memory request Linux

Question

I have a c program that makes memory request. After that run my program i use the free -g -t command to see the memory used. The program call to the malloc function, but it does not call the free function. For example: pointer=malloc(900000000*sizeof(double)) I thought that that called would occupy ram memory but when i used the free command the free memory does not change . About 6 mega. I run the program a lot of times. The program don t free the memory but the free memory does not change. i thouhgt that the operation free memory was in charge of the programmer in c. My program is some more complicated than that, but the main operation is like that.

I don t underestand why the free (and occupped) memory does not change.

Scribble on the allocated memory. That should end up allocating it. Linux assumes people will over-allocate and is lazy about committing the memory to your program. — Jonathan Leffler, Sep 10 '17 at 19:43
I am not sure if i underestand your comment. You say that the output of the linux free command show me is not updated o something like that? — Patricio, Sep 10 '17 at 20:09
Probably. I've not formally experimented but your symptoms match my suggested diagnosis. — Jonathan Leffler, Sep 10 '17 at 20:47
malloc() does *not* allocate memory. It allocates **address space**. The memory will be *faulted in* once referenced. — wildplasser, Sep 10 '17 at 21:39

user1048576 · Answer 1 · 2017-09-10T22:00:39.020

As Jonathan Leffler says in his comment, malloc() does not actually allocate the memory you ask for in physical memory, that happens first when you use the memory. If you write to the memory you got from malloc() then you will see that memory usage as seen by "free" will increase.

From malloc man page under Notes:

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.

"free" reports the physical memory usage, as mentioned in free man page

memory request Linux

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