C malloc, memory usage only when populating

Question

I'm allocating some space with malloc when my app starts. If I don't populate this variable top shows 0% of my memory used by this app, but if I start to populate this variable top begins to show increase usage of ram by the way I'm populating this array.

So my question is: shouldn't top show this space allocated by malloc as an used space of my app? Why it only show increase of RAM usage from my app when I populate this variable?

I'm at Ubuntu 10.10 64bits. Here is the code that populates it:

char pack(uint64_t list, char bits, uint64_t *list_compressed, char control, uint64_t *index){
uint64_t a, rest;   

if(control == 0){
    a = list;
}
else{
    rest = list >> (64 - control);

    a = (control == 64 ? list_compressed[*index] : list_compressed[*index] + (list << control));

    if(control + bits >= 64){
        control = control - 64;
        //list_compressed[*index] = a;
        (*index)++;
        a = rest;
    } 
}

//list_compressed[*index] = a;
control = control + bits;

return control;
}

The "malloqued" variable is list_compressed.

If I uncomment the list_compressed population the ram usage is increased, if I keep it commented the usage is 0%.

en.wikipedia.org/wiki/Virtual_memory

Hans Passant
– Hans Passant

2012-04-21 20:37:09 +00:00
Commented Apr 21, 2012 at 20:37 — Hans Passant
– Hans Passant, Commented Apr 21, 2012 at 20:37

MByD · Accepted Answer · 2012-04-21 20:40:40Z

6

Short answer, no. On many OSs, when you call malloc, it doesn't directly allocate you the memory, but only when you access it.

From malloc man page:

By default, Linux follows an optimistic memory allocation strategy. This means that when malloc() returns non-NULL there is no guarantee that the memory really is available.

edited Apr 21, 2012 at 20:40

answered Apr 21, 2012 at 20:34

MByD

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2 Comments

Jonathan Leffler Over a year ago

Which OS other than Linux does optimistic memory allocation and has an OOM Killer analogue?

MByD Over a year ago

@JonathanLeffler - AFAIK, delay with memory allocation until actual usage is done on Windows and iOs (but I might be wrong...)

slartibartfast · Accepted Answer · 2012-04-21 20:34:05Z

2

Modern operating systems may just return a virtual memory address when you allocate, which doesn't actually point to the chunk of memory. It is only 'allocated' when you want to use it.

answered Apr 21, 2012 at 20:34

slartibartfast

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1 Comment

Frederico Schardong Over a year ago

That makes sense. So how can I know how much memory was virtually allocated?

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