Databases
StackOverflow2013
Postsdbo
PODoes Malloc only use the heap if requested memory space is large?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PODoes Malloc only use the heap if requested memory space is large?
    primarykey
    data
    text
    <p>Whenever you study the memory allocation of processes you usually see it outlined like this:</p> <p><img src="https://i.stack.imgur.com/wn0oN.jpg" alt="enter image description here"></p> <p>So far so good.</p> <p>But then you have the sbrk() system call which allows the program to change the upper limit of its <strong>data section</strong>, and it can also be used to simply check where that limit is with sbrk(0). Using that function I found the following patterns:</p> <p><strong>Pattern 1 - Small malloc</strong></p> <p>I run the following program on my Linux machine:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;unistd.h&gt; int globalVar; int main(){ int localVar; int *ptr; printf("localVar address (i.e., stack) = %p\n",&amp;localVar); printf("globalVar address (i.e., data section) = %p\n",&amp;globalVar); printf("Limit of data section = %p\n",sbrk(0)); ptr = malloc(sizeof(int)*1000); printf("ptr address (should be on stack)= %p\n",&amp;ptr); printf("ptr points to: %p\n",ptr); printf("Limit of data section after malloc= %p\n",sbrk(0)); return 0; } </code></pre> <p>And the output is the following:</p> <pre><code>localVar address (i.e., stack) = 0xbfe34058 globalVar address (i.e., data section) = 0x804a024 Limit of data section = 0x91d9000 ptr address (should be on stack)= 0xbfe3405c ptr points to: 0x91d9008 Limit of data section after malloc= 0x91fa000 </code></pre> <p>As you can see the allocated memory region was right above the old data section limit, and after the malloc that limit was pushed upward, so the allocated region is actually inside the new data section. </p> <p><strong>Question 1</strong>: Does this mean that small mallocs will allocate memory in the data section and not use the heap at all? </p> <p><strong>Pattern 2 - Big Malloc</strong></p> <p>If you increase the requested memory size on line 15:</p> <pre><code>ptr = malloc(sizeof(int)*100000); </code></pre> <p>you will now the following output:</p> <pre><code>localVar address (i.e., stack) = 0xbf93ba68 globalVar address (i.e., data section) = 0x804a024 Limit of data section = 0x8b16000 ptr address (should be on stack)= 0xbf93ba6c ptr points to: 0xb750b008 Limit of data section after malloc= 0x8b16000 </code></pre> <p>As you can see here the limit of the data section has not changed, and instead the allocated memory region is in the middle of the gap section, between the data section and the stack.</p> <p><strong>Question 2</strong>: Is this the large malloc actually using the heap?</p> <p><strong>Question 3</strong>: Any explanation for this behavior? I find it a bit insecure, cause on the first example (small malloc) even after you free the allocated memory you'll still be able to use the pointer and use that memory without getting a seg fault, as it will be inside your data section, and this could lead to hard to detect bugs.</p> <p><strong>Update with Specs</strong>: Ubuntu 12.04, 32-bits, gcc version 4.6.3, Linux kernel 3.2.0-54-generic-pae.</p> <p>Update 2: Rodrigo's answer below solved this mystery. <a href="http://en.wikipedia.org/wiki/Data_segment" rel="noreferrer">This Wikipedia link</a> also helped.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USDaniel Scocco
    1. USDaniel Scocco
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. PODoes Malloc only use the heap if requested memory space is large?
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PODoes Malloc only use the heap if requested memory space is large?
      1. USClaudiu
      1. VTFavorite
    3. VO
      singulars
      1. PODoes Malloc only use the heap if requested memory space is large?
      1. This table or related slice is empty.
      1. VTDownMod
    1. COAll of these "will stuff X happen if stuff Y happens" questions are theoretical and practically unanswerable without mentioning a specific implementation. What Linux? Which compiler? Which standard library implementation? What CPU?
      singulars
      1. PODoes Malloc only use the heap if requested memory space is large?
      1. This table or related slice is empty.
    2. CO@H2CO3, so are you saying you are sure the above behavior is implementation-dependent and not standard of the Linux kernel, for instance? Cause if this was standard of the Linux kernel then specs wouldn't matter right? Either way I included them for the sake of completeness.
      singulars
      1. PODoes Malloc only use the heap if requested memory space is large?
      1. USDaniel Scocco
    3. COThanks. Since the behavior you are asking about is not specified by the C standard, the underlying implementation is free to do whatever it wants. Of course, the Kernel's documentation does matter, but I'd anticipate that at least the used standard library implementation matters as well, not to mention Linuxes that run on a different, more "exotic" architecture than the ones you'd "intuitively" expect as exclusive hosts (microcontrollers with no heap, anyone?). (Now I've upvoted because this is otherwise a good question.)
      singulars
      1. PODoes Malloc only use the heap if requested memory space is large?
      1. This table or related slice is empty.
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload