how to fix a memory size for my application in C?

Question

Hi,

I would like to allocate a fixed memory for my application (developed using C). Say my application should not cross 64MB of memory occupation. And also i should avoid to use more CPU usage. How it is possible?

Regards Marcel.

Answer 1

One fairly low-tech way I could ensure of not crossing a maximum threshold of memory in your application would be to define your own special malloc() function which keeps count of how much memory has been allocated, and returns a NULL pointer if the threshold has been exceeded. This would of course rely on you checking the return value of malloc() every time you call it, which is generally considered good practice anyway because there is no guarantee that malloc() will find a contiguous block of memory of the size that you requested.

This wouldn't be foolproof though, because it probably won't take into account memory padding for word alignment, so you'd probably end up reaching the 64MB memory limit long before your function reports that you have reached it.

Also, assuming you are using Win32, there are probably APIs that you could use to get the current process size and check this within your custom malloc() function. Keep in mind that adding this checking overhead to your code will most likely cause it to use more CPU and run a lot slower than normal, which leads nicely into your next question :)

And also i should avoid to use more CPU usage.

This is a very general question and there is no easy answer. You could write two different programs which essentially do the same thing, and one could be 100 times more CPU intensive than another one due to the algorithms that have been used. The best technique is to:

1. Set some performance benchmarks.
2. Write your program.
3. Measure to see whether it reaches your benchmarks.
4. If it doesn't reach your benchmarks, optimise and go to step (3).

You can use profiling programs to help you work out where your algorithms need to be optimised. Rational Quantify is an example of a commercial one, but there are many free profilers out there too.

Answer 2

Hmm; good question. I can see how you could do this for memory allocated off the heap, using a custom version of malloc and free, but I don't know about enforcing it on the stack too.

Managing the CPU is harder still...

Interesting.

Answer 3

Under unix: "ulimit -d 64M"

Answer 4

If you are on POSIX, System V- or BSD-derived system, you can use setrlimit() with resource RLIMIT_DATA - similar to ulimit -d.

Also take a look at RLIMIT_CPU resource - it's probably what you need (similar to ulimit -t)

Check man setrlimit for details.

Answer 5

For CPU, we've had a very low-priority task ( lower than everything else ) that does nothing but count. Then you can see how often that task gets to run, and you know if the rest of your processes are consuming too much CPU. This approach doesn't work if you want to limit your process to 10% while other processes are running, but if you want to ensure that you have 50% CPU free then it works fine.

For memory limitations you are either stuck implementing your own layer on top of malloc, or taking advantage of your OS in some way. On Unix systems ulimit is your friend. On VxWorks I bet you could probably figure out a way to take advantage of the task control block to see how much memory the application is using... if there isn't already a function for that. On Windows you could probably at least set up a monitor to report if your application does go over 64 MB.

The other question is: what do you do in response? Should your application crash if it exceeds 64Mb? Do you want this just as a guide to help you limit yourself? That might make the difference between choosing an "enforcing" approach versus a "monitor and report" approach.