mmap allocates memory in heap ?

Question

I was reading about mmap in wikipedia and trying out this example http://en.wikipedia.org/wiki/Mmap#Example_of_usage. I have compiled this program with gcc and ran valgrind overit.

Here is valgrind output:

# valgrind a.out 
==7018== Memcheck, a memory error detector
==7018== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==7018== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==7018== Command: a.out
==7018== 
PID 7018:   anonymous string 1, zero-backed string 1
PID 7019:   anonymous string 1, zero-backed string 1
PID 7018:   anonymous string 2, zero-backed string 2
==7018== 
==7018== HEAP SUMMARY:
==7018==     in use at exit: 0 bytes in 0 blocks
==7018==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==7018== 
==7018== All heap blocks were freed -- no leaks are possible
==7018== 
==7018== For counts of detected and suppressed errors, rerun with: -v
==7018== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)
PID 7019:   anonymous string 2, zero-backed string 2
==7019== 
==7019== HEAP SUMMARY:
==7019==     in use at exit: 0 bytes in 0 blocks
==7019==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==7019== 
==7019== All heap blocks were freed -- no leaks are possible
==7019== 
==7019== For counts of detected and suppressed errors, rerun with: -v
==7019== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)

My Question is

Does mmap allocate memory on Heap ? if not what does munmap do ?

Oliver Charlesworth · Accepted Answer · 2013-02-19T09:46:35.520

24

On a Unix-like system, your program's address space consists of one or more virtual memory regions, each of which is mapped by the OS to physical memory, to a file, or to nothing at all.

The heap is, generally speaking, one specific memory region created by the C runtime, and managed by malloc (which in turn uses the brk and sbrk system calls to grow and shrink).

mmap is a way of creating new memory regions, independently of malloc (and so independently of the heap). munmap is simply its inverse, it releases these regions.

edited Feb 19 '13 at 09:46

answered Feb 19 '13 at 09:34

Oliver Charlesworth

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However, sometimes `malloc` may itself use `mmap` instead of `brk`/`sbrk`.. – benjimin Mar 05 '20 at 06:25

score 9 · Answer 2 · answered Feb 19 '13 at 09:35

9

mmapped memory is neither heap nor stack. It is mapped into virtual address space of the calling process, but it's not allocated on the heap.

answered Feb 19 '13 at 09:35

piokuc

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mmap allocates memory in heap ?

2 Answers2