I have a program that calls a shell script tool that I made that goes through a directory and zips up files and gets the checksum value and calls some other tools to upload the files. The operation takes roughly 3 to 4 minutes.
I call the script like this:
int result = system("/bin/sh /path/to/my/script");
I've also got the same result by using the exec() family of functions:
int child = fork();
if(child == 0) {
execl( "/bin/sh", "sh", "/path/to/my/script", (char*)0 );
}
I know with exec you can redirect output to the parent program so it can read the output of the command line tools, but other than that when should you use system as opposed to exec?
Ignoring for the time being that use of system is portable while use of exec family of functions is not portable...
When you combine use of exec family of functions with other POSIX functions such as pipe, dup, wait, you get a lot more control over how to pass data between the parent process and the child process.
When you don't need any of those controls, i.e. you just want to execute a command, then using system is preferable, IMO.
The first system call in your question will do the same, what you are doing in the next piece of code (fork and execl)
From documentation:
The system() library function uses fork(2) to create a child process
that executes the shell command specified in command using execl(3)
http://man7.org/linux/man-pages/man3/system.3.html
Related
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 have an interface where i use to execute the mml command in my solaris unix like below:
> eaw 0004
<RLTYP;
BSC SYSTEM TYPE DATA
GSYSTYPE
GSM1800
END
<
As soon as i do eaw <name> on the command line.It will start an interface where in i can execute mml commands and i can see the output of those commands executed.
My idea here is to parse the command output in c++.
I can do away with some logic for parsing.But to start with How can get the command to be executed inside c++ ? Is there any predefined way to do this.
This should be similar to executing sql queries inside c++.But we use other libraries to execute sql queries.I also donot want to run a shell script or create temporary files in between.
what i want is to execute the command inside c++ and get the output and even that in c++.
could anybody give me the right directions?
You have several options. From easiest and simplest to hardest and most complex to use:
Use the system() call to spawn a shell to run a command
Use the popen() call to spawn a subprocess and either write to its standard input stream or read from its standard output stream (but not both)
Use a combination of pipe(), fork(), dup()/dup2(), and exec*() to spawn a child process and set up pipes for a child process's standard input and output.
The below code is done with the sh command. This redirects stdout to a file named "out" which can be read later to process the output. Each command to the process can be written through the pipe.
#include <stdio.h>
int main()
{
FILE *fp;
fp = popen("sh > out", "w");
if (fp) {
fprintf(fp, "date\n");
fprintf(fp, "exit\n");
fclose(fp);
}
return 0;
}
I have a simple C/CPP process running on a linux system. This is a.out.
Another process is capable to start a.out inside its code. This is b.out.
What code do I need inside a.out to understand that it is executed from the command line?
Eg ./a.out
Is there a way a process to know if it started from the cmd or started from another process?
You can't find out in general whether a program was started "from the command line" (by a user's explicit command), but you can find out whether its standard input and output are talking to a terminal/command window:
#include <unistd.h>
isatty(fileno(stdin))
and stdout return whether standard input/standard output are terminals.
If you need to know what process starting your program, use the getppid system call to get the parent's process ID (ppid), then read the pseudo-file /proc/ppid/cmdline.
You can check its parent task ID, using getppid()
You can do multiple things, but none will be 100% reliable:
isatty(0) to check whether standard input is a TTY terminal,
check for the parent task ID (getppid()), then lookup the parent's PID to match it against its executable's path (using whatever you want. a call to ps and some parsing could do, or have fun using /proc/)
you could also just have a look at the environment variables set up. do a printout of all the values contained in the env. To do that, either use the extern environ variable:
extern char **environ;
or modify your main() prototype to be:
int main(int ac, char **av, char **environ)
I would set an environment variable, in the parent process, to some value (say the parent pid), and have the child process check for it.
It is unlikely that a shell user would set this variable (call it something unlikely to name-clash), so if this variable is set to the expected value, then you know that it is being started from the parent process.
You can check whether its standard input is a terminal:
if(isatty(0)) { ... }
In short: you can't doing it directly.
In long: look you can check the getppid() value and compare it with the bash PID orb.out PID
TO search for a process inside the process table with Known PID with C you can do this:
1) get the PPID of a.out and search with this value in /porc and then if you find the folder check the cmdline file and check if this process is b.out or shell process.
2) you can deal with sysctl system call and dealing with kernel param's(you can google it)
3)
pid_t ppid = getppid();
system("pidof bash > text.in");
the system will get the pid of any shell process and write the result to text.in file
it contains all bash PID's space separated you can compare this values with getppid() value.
Good Luck.
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
I've got a GUI C++ program that takes a shell command from the user, calls forkpty() and execvp() to execute that command in a child process, while the parent (GUI) process reads the child process's stdout/stderr output and displays it in the GUI.
This all works nicely (under Linux and MacOS/X). For example, if the user enters "ls -l /foo", the GUI will display the contents of the /foo folder.
However, bash niceties like output redirection aren't handled. For example, if the user enters "echo bar > /foo/bar.txt", the child process will output the text "bar > /foo/bar.txt", instead of writing the text "bar" to the file "/foo/bar.txt".
Presumably this is because execvp() is running the executable command "echo" directly, instead of running /bin/bash and handing it the user's command to massage/preprocess.
My question is, what is the correct child process invocation to use, in order to make the system behave exactly as if the user had typed in his string at the bash prompt? I tried wrapping the user's command with a /bin/bash invocation, like this: /bin/bash -c the_string_the_user_entered, but that didn't seem to work. Any hints?
ps Just calling system() isn't a good option, since it would cause my GUI to block until the child process exits, and some child processes may not exit for a long time (if ever!)
If you want the shell to do the I/O redirection, you need to invoke the shell so it does the I/O redirection.
char *args[4];
args[0] = "bash";
args[1] = "-c";
args[2] = ...string containing command line with I/O redirection...;
args[4] = 0;
execv("/bin/bash", args);
Note the change from execvp() to execv(); you know where the shell is - at least, I gave it an absolute path - so the path-search is not relevant any more.