I wrote a shared library in C++ on Linux, which contains a function f(). This library is used from multiple programs, calling the function.
Now I would like to conduct some debugging with that function, while calling it from program A. When calling it from any other program, it should fail, as long as I debug the function. After I do not have complete control over when the other programs are run, I would like to add an exception, which stops every program except program A when calling function f during Debug sessions.
How could I solve that?
The only way I can think of is by checking the information of the currently running process. You can get the pid by calling the getpid() function. All the information on all processes on a linux system can be found in the /proc/<pid> directory. When calling function f, you can check this information and decide whether to throw an exception or not.
There isn't a way that can't be gotten around. If another process has PTRACE on yours, it can make anything at all happen in your process, so any check you perform to try to make the function not work can be disabled.
I want to change the timezone for Linux system. I know there are many ways.
One way is to use tzset() function and another is to call 'timedatectl' command from 'popen()' function.
I am using second approach i.e, using "popen()".
I just want to ask is it a good programming practice to use "popen()" in your code?
Also, I am carefully calling "pclose()" for every "popen()".
There is nothing wrong about popen in general, if you really need a child process to do a specific job for you.
popen creates a pipe allowing you to either read the output (what it wrote to stdout) of the child process or write input to its stdin - but not both at the same time.
If you are not interested in either option, you possibly might prefer calling system instead (however, system will wait for the process to terminate, in contrast to popen - pclose waits for).
But why would you want to create a separate process, if you can do the same job by simply calling an ordinary function (system call or not)? You are creating a lot of overhead using a process then (process must be initialised and hooked into OS, it needs its own memory for executable code, heap and stack, ...)!
It gets a little more complicated, if the job in question requires a considerable amount of time and you cannot afford to wait for the function to complete, but need to do some other stuff. However, in such a case, I'd rather create a thread only and again call the function from there...
popen() invokes a shell to run the command which is an extra unnecessary layer of indirection. Plus there are all sorts of security pitfalls, for instance, you don't have control over the environment - or which shell actually gets invoked.
I'd say it's fine for prototypes and proofs of concept, but for production code you should use fork(), one of the execs and pipes for IO.
EDIT
If there is a function equivalent to doinf something by invoking a command, always use that first. For example, if you can achieve what you want with tzset(), always use that in preference to spawning a new process.
I'm a little new to it and I don't understand the threading term completely yet, Allthough I know how to make a thread and run programs with multiple threads. What I'm wondering about is that when you create a dll file (c++) and inject it into a process (lets say gamehacking) for instance. Would you need to create threads in the dll file, or is that not going to work? After my understanding the main thread will be running from the host process right? Or how does it work?
Well it depends on what you are planning to achieve using the DLL. If that particular DLL has some static functions / utility class, that just takes an input, doing some calculations / processing and produce an output, then there is no need of threading here.
But if that DLL is going to listen on a socket or write to a file or do the actual work that going to need some parallelism, then you might want to create threads inside that DLL.
Basically you must need to understand, what is that task, that is accomplished by this DLL. A DLL can be linked during compile time as a normal library or it can be loaded dynamically in run-time based on your need / use-case.
To answer your question,
Would you need to create threads in the dll file, or is that not going to work?
Ans : Not always. You need to create thread to accomplish some task. That being said, this is not the case always. It is perfectly feasible to run a DLL inside a process, without having any threads.
After my understanding the main thread will be running from the host process right? Or how does it work?
Ans : That's right. Any process you run, there will be one thread by default. If your application is simple enough to be processed by a single thread, then it is a blessing. Keep up with it :)
Since you are specifically refering to injecting the DLL, I have some input for you additionally to what has already been said.
First, let's make sure that the concepts of threads, processes and modules are clear.
A thread is basically the immediate environment in which code runs. Things like the current state of processor registers and stack variables (e.g. your local variables in functions, in most cases, but also where in the code the execution currently stands) belong to a thread. There are also other resources which often have thread-affinity, such as windows. It depends a lot on the resource in question whether and what kind of thread affinity they have.
Let's assume you write a simple hello world program. It will run in one thread which goes through your program from the beginning to the end and print "Hello World". Now let's assume you would want to write a program which slowly writes "Hello World", one character per second, but in the meantime download a file. Then you could create a second thread and have one thread output "Hello World" slowly, and one thread download the file. This means execution can happen in parallel, with different local state - one thread is currently inside your printHelloWorld function and one thread is inside downloadFile.
A process is basically a container for one or more threads. It bundles them together in a shared environment which uses the same virtual memory (this means that for example global variables in your code would be accessible from all threads, but this would require careful synchronization to avoid race conditions) and shares resources such as file handles the threads in the process create. So, your hello-world-and-download program from before would have 2 threads in 1 process, sharing the console for example, and being seen in the task manager as one entity.
A module is a file which contains executable code (in most cases, that is) and is loaded into a process. Usually, in a process there are one EXE file and several DLL files loaded as modules. DLL files and EXE files are technically almost the same, but EXE files are meant to be the basis from which a processes starts, and DLL files are meant to be libraries exporting certain functions which can be used by other modules. Since I said modules are loaded into processes, it means that a module is accessible by all threads in the process, and it doesn't have thread-affinity by its own - in our previous example, when the second thread downloads the file, it may be calling into a HTTP-networking DLL, whose code would then run in the second thread. There is a number of modules which is loaded automatically into each process by Windows, and others are probably loaded by certain features of your compiler.
OK, so, back to your question:
Would you need to create threads in the dll file[...]?
Per se, using a DLL has nothing to do with whether you need to create new threads or not. It depends on what you want to do - if you need to do some time-consuming task in parallel to whatever other code is running, then you would need to create a new thread for it, otherwise there is no need.
[...]or is that not going to work?
As said, you can create new threads if you want (it will work), but it's not a necessity coming with using a DLL.
After my understanding the main thread will be running from the host process right?
The main thread of the host process will of course be in the host process. (Although there is technically no "main thread", since it's perfectly valid to have the first thread in a process create a second one and then terminate, so only the second one would be running anymore, you usually do have the first thread live through the whole lifetime of the process, and you can probably call it the "main thread" in this case.) In which module the currently-running code is located, though, will depend on what the thread is currently doing.
Let me get back to the matter of "injecting": The previous answers appear to have assumed a more "normal" environment where your DLL is just linked to the program and meant to be loaded by it. In this case, your DLL's initialization routine (which is automatically run when a module is loaded into a process) would just be run in the "main thread", probably before the actual work of the process begins.
However, things are a bit different when you inject a DLL. It depends on how you do the injection:
If you inject the DLL by modifying the imports table of the host EXE, then your DLL will be loaded the "normal" way I just said. So you can expect your initialization routine to run during the process' startup, in the main thread.
If you inject the DLL by using the AppInit_DLLs registry key, it would be the same.
Same thing if you inject the DLL by starting the host process suspended, then writing a stub to load the DLL into the processes' memory and using SetThreadContext to point the instruction pointer to it.
If you inject the DLL through means of remotely calling LoadLibrary inside the target process, using CreateRemoteThread, then however, as the name suggests, you are creating a new thread inside the process. In this thread, LoadLibrary will load your DLL and also call your initialization routine, so in this case, your initialization routine would indeed run in a new thread other than the "main thread".
Every process has at least one thread. When that process starts, it's possible to link a bunch of functions, or a library, to the memory space of that process. That's what a dll is. The advantage compared to linking directly to the binary is the library only has to exist in one place in the file system and one place in memory while being used by multiple processes. It's a linking technique, similar to how .so files are used in Linux. It has nothing to do with threading.
Would you need to create threads in the dll file, or is that not going to work?
There wouldn't be any point loading a DLL that didn't contain code that would be run. That said, there are several ways the DLL code might get run:
when the DLL is loaded it gets a chance to run some initialisation code
during initialisation, it might:
start one or more threads, which can keep running - perhaps watching for some event that triggers some action on their part
register for callbacks from the OS or application, such as setting up signal handlers, keystroke handlers, any type of event handler....
it might contain functions that the program will look for dynamically and run, mistaking your DLL code for the original versions of those functions that the program came with
Which of these suits your needs depends entirely on what your DLL is trying to achieve, and what's technically necessary to achieve it. For example, if watching for some memory to have specific content, then modifying it further, it might suffice to have a function in your DLL called by an OS alarm service, resetting itself to go off again later if the triggering memory content is not found. But, the trigger might be existence of a file, or shared memory service, a socket being created etc..
After my understanding the main thread will be running from the host process right? Or how does it work?
Yes - threads started within a process - including any DLL initialisation routines - are also within the process. There are some library functions that may create other processes - such as fork, popen, system - that may contain their own threads.
The scenario is here:
If a program is executed, at runtime assume it will link to some DLL files, the (master) program/process may or may not create multi-threading function-calls to the functions in DLLs.
Then is there a way that the DLL, of cause besides parameter-passing, can tell whether the master process, who calls the functions within the DLL at runtime, is in a single or multi-thread manner (For instance, by Open MP)?
You can check and compare the current thread ID to detect calls from different threads. You could also implement a DLLMain() function that gets called for every started and terminated thread. I'm pretty sure you can also retrieve a handle to the current process and enumerate the threads running in it. Only the first version will actually tell you if your code is run from different threads though, I think that e.g. WinSock will create a thread for you, even though your program is single-threaded otherwise.
BTW: Consider adding win32api tag and removing C++ tag.
does anyone know of a way to define some behavior to be called on every function or line in a C++ program?
i would like to validate my software by essentially causing it to exit at certain points in the application, and make sure that the next boot up of the process can handle recovering in all of the points at which the process previously died.
essentially, im looking for a way to script failures in C++, so that I don't have to define points using some macros, and just tell the application to essentially cycle through all of these "Death points" and confirm that the process can recover from every single one of them, doing all of this in an automated fashion.
i could easily create a macro called DEATH_POINT() and it could essentially check whether or not it should exit the process or not, but I was hoping there was something a bit more elegant than having a bunch fo macros sitting in the code.
Depending on the compiler you are using there is probably a way to have it call a function automatically every time it enters a function. That might do what you need.
For Visual Studio there's info here: http://msdn.microsoft.com/en-us/library/c63a9b7h.aspx
And the top answer here has information about doing it in GCC: Automatically adding Enter/Exit Function Logs to a Project
I suggest you go one step beyond that, and test on an instruction-by-instruction basis, rather than entire expressions, statements, or lines.
You should be able to use the Debugger API to set a breakpoint programmatically (you'll need a helper process though, IIRC). The process will be suspended when the breakpoint is reached. And you can receive an event when the breakpoint is hit and terminate the process.