I have a method in my process that should be run only if the process is not in background.
How can I dynamically test if the current process is in background ?
Thanks
Here is what I use, for a program launched from a shell with job control (most of the shell, see below):
/* We can read from stdin if :
* - we are in foreground
* - stdin is a pipe end
*/
static int validate_stdin(void) {
pid_t fg = tcgetpgrp(STDIN_FILENO);
int rc = 0;
if(fg == -1) {
debug_printf("Piped\n");
} else if (fg == getpgrp()) {
debug_printf("foreground\n");
} else {
debug_printf("background\n");
rc = -1;
}
return rc;
}
If a session has a controlling terminal, there can be only process group in the foreground, and tcget/setpgrp is used for setting this process group id. So if your process group Id is not the process group Id of the foreground process group, then you are not in foreground.
It works if the shell has job control, as the link pointed by mouviciel says.
However, it is not always the case. For example, on embedded system using busybox, the shell can be configured with or without job control.
Check out Unix FAQ: How can a process detect if it's running in the background?
General answer is: You can't tell if you're running in the background.
But you can check if stdin is a terminal: if(isatty(0)) { ... }
Try to check availability of DISPLAY. There shown source code of xset command
How to check if Linux console screensaver has blanked screen
This sounds like a bad design. Can you tell us something about this method you're mentioning in your question? As mouviciel said, there's no reliable way.
One suggestion I have is to use the "foreground behaviour" by default and keep the "background behaviour" under a switch like -d (for daemon mode) or vice versa if your program usually runs in the background. One example of such usage is fetchmail.
Related
I'm writing some executables that use the Windows console, in C and C++.
I'm trying to get the console to not close after the logic of my program finishes... But not just merely pause or wait, I'd like it to become a cmd.exe command line console itself, ready to accept new prompts.
Essentially I'd like the behavior of running my program via double-clicking or drag-and-dropping to be equivalent to hitting winkey + r and running :
cmd /k "program.exe [list of drag+drop files if any]"
(While not opening a new console if run from a command-line itself.)
Is this possible at all?
Edit
I've been tinkering with this and arrived to a solution that seems to work:
std::getenv("PROMPT") will return 0 when not run from the commandline (I think anyway, not sure if that holds in all cases), and so that can be used to fork the logic depending on how the executable is run.
The following code works for me at least, in my limited experimentation with it. If it's run from the explorer, it uses it's first instance to invoke cmd.exe with parameters that lets THAT instance invoke our program again, with the original parameters.
int main(int argc, char * argv[]) {
// checks if we're in the explorer window, if so delegates to a new instance
if (std::getenv("PROMPT") == NULL) {
printf("Starting from explorer...\n");
std::string str("cmd /Q /k \"");
for (uint32 n = 0; n < argc; ++n) {
str.append("\"");
str.append(argv[n]);
str.append("\"");
if(n < argc-1)
str.append(" ");
}
str.append("\"\n");
system(str.c_str());
return 0;
}
// actual code we want to run
uint32 fileCount = 0;
for (uint32 n = 0; n < argc - 1; ++n) {
fileCount++;
printf("file %02u >> %s\n", n, argv[n+1]);
}
if (fileCount == 0) {
printf("No inputs...\n");
}
return 0;
}
So I guess conceptually, it looks like this.
____stays open_______________________ __closes when finished___
program.exe [paramList] ---> cmd.exe -+-> program.exe [paramList]
|
+-> any subsequent commands
|
etc
In my opinion, you have linked your program as a Windows console program, so it will always open a console terminal when you run it. In that case, your program is presenting the information it outputs in the console (as if the standard output had been redirected to the opened window) this means you cannot use it as a UNIX filter like dir or copy commands. (indeed, you are not in a unix system, so the console is emulated with a special windows library that is linked to your program).
To be able to run your program inside a cmd.exe invocation in a normal window terminal (as you do with the dir command --well, dir is internal to cmd.exe, but others, like xcopy.exe aren't), you need to build your program as a different program type (a unix filter command or a windowless console program, I don't remember the program type name as I'm not a frequent windows developer) so the standard input and the standard output (these are things that Windows hinerits from MS-DOS) are preserved on the program that started it, and you program is capable of running with no window at all.
Windows console program is a different thing that a windows filter program that doesn't require a console to run. The later is like any other ms-dos like command (like dir or copy) and they have an interface more similar to the unix like counterparts.
If you do this, you will be able to run your program from cmd in another window, and it will not create a Windows terminal console to show your program output.
You could simply insert the line
system( "cmd" );
at the end of your program, which will call the command prompt after your program finished executing.
However, this may not fulfil your following requirement:
(While not opening a new console if run from a command-line itself.)
Although using system( "cmd" ); will not open a new console window (it will use the existing one), it will create a new process cmd.exe, which means that you will now have 2 cmd.exe processes if your program was invoked by cmd.exe. Also, if the original cmd.exe process was invoked by your own program, then you will now have 2 processes running your program. If you now call your program again from this new command prompt, you will now have 3 cmd.exe processes and 3 processes running your program. This could get ugly very quickly, especially if you are repeatedly calling your program from a batch file.
In order to prevent this, then your program could try to somehow detect whether its parent process already is cmd.exe, and if it was, it should exit normally instead of invoking cmd.exe again.
Unfortunately, the Windows API does not seem to offer any official way for a child process to obtain the process ID of its parent process. However, according to this page, it is possible to accomplish this using undocumented functions.
Using undocumented functions is generally not advisable, though. Therefore, it would probably be better if you always called your program from a command prompt, so that it could simply exit normally.
I wrote a daemon program on Linux according to the guide at http://linux.die.net/man/1/daemonize, but the process crashed several times and I cannot find the reason. It has troubled me for a few days.
Today I happened to have read 'UNIX network Programming
volume 1, Third Edition' by W.Richard Stevens. And in this book, it shows an example to write daemon program. After reading the example, I realized 'Disassociate from the control terminal' is missing from my code.
Now my question is to daemonize a process, why we need disassociate from the control terminal? And does it related to the crash of the process? Is there any other place is missing in my code for daemonize?
Appreciate your replies.
Here is my code:
bool daemonize()
{
// http://linux.die.net/man/1/daemonize
// change working dir to root
(void) uchdir("/");
// close stdin, stderr, stdout
if (int fdnull = open("/dev/null", O_RDWR))
{
dup2 (fdnull, STDIN_FILENO);
dup2 (fdnull, STDOUT_FILENO);
dup2 (fdnull, STDERR_FILENO);
close(fdnull);
}
else
{
Log (ERR, "Failed to open /dev/null");
return false;
}
// detach from previous process group
if (setsid () == -1) /* request a new session (job control) */
{
Log (ERR, "Failed to detach from previous process group");
return false;
}
// inhibit others completely and group write
umask(027);
// it's dameonized!
return true;
}
The basic steps to deamonize a C or C++ program have already been mentioned in this question: Creating a daemon in Linux
Yes, the question there was for C and not for C++, but since the system calls you need to daemonize a program are C functions in both cases, that really does not make a difference.
I found this github repository useful, it has what you need to build a daemon:
Simple example of daemon for Linux
And here is a stack overflow thread why double fork is nessesary.
Not addressing your actual question but…
I wrote a daemon program on Linux according to the guide at [http://linux.die.net/man/1/daemonize][1],
please don't!
Programs that daemon-ize are the bane of any sensible init system and service tracker. The problem is, that after a program detaches from its controlling terminal and parent process it gets difficult to keep track about its state and regain control over it. The usual way of using PID files is prone to race conditions, which is especially bad if the intention is to automatically respawn such a process.
The use of daemon led to the development of several hacks, some outright ugly some kind of okay, but none of them being beautiful. Do yourself and the rest of the world a favour and don't daemonize your program.
I wrote a c++ program to check if a process is running or not . this process is independently launched at background . my program works fine when I run it on foreground but when I time schedule it, it do not work .
int PID= ReadCommanOutput("pidof /root/test/testProg1"); /// also tested with pidof -m
I made a script in /etc/cron.d/myscript to time schedule it as follows :-
45 15 * * * root /root/ProgramMonitor/./testBkg > /root/ProgramMonitor/OutPut.txt
what could be the reason for this ?
string ReadCommanOutput(string command)
{
string output="";
int its=system((command+" > /root/ProgramMonitor/macinfo.txt").c_str());
if(its==0)
{
ifstream reader1("/root/ProgramMonitor/macinfo.txt",fstream::in);
if(!reader1.fail())
{
while(!reader1.eof())
{
string line;
getline(reader1,line);
if(reader1.fail())// for last read
break;
if(!line.empty())
{
stringstream ss(line.c_str());
ss>>output;
cout<<command<<" output = ["<<output<<"]"<<endl;
break;
}
}
reader1.close();
remove("/root/ProgramMonitor/macinfo.txt");
}
else
cout<<"/root/ProgramMonitor/macinfo.txt not found !"<<endl;
}
else
cout<<"ERROR: code = "<<its<<endl;
return output;
}
its output coming as "ERROR: code = 256"
thanks in advacee .
If you really wanted to pipe(2), fork(2), execve(2) then read the output of a pidof command, you should at least use popen(3) since ReadCommandOutput is not in the Posix API; at the very least
pid_t thepid = 0;
FILE* fpidof = popen("pidof /root/test/testProg1");
if (fpidof) {
int p=0;
if (fscanf(fpidof, "%d", &p)>0 && p>0)
thepid = (pid_t)p;
pclose(fpidof);
}
BTW, you did not specify what should happen if several processes (or none) are running the testProg1....; you also need to check the result of pclose
But you don't need to; actually you'll want to build, perhaps using snprintf, the pidof command (and you should be scared of code injection into that command, so quote arguments appropriately). You could simply find your command by accessing the proc(5) file system: you would opendir(3) on "/proc/", then loop on readdir(3) and for every entry which has a numerical name like 1234 (starts with a digit) readlink(2) its exe entry like e.g. /proc/1234/exe ...). Don't forget the closedir and test every syscall.
Please read Advanced Linux Programming
Notice that libraries like Poco or toolkits like Qt (which has a layer QCore without any GUI, and providing QProcess ....) could be useful to you.
As to why your pidof is failing, we can't guess (perhaps a permission issue, or perhaps there is no more any process like you want). Try to run it as root in another terminal at least. Test its exit code, and display both its stdout & stderr at least for debugging purposes.
Also, a better way (assuming that testProg1 is some kind of a server application, to be run in at most one single process) might be to define different conventions. Your testProg1 might start by writing its own pid into /var/run/testProg1.pid and your current application might then read the pid from that file and check, with kill(2) and a 0 signal number, that the process is still existing.
BTW, you could also improve your crontab(5) entry. You could make it run some shell script which uses logger(1) and (for debugging) runs pidof with its output redirected elsewhere. You might also read the mail perhaps sent to root by cron.
Finally I solved this problem by using su command
I have used
ReadCommanOutput("su -c 'pidof /root/test/testProg1' - root");
insteadof
ReadCommanOutput("pidof /root/test/testProg1");
I am writing a small shell program that takes a command and executes it. If the user enters a not valid command the if statement returns a -1. If the command is correct it executes the command, however once it executes the command the program ends. What am I doing wrong that is does not execute the lines of code after it? I have tested execvp( command.argv[0], command.argv) with ls and cat commands so I am pretty sure it works. Here is my code.
int shell(char *cmd_str ){
int commandLength=0;
cmd_t command;
commandLength=make_cmd(cmd_str, command);
cout<< commandLength<<endl;
cout << command.argv[0]<< endl;
if( execvp( command.argv[0], command.argv)==-1)
//if the command it executed nothing runs after this line
{
commandLength=-1;
}else
{
cout<<"work"<<endl;
}
cout<< commandLength<<endl;
return commandLength;
}
From man page of execvp(3)
The exec() family of functions replaces the current process image with
a new process image
So your current process image is overwritten with the image of your command! Hence you need to use a fork+exec combination always so that your command executes in the child process and your current process continues safely as a parent!
On a lighter note I want to illustrate the problem with a picture as a picture speaks a thousand words. No offence intended :) :)
From the documentation on exec
The exec() family of functions replaces the current process image with a new process image. The functions described in this manual page are front-ends for execve(2). (See the manual page for > execve(2) for further details about the replacement of the current process image.)
If you want your process to continue, this is not the function you want to use.
#Pavan - Just for nit-pickers like myself, technically the statement "current process is gone" is not true. It's still the same process, with the same pid, just overwritten with a different image (code, data etc).
I'm working on a init for an initramfs in C++ for Linux. This script is used to unlock the DM-Crypt w/ LUKS encrypted drive, and set the LVM drives to be available.
Since I don't want to have to reimplement the functionality of cryptsetup and gpg I am using system calls to call the executables. Using a system call to call gpg works fine if I have the system fully brought up already (I already have a bash script based initramfs that works fine in bringing it up, and I use grub to edit the command line to bring it up using the old initramfs). However, in the initramfs it never even acts like it gets called. Even commands like system("echo BLAH"); fail.
So, does anyone have any input?
Edit: So I figured out what was causing my errors. I have no clue as to why it would cause errors, but I found it.
In order to allow hotplugging, I needed to write /sbin/mdev to /proc/sys/kernel/hotplug...however I ended up switching around the parameters (on a function I wrote myself no less) so I was writing /proc/sys/kernel/hotplug to /sbin/mdev.
I have no clue as to why that would cause the problem, however it did.
Amardeep is right, system() on POSIX type systems runs the command through /bin/sh.
I doubt you actually have a legitimate need to invoke these programs you speak of through a Bourne shell. A good reason would be if you needed them to have the default set of environment variables, but since /etc/profile is probably also unavailable so early in the boot process, I don't see how that can be the case here.
Instead, use the standard fork()/exec() pattern:
int system_alternative(const char* pgm, char *const argv[])
{
pid_t pid = fork();
if (pid > 0) {
// We're the parent, so wait for child to finish
int status;
waitpid(pid, &status, 0);
return status;
}
else if (pid == 0) {
// We're the child, so run the specified program. Our exit status will
// be that of the child program unless the execv() syscall fails.
return execv(pgm, argv);
}
else {
// Something horrible happened, like system out of memory
return -1;
}
}
If you need to read stdout from the called process or send data to its stdin, you'll need to do some standard handle redirection via pipe() or dup2() in there.
You can learn all about this sort of thing in any good Unix programming book. I recommend Advanced Programming in the UNIX Environment by W. Richard Stevens. The second edition coauthored by Rago adds material to cover platforms that appeared since Stevens wrote the first edition, like Linux and OS X, but basics like this haven't changed since the original edition.
I believe the system() function executes your command in a shell. Is the shell executable mounted and available that early in your startup process? You might want to look into using fork() and execve().
EDIT: Be sure your cryptography tools are also on a mounted volume.
what do you have in initramfs ? You could do the following :
int main() {
return system("echo hello world");
}
And then strace it in an initscript like this :
strace -o myprog.log myprog
Look at the log once your system is booted