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my os is arch linux 64 bit.
free -m result below
total used free shared buffers cached
Mem: 32172 31909 262 0 119 13054
-/+ buffers/cache: 18735 13436
Swap: 258 258 0
ulimit -a core file size (blocks, -c) unlimited data seg size (kbytes, -d) unlimited scheduling priority (-e) 20 file size (blocks, -f) unlimited pending signals (-i) 257329 max locked memory (kbytes, -l) 64 max memory size (kbytes, -m) unlimited open files (-n) 1024 pipe size (512 bytes, -p) 8 POSIX message queues (bytes, -q) 819200 real-time priority (-r) 0 stack size (kbytes, -s) 8192 cpu time (seconds, -t) unlimited max user processes (-u) 257329 virtual memory (kbytes, -v) unlimited file locks (-x) unlimited
the kenerl option overcommit_memroy is 0
But when i try to use malloc to allocate 2G bytes ,i fail for the reason:Cannot allocate memory?
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Malloc(12) and malloc(16) allocate 16 bytes for the user, plus an extra 8 bytes for bookkeeping for a total of 24 bytes. Malloc(100) allocates 104 bytes for the user, plus an extra 8 bytes for bookkeeping.
The restriction concerning the size of statically allocated memory is 2 Gbytes both for 32-bit and 64-bit programs. Dynamic data. This is data for which memory is being allocated while executing the program. In C++, this allocation is usually performed by the malloc function or new operator.
Malloc allocates memory more than RAM.
The malloc line allocates a block of memory of the size specified -- in this case, sizeof(int) bytes (4 bytes). The sizeof command in C returns the size, in bytes, of any type. The code could just as easily have said malloc(4), since sizeof(int) equals 4 bytes on most machines.
In this case memory “allocated” can be a few bytes more (usually a word or 2) but also could be less depending on the size and implementation of malloc. Writing an efficient malloc is definitely an interesting piece of work and something you should try.
Malloc takes a size_t, which can be up to SIZE_MAX, which on a 32-bit system is 2 to the 32° power (4,294,967,296), and on a 64-bit system, 2 to the 64° (18,446,744,073,709,551,616) bytes. (4GiB resp. 16EiB ≈ 18EB) Virtual memory size available on the host, less that in use by other processes, plus overcommit space.
malloc does its own memory management, managing small memory blocks itself, but ultimately it uses the Win32 Heap functions to allocate memory. You can think of malloc as a "memory reseller". The windows memory subsystem comprises physical memory (RAM) and virtual memory (HD).
You can, however, find it out by doing a hunt+halving search, caling malloc with ever larger arguments until it fails, and then homing in on the value it balks at. Something like By this point you know that i/2 bytes is ok, while i bytes is not ok.
It could be anything, actually. It may have worked before because the free memory was all contiguous, and now the free memory isn't, or it might be that your memory request fails the heuristics being used.
see this link here - http://linuxtoolkit.blogspot.com/2011/08/tweaking-linux-kernel-overcommit.html - for more information on turning the overcommit know.
Basically:
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