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When you exit a C application, is the malloc-ed memory automatically freed?
Pretty much the title. Do (modern) OSs automatically reclaim heap-allocated memory on program termination? If so, is freeing the memory a strict must or more of a just-follow-the-damn- standard-you-fool kinda thing?
What is the worst thing than can happen if I do not explicitly free the memory except that I end up wasting it while the application is in run?
When an application terminates, all memory will be reclaimed by the OS. The reason people preach memory management techniques is because so long as the app is running, any memory it's asked for will remain in its posession (even if it's no longer being used). Unless, of course, you were nice enough to call free.
if the app terminates the os will clean up the memory and resources.
if you do not clean up memory with free( ) then heap-allocated memory will be used up as you request more and more. if it continues, the os will start using swap/page file to allocate. it u still continue os will grind down to a halt, or terminate the app with some kind of error
Does OS reclaim memory on application exit in C? Yes.
If you don't free memory: your application may run out of memory & crash.
Freeing memory before exiting is optional because the o/s will clean up for you.
Freeing unused memory while running is a good idea; it prevents the application from running out of memory (and having to exit). The difficulty is often working out which memory is unused.
Yes. But if your program is a long running program, you not only crash your program eventually, but as the time progresses, you will also slowdown the system, assuming you use too much memory.
Related
I am learning to code in C. I came across the term 'memory leak'. So, I want to ask, if I wrote a simple program in C, in which malloc() is used and if I didn't free it (though I know freeing the dynamically allocated memory is a good programming practice).
Will that memory leak would be there in the system permanently?
Will the OS never use that memory again as it is sort of lost maybe?
Just consider the case where small amount of memory(100 bytes or so) is allocated, which we do while practicing.
I'm asking this because I'm running the same program multiple times for debugging, are those memory leaks harmful?
Or the OS detects the memory leak and restore it?
Any help regarding the above and related topics is appreciated.
General-purpose operating systems are designed to protect against all sorts of issues caused by misbehaving programs. That includes managing memory. The operating system maintains its own records about what memory has been provided to each process, and it reclaims that memory when the process terminates (and no other process is using it, as occurs with various shared memory).
Special-purpose “embedded” operating systems might not provide this function.
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In C program, if there are dynamically allocated memories remained not freed after the execution of the program exit with exit(100);, do we get memory leaks problem? For example:
int main (void) {
char str1[] = "Hello World"
char *str2;
str2 = malloc(strlen(str1 + 1));
if (str2)
exit(101); // memory leaks?
free(str2);
return 0;
}
Not under modern operating systems, no. The OS automatically collects all the memory when the process dies.
In fact freeing memory can actually be detrimental for the performance if the program is exiting anyway. The reason is that calling free sometimes involves a lot of work - updating a lot of structures, touching cache lines etc. By simply exiting you don't do all this userspace nonsense and the OS takes care of actually unmapping your data.
All dyanmically allocated memory allocated using malloc needs to be freed explicitly by calling free. While your program keeps running the memory unallocated this way might be called a leak(provided it is not being used at all).However, once your program/process returns the OS simply reclaims the memory it allocated to the process. The OS does not understand leak it simply reclaims back what it gave to the process.
This depends on the operating system. All modern operating systems (to my knowledge) will free memory not explicitly freed by the C program when it has completed execution. Thus, you can get away with this without consequences under normal circumstances. In fact, there are some schools of thought that do not recommend releasing memory when program execution will end soon as it is unnecessary. However, if you happen to be dealing with old or unusual operating systems that can be dangerous. In some of those systems, it can take a restart to free the memory again.
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What REALLY happens when you don't free after malloc?
Is freeing allocated memory needed when exiting a program in C
During my first C class, it was drilled into me that I should call free() after malloc() or calloc() as it releases the space after dynamically allocating memory or otherwise it will be gone (I assume until system reboot). However, recently I started reading on various coding sites that this memory will be released back when the program ends.
Which one is the correct statement?
I would look in the official spec, but I have no idea where to get it (tried Googling).
Thanks.
Really short answer..
Both of the statements are correct.
A longer more elaborate one..
Both of them are correct, when your application fully terminates the allocated memory will (probably) be released back to the system.
I wrote "probably" since all modern/widely-used OSes will deallocate the memory used, but some primitive systems might not have this "feature" available.
Because of the above you should always use free to deallocate memory of allocated but unused variables, especially during run-time (otherwise you'll have a memory leak that might/will slowly eat up all available memory).
If you wanna live on the wild side and not deallocate memory before the application terminates, that's fine by me and most people.B ut, please make sure that the platform you are going to run the application in really does release the memory back to the operating system afterwards.
Regarding looking in the standard ("official spec")
The standard most often/always leaves out implementation details, therefor there is nothing in there saying what will happen to allocated memory that hasn't been deallocated when the running application terminates.
That's implementation specific, and therefor depends in/on/at what you are running it.
On any system with protected memory (e.g. Windows 4.x+, Linux, Unix, …), the memory is released when the program terminates.
Some embedded systems may not always do so, however.
Regardless, it's definitely good form to free() everything correctly.
This is valid within the program, that's while the process is running.
Out of it, this is OS dependent. There is no spec, but the big majority of systems will clean up all the memory used by the process after it is finished.
All memory for a given process is released back to the system at its termination. However, before a program exits, it's important to always free() your memory, especially for long-running programs. In short-running programs, it's still good practice. It's always nice when valgrind spits out a clean bill of health!
If I allocated memory in my C program using malloc and now I want to exit, do I have to free the allocated memory, or can I assume that since my entire program terminates, it will be freed by the OS?
I run in Linux environment.
Any modern operating system will clean up everything after a process terminates, but it's generally not a good practice to rely on this.
It depends on the program you are writing. If it's just a command line tool that runs and terminates quickly, you may not bother cleaning up. But be aware that it is this mindset that causes memory leaks in daemons and long-running programs.
It can be a good design and very efficient to simply exit and allow the operating system to clean everything up. Apple OS X now does this by default: applications are killed without notice unless the application sets a "don't kill me" flag.
Often, freeing every memory allocation takes significant time. Some memory pages may have been swapped out and must be read back in so they can be marked as free. The memory allocator has to do a lot of work updating free memory tracking data. All of this effort is a waste because the program is exiting.
But this must be done by design and not because the programmer has lost track of allocated memory!
In any case it will be freed by the operating system upon process termination. So you don't need it, but since it is a good practice, why don't you do it anyway? :)
Actually with complex code I wouldn't risk to don't release something which I'm not sure at 100% that will be useless because program exits afterwards. So for any minimal doubt just free it.
Yes you can assume that.
Although it is a good practice to deallocate the memory immediately after it is not needed, even for software that runs for a short time only.
The operating system will reclaim the memory so you don't need to free it.
Most programs do free memory though because if you don't free any memory then you are liable to have problems caused by these intentional leaks.
Linux will free the allocated memory and close the file descriptors on process termination.
The OS will reclaim the memory, however it's good practice to free things if you expect they'll run out of scope before you malloc something else. However, you can more or less rely upon the termination of the program to deal with memory management for you.
Always free your allocated memory since that the operating system will hold less memory for no reason. It is very noticed in small operating systems that holds small memory size.
I'm developing on Ubuntu 9.10
I'm writing a C program, during my tests & debugs I'm calling malloc and always remember to call free() - This is obviously just during debug.
I'm curious: Am I eating up the free memory the system has which each debugging session? Or does the kernel cleans up the process memory after I shutdown my application from the IDE? Logically reasoning I'm pretty sure that the kernel knows about the whole process being killed and thus knows what memory it has allocated, and thus even though the application did not call free the memory is still being released.
I would appreciate an explanation.
Thank you,
Maxim.
Yes, the OS will reclaim all the memory allocated to your program when it stops running.
You are correct.
This is not a serious concern, because any 'leaked' memory is immediately freed once the program you're debugging is terminated.
Memory leaks are generally only a concern when a process is long running.
The kernel has a collection of process records within the kernel's memory and keeps track of each process, the amount of memory consumed, resources such as i/o, file handles or inodes. The process records are usually held in a queue in which the kernel's task pointer points to the process record in a never ending fashion (that explains why the perception of 'multi-tasking', it is doing that a blink of an eye - so fast, really, it is doing single tasking in the eyes of the kernel). There is a field in the process record that tells how much memory is chewed up by said process.
Yes the kernel does obtain the memory back to its own pool ready for consumption by another process. Furthermore, you are absolutely 100% correct in relation to memory leaks as John Weldon above pointed out. I mentioned this in another posting, for every malloc there is a free, if there isn't you have a memory leak. So don't worry about your debugging session. That is perfectly ok as the kernel has a responsibility to ensure the memory is reclaimed.
Some applications (especially daemons) must be debugged throughly and that no memory leaks occur in it as the daemon will be running for a long time before the next reboot. Incidentally, it was mentioned in my favourite book 'Expert C Programming, Deep C Secrets - Peter Van Der Linden', that at one stage when he was in Sun, there was a tool called 'printtool' for printing, but every so often the queue jammed up because there was a memory leak in the print spooler program and the reboot of the Sun machine cured it, he describes that in detail in relation to memory leaks.
Hope this helps,
Best regards,
Tom.
A lot of old Unix apps leaked memory badly, and counted on the end-of-process cleanup. Even in the limited address space of those days, it generally worked reasonably well. This doesn't work with long-running apps, of course. I wouldn't worry about the effects of memory leaks when debugging (the leaks themselves are bugs, so you'll want to remove them before releasing).
What happens in Unix, and all other current OSs that I'm actually familiar with, is that the OS assigns memory to a process. malloc() grabs memory out of the process memory, and if it asks for more than the process can provide will ask for more process memory from the OS. If the program's got a memory leak, the process memory can grow as large as the system will allow, but it's all process memory, and all the allocation will go away when the process ends.
I understand there have been OSs that didn't recover memory from a terminated process, but I haven't seen one or worked on one. The only OSs for personal computers (as opposed to special purpose or enterprise computers) that significant numbers of people use are Windows and Unix variants, and those will release all memory at the end of the process.