Z, the kernel stopping your current fg-job, and again by your shell you can send it into background.
Youre looking for the way how to remote control the shell what running some program in fg. I don't know any 'remote-controling' way.
Ofc, here are alternative solutions, for example:
use the screen command, and you can recall the specific screen into your terminal, and can manually send process into bg.
or you can use some screen-sharing utility, to overtake a specific terminal and CTRL-Z, bg
or, you can patch bash and adding remote control functionality. ;)
or, here is something what i don't know. ;) - hm, maybe trap some user-signal handling code in the /etc/profile?
You can read a bit about here: http://en.wikipedia.org/wiki/Process_group
Honestly, after a half hour of thinking I don't get any idea why you want remotely (from the another terminal - by its PID) send some processes from the fg into the bg. Give me no sense.
Can you please tell, what you want achieve?
You probably want to reduce process priority, but I not sure it's good idea.
We send process to background generally to free shell's prompt.
The "+" means that the program "is in the foreground process group". I don't believe, however, that this state at all affects the process's scheduling.
However, you can change it with tcsetpgrp.
From the man page: "The function tcsetpgrp() makes the process group with process group ID pgrp the foreground process group on the terminal associated to fd, which must be the controlling terminal of the calling process, and still be associated with its session. Moreover, pgrp must be a (non-empty) process group belonging to the same session as the calling process."
By my reading, you just call this function and make the shell (or some other program) be the foreground process.
I know that in a DOS/Windows application, you can issue system commands from code using lines like:
system("pause");
or
system("myProgram.exe");
...from stdlib.h. Is there a similar Linux command, and if so which header file would I find it in?
Also, is this considered bad programming practice? I am considering trying to get a list of loaded kernal modules using the lsmod command. Is that a good idea or bad idea? I found some websites that seemed to view system calls (at least system("pause");) in a negative light.
system is a bad idea for several reasons:
Your program is suspended until the command finishes.
It runs the command through a shell, which means you have to worry about making sure the string you pass is safe for the shell to evaluate.
If you try to run a backgrounded command with &, it ends up being a grandchild process and gets orphaned and taken in by the init process (pid 1), and you have no way of checking its status after that.
There's no way to read the command's output back into your program.
For the first and final issues, popen is one solution, but it doesn't address the other issues. You should really use fork and exec (or posix_spawn) yourself for running any external command/program.
Not surprisingly, the command is still
system("whatever");
and the header is still stdlib.h. That header file's name means "standard library", which means it's on every standard platform that supports C.
And yes, calling system() is often a bad idea. There are usually more programmatic ways of doing things.
If you want to see how lsmod works, you can always look-up its source code and see what the major system calls are that it makes. Then use those calls yourself.
A quick Google search turns up this link, which indicates that lsmod is reading the contents of /proc/modules.
Well, lsmod does it by parsing the /proc/modules file. That would be my preferred method.
I think what you are looking for are fork and exec.
I have a C++ windows service that sometimes crashes when I call the system() function to shell out a command. I've run the exact text of the command in the windows command-line, and it runs just fine, but for some reason it fails when system() is run.
To make matters worse, I can't seem to get any information as to why system() is failing. There doesn't seem to be an exception raised, because I'm doing a catch(...) and nothing's getting caught. My service just stops running. I know that it's the call to system() that is failing because I've put logging information before and after the call, and anything after just doesn't log anything.
So, is there a different way that I can shell out my command? At the very least, something that will give me some information if things go wrong, or at least let me handle an exception or something.
I belive system() is technically part of the C standard library, and therefore wouldn't throw exceptions. You should be able to check the return code or the ERRNO variable to get some information about what happened. This MSDN link has some information about the possible return codes on Windows.
I've also seen system() fail for other external reasons, such as virus scanners, so you might investigate that as well.
I don't know of a better way to run shell commands, but I could be wrong.
EDIT: If it still just seems to crash for no reason, you might try using process monitor to see what is going on at a lower level. Since the output from process monitor can be kind of overwhelming, a trick I like to use is to add a statement right before the call to system() to your program to open a nonexistent file like "C:\MARKER.TXT" or something, then you can search the process monitor output for the name of the file and look at the entries right afterward that may have something to do with the problem.
Ordinary catch() will not catch fatal exceptions (e.g. segmentation fault). You have to use structured exception handling. Better yet, enable post-mortem debugging; this article explains how you can enable post-mortem debugging of services.
You could use fork/exec, but I think that is what the system is doing.
I think your problem could be the user account associated on your service.
Either there's an environment problem (missing entry in path) or the account the service is using to run doesn't have the rights to exec whatever you're trying to run.
Run services.msc and look at the properties for your service.
On the logon page, as a test, change the setup so it uses your account to run the service. If it succeeds, you know what the problem is.
Another thing to look at is the path while inside the service. Use getenv( "PATH" ) and see if a path you might be reliant on is missing.
Hope this helps...
I ended up using CreateProcess. It's been working out so far.