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What is a practical relevance of signals in C programs ?
Where do we need signals. I am getting confused and feeling traped in this topic, Who generates signals ?
Signals are a lightweight way for processes to communicate with each other asynchronously; as such, it shouldn't be a surprise that processes generate signals.
Signals are in C because signals were how you communicated with a process in the original version of UNIX.
They are a fairly simple way of allowing your process to respond to external requests such as "re-read my config file", for example.
The K&R chapter is a good intro, but read the C standard to see the minimum for what is defined. In particular, the only portable thing you can do in a signal handler is either abort the process, or set an atomic flag which your main thread of execution will check in due course. In practice, people do more complicated things in signal handlers such as making system calls.
Signals provide a way of supporting asynchronous interrupts in a C program. That said, the C spec is almost completely void of specified behavior for signals, leaving them essentially entirely up to the implementation, so writing portable code that uses signals is almost impossible. You need to carefully read the documentation for your implementation to see what exactly signals can do.
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I know that pthread_kill requires Signal as a parameter but I am just curious about that what will be happen if I will give 0 as a signal (no SIGUSR1 or any other) .
Thread will be still killed in above situation ?
I am trying to learn these stuffs I am putting my hands in threading in C.
It will happen exactly what is described in the man page :
If sig is 0, then no signal is sent, but error checking is still performed.
That means it will act like pthread_kill has send the signal, but without sending it. It's usefull when you try to check wether a particular PID respond to signal or not. It's a "dry-run".
Edit : Well, out of habit I've writted "PID", but it's more the "TreadId" than any other things.
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As we know that exec family function calls eventually calls execve() which is a system call and remaining are library functions. So, typically, whatever implications applies on execve() are true for other exec*() functions as well.
I would like to know if, during execution of execve(), all signals are blocked until it succeed, or if there is a way to pass signal to that pid which corresponds to exec? (I know it does not return on success and further execution of calling function does not happen)
I am not sure I got your question right, so feel free to correct me if I am wrong.
But, basically, yes, system calls can be considered as 'atomic' from the process point of view. So, once the execve() system call is started, only the kernel has the hand on it and it won't release the process until running the new command or failing with an error code or raise the SIGKILL signal (as SIGKILL is the only unblockable signal).
But, once the execve() spawned a new process (and returned from the kernel), it is perfectly interruptible with any signal.
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man for int kill(pid_t pid, int sig); says:
If pid equals -1, then sig is sent to every process for which the
calling process has permission to send signals, except for process 1
(init), but see below.
Does this mean that if my program is run with root permissions and it accidentally (due to memory corruption or a hack) provides -1 as pid argument - this will cause a complete DoS for the entire system?
If so, is it recommended to always perform a double check for the pid argument value before calling this potentially disasterous call? (just sayin')
Does this mean that if my program is run with root permissions and it
accidentally (due to memory corruption or a hack) provides -1 as pid
argument - this will cause a complete DoS for the entire system?
Yes, such a scenario is possible. But the chances of it happening is very less. Because no program that run with root permissions would do such a thing. If a malicious user/binary has somehow got gained root privilege, then sending signals is just one of the problems.
If so, is it recommended to always perform a double check for the pid
argument value before calling this potentially disastrous call?
That's just super paranoid thinking. There are thousands of ways to do disastrous activities.You might as well worry about:
What if there's not malicious daemon, ran at system startup, does:
kill(-1, SIGKILL);
How do you know if library function you make wouldn't call reboot(2) and reboot your system?
etc.
Besides, PIDs are not just user provided values that need to be sanitized. PIDs are mostly values acquired within the program using system calls or library calls. So the chances of "accidentally" using -1 is zero. Basically, you someone/program has root privilege and decided to screw your system then there's not much you can do.
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I need to do some operation at regular intervals in my thread. What is the best method to do this?
Like if, i have a local socket communication between linux deamon and a android application. In this case, if i want to send data periodically to the android app from deamon, how can i proceed?
If you use Linux why not try sleep() or usleep() functions of unistd.h?
sleep(5);
Will pause the thread for five seconds and then resume execution.
sleep(sec)
The sleep() function shall cause the calling thread to be suspended from execution until either the number of realtime seconds specified by the argument seconds has elapsed or a signal is delivered to the calling thread and its action is to invoke a signal-catching function or to terminate the process.
usleep(usec)
usleep() function suspends execution of the calling thread for (at least) usec microseconds.
Emphasis mine.
Taken from here and here.
You should use Simple Signals - C programming and alarm function if you want truely accurate timing. If you don't do this, using sleep() etc will eventually result in your timer shifting due to the overhead of your code, the OS, etc.
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I don't really understand re-entrant code.
Can somebody show me an algorithm for re-entrant code so I can look at it? An explanation of how it works would be nice too. Thanks.
If you, as a developer of some function, can be sure that your function is not called during its execution time, then you may not to pay attention to make it reentrant.
However if your function can be called from different task, threads, or just by interruption signal, there can be a situation, when the function reenters itself during its execution, if it is not reentrant-compliant , not protected from multiple entrances it may lead to unpredictable behaviour, such as data corruption\overwriting or corruption of chronology of the actions in the function.
Here may be a number of approaches. Sometimes it is enough to write a function, for example, working only with copies of the external variables. However sometimes using of mutexes is inevitable. When mutexes are used, the caller of the function will be waiting for mutex unlocking. You just place mutex locking before the critical section and mutex unlocking after the critical section. Whole function body can be wrapped between lock and unlock couple.