How to know the available virtual memory inside an

Question

As the title asks,

Is it possible to know the currently available virtual memory (which is allocatable by malloc/mmap) inside an application by programmatically?

What I'm looking for is some kind of a function

unsigned int free_mem();

which will return the size of memory allocatable by malloc or mmap.

Answer 1

Since you are explicitly talking about available virtual memory, you need to find the parts of the virtual address space which are currently unallocated. Segment information is available in proc/self/smaps.

However, the size of the virtual address space is determined by the cpu your program is executing on. This is not 64 bits on a 64 bit processor, but can be determined by cat /proc/cpuinfo | grep "address sizes" which yields

address sizes   : 42 bits physical, 48 bits virtual
address sizes   : 42 bits physical, 48 bits virtual
address sizes   : 42 bits physical, 48 bits virtual
address sizes   : 42 bits physical, 48 bits virtual
address sizes   : 42 bits physical, 48 bits virtual
address sizes   : 42 bits physical, 48 bits virtual
address sizes   : 42 bits physical, 48 bits virtual
address sizes   : 42 bits physical, 48 bits virtual

on the VM I am currently testing it on.

Note that the total amount of available virtual address space is different from the largest amount allocatable in one go, since you can only mmap a sequential amount of memory.

For example consider the simple case of 32 bit pointers with the following segments:

00020000-000204ff /usr/bin/executable
00030000-0003ffff [heap]
7ff80000-7ff9ffff [stack]

In this case the largest continuous segment is 7ffA000-ffffffff, but the total amount of available virtual memory is only slightly less than 4 GB.

Note also that malloc may be able to satisfy your request without requiring more virtual memory by reusing already requested pages. The amount available this way depends on a many things (size of the allocation or allocations, state of the heap) and cannot be queried in any portable way. This holds double if someone LD_PRELOADs another malloc implementation.