malloc() in C And Memory Usage

Question

I was trying an experiment with malloc to see if I could allocate all the memory available.

I used the following simple program and have a few questions:

int main(void)
{
    char * ptr;
    int x = 100;

    while(1)
    {
        ptr = (char *) malloc(x++ * sizeof(char) / 2);
        printf("%p\n",ptr);
    }

    return 0;
}

1) Why is it that when using larger data types(int, unsigned long long int, long double) the process would use less memory but with smaller data types (int, char) it would use more?

2) When running the program, it would stop allocating memory after it reached a certain amount (~592mb on Windows 7 64-bit with 8GB RAM swap file set to system managed). The output of the print if showed 0 which means NULL. Why does it stop allocating memory after a reaching this threshold and not exhaust the system memory and swap?

I found someone in the following post trying the same thing as me, but the difference they were not seeing any difference in memory usage, but I am. Memory Leak Using malloc fails

I've tried the code on Linux kernel 2.6.32-5-686 with similar results.

Any help and explanation would be appreciated.

Thanks,

Using long long it should be much more memory. What have you tried? What you are putting out is just the place in memory where it is stored, not the allocated memory. In your example you are allocating 50 bytes. — Gandaro
Just wanted to say this before somebody else did, don't cast the return value of malloc. OOOssshh!! — user681007
There's a difference between local heap and system heap. In Windows, apps don't get a very large local heap, so malloc will run out of memory faster. Use VirtualAlloc() to allocate from the sysetem heap. Then the issue can become one of running out of handles/pages for small allocations not actual memory. — BitBank

iabdalkader iabdalkader · Accepted Answer · 2012-02-17T16:15:09

1)Usually memory is allocated in multiples of pages, so if the size you asked for is less than a page malloc will allocate at least one page.

2)This makes sense, because in a multitasking system, you're not the only user and your process is not the only process running, there are many other processes that share a limited set of resources, including memory. If the OS allowed one process to allocate all the memory it needs without any limitation, then it's not really a good OS, right ?

Finally, in Linux, the kernel doesn't allocate any physical memory pages until after you actually start using this memory, so just calling malloc doesn't actually consume any physical memory, other than what is required to keep track of the allocation itself of course. I'm not sure about Windows though.

Edit: The following example allocates 1GB of virtual memory

#include <stdio.h>
int main(int agrc, char **argv)
{
    void *p = malloc(1024*1024*1024);
    getc(stdin);
}

If you run top you get

top -p `pgrep test`
PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
20   0 1027m  328  252 S    0  0.0   0:00.00 test

If you change malloc to calloc, and run top again you get

top -p `pgrep test`
PR   NI VIRT  RES  SHR S %CPU %MEM    TIME+ COMMAND              
20   0  1027m 1.0g 328 S    0  1.3   0:00.08 test

malloc() in C And Memory Usage

4 Answers