Vikas Vikas - 3 months ago 10
C Question

maximum memory which malloc can allocate

I was trying to figure out how much memory I can malloc to maximum extent on my machine
(1 Gb RAM 160 Gb HD Windows platform).

I read that the maximum memory malloc can allocate is limited to physical memory (on heap).

Also when a program exceeds consumption of memory to a certain level, the computer stops working because other applications do not get enough memory that they require.

So to confirm, I wrote a small program in C:

int main(){
int *p;
while(1){
p=(int *)malloc(4);
if(!p)break;
}
}


I was hoping that there would be a time when memory allocation would fail and the loop would break, but my computer hung as it was an infinite loop.

I waited for about an hour and finally I had to force shut down my computer.

Some questions:


  • Does malloc allocate memory from HD also?

  • What was the reason for above behaviour?

  • Why didn't loop break at any point of time?

  • Why wasn't there any allocation failure?


Answer

I read that the maximum memory malloc can allocate is limited to physical memory (on heap).

Wrong: most computers/OSs support virtual memory, backed by disk space.

Some questions: does malloc allocate memory from HDD also?

malloc asks the OS, which in turn may well use some disk space.

What was the reason for above behavior? Why didn't the loop break at any time?

Why wasn't there any allocation failure?

You just asked for too little at a time: the loop would have broken eventually (well after your machine slowed to a crawl due to the large excess of virtual vs physical memory and the consequent super-frequent disk access, an issue known as "thrashing") but it exhausted your patience well before then. Try getting e.g. a megabyte at a time instead.

When a program exceeds consumption of memory to a certain level, the computer stops working because other applications do not get enough memory that they require.

A total stop is unlikely, but when an operation that normally would take a few microseconds ends up taking (e.g.) tens of milliseconds, those four orders of magnitude may certainly make it feel as if the computer had basically stopped, and what would normally take a minute could take a week.