When you exit a C application, is the malloc-ed memory automatically freed?

user_44084 0
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Let's say I have the following C code:

int main () {
  int *p = malloc(10 * sizeof *p);
  *p = 42;
  return 0;  //Exiting without freeing the allocated memory
}

When I compile and execute that C program, ie after allocating some space in memory, will that memory I allocated be still allocated (ie basically taking up space) after I exit the application and the process terminates?

memory-management c
9 Answers

It depends on the operating system. The majority of modern (and all major) operating systems will free memory not freed by the program when it ends.

Relying on this is bad practice and it is better to free it explicitly. The issue isn't just that your code looks bad. You may decide you want to integrate your small program into a larger, long running one. Then a while later you have to spend hours tracking down memory leaks.
Relying on a feature of an operating system also makes the code less portable.

user_1181450

In general, modern general-purpose operating systems do clean up after terminated processes. This is necessary because the alternative is for the system to lose resources over time and require rebooting due to programs which are poorly written or simply have rarely-occurring bugs that leak resources.

Having your program explicitly free its resources anyway can be good practice for various reasons, such as:

  • If you have additional resources that are not cleaned up by the OS on exit, such as temporary files or any kind of change to the state of an external resource, then you will need code to deal with all of those things on exit, and this is often elegantly combined with freeing memory.
  • If your program starts having a longer lifetime, then you will not want the only way to free memory to be to exit. For example, you might want to convert your program into a server (daemon) which keeps running while handling many requests for individual units of work, or your program might become a small part of a larger program.

However, here is a reason to skip freeing memory: efficient shutdown. For example, suppose your application contains a large cache in memory. If when it exits it goes through the entire cache structure and frees it one piece at a time, that serves no useful purpose and wastes resources. Especially, consider the case where the memory pages containing your cache have been swapped to disk by the operating system; by walking the structure and freeing it you're bringing all of those pages back into memory all at once, wasting significant time and energy for no actual benefit, and possibly even causing other programs on the system to get swapped out!

As a related example, there are high-performance servers that work by creating a process for each request, then having it exit when done; by this means they don't even have to track memory allocation, and never do any freeing or garbage collection at all, since everything just vanishes back into the operating system's free memory at the end of the process. (The same kind of thing can be done within a process using a custom memory allocator, but requires very careful programming; essentially making one's own notion of “lightweight processes” within the OS process.)

user_996920

What's happening here (in a modern OS), is that your program runs inside its own "process." This is an operating system entity that is endowed with its own address space, file descriptors, etc. Your malloc calls are allocating memory from the "heap", or unallocated memory pages that are assigned to your process.

When your program ends, as in this example, all of the resources assigned to your process are simply recycled/torn down by the operating system. In the case of memory, all of the memory pages that are assigned to you are simply marked as "free" and recycled for the use of other processes. Pages are a lower-level concept than what malloc handles-- as a result, the specifics of malloc/free are all simply washed away as the whole thing gets cleaned up.

It's the moral equivalent of, when you're done using your laptop and want to give it to a friend, you don't bother to individually delete each file. You just format the hard drive.

All this said, as all other answerers are noting, relying on this is not good practice:

  1. You should always be programming to take care of resources, and in C that means memory as well. You might end up embedding your code in a library, or it might end up running much longer than you expect.
  2. Some OSs (older ones and maybe some modern embedded ones) may not maintain such hard process boundaries, and your allocations might affect others' address spaces.
user_732970

Yes. The OS cleans up resources. Well ... old versions of NetWare didn't.

Edit: As San Jacinto pointed out, there are certainly systems (aside from NetWare) that do not do that. Even in throw-away programs, I try to make a habit of freeing all resources just to keep up the habit.

user_234780

Yes, the operating system releases all memory when the process ends.

user_263340

It depends, operating systems will usually clean it up for you, but if you're working on for instance embedded software then it might not be released.

Just make sure you free it, it can save you a lot of time later when you might want to integrate it in to a large project.

user_1090970

That really depends on the operating system, but for all operating systems you'll ever encounter, the memory allocation will disappear when the process exits.

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