Why does malloc initialize the values to 0 in gcc?

Question

Maybe it is different from platform to platform, but

when I compile using gcc and run the code below, I get 0 every time in my ubuntu 11.10.

#include         


        

Answer 1

Why do you assume that malloc() initializes to zero? It just so happens to be that the first call to malloc() results in a call to sbrk or mmap system calls, which allocate a page of memory from the OS. The OS is obliged to provide zero-initialized memory for security reasons (otherwise, data from other processes gets visible!). So you might think there - the OS wastes time zeroing the page. But no! In Linux, there is a special system-wide singleton page called the 'zero page' and that page will get mapped as Copy-On-Write, which means that only when you actually write on that page, the OS will allocate another page and initialize it. So I hope this answers your question regarding performance. The memory paging model allows usage of memory to be sort-of lazy by supporting the capability of multiple mapping of the same page plus the ability to handle the case when the first write occurs.

If you call free(), the glibc allocator will return the region to its free lists, and when malloc() is called again, you might get that same region, but dirty with the previous data. Eventually, free() might return the memory to the OS by calling system calls again.

Notice that the glibc man page on malloc() strictly says that the memory is not cleared, so by the "contract" on the API, you cannot assume that it does get cleared. Here's the original excerpt:

malloc() allocates size bytes and returns a pointer to the allocated memory.
The memory is not cleared. If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().

If you would like, you can read more about of that documentation if you are worried about performance or other side-effects.

Answer 2

Never ever count on any compiler to generate code that will initialize memory to anything. malloc simply returns a pointer to n bytes of memory someplace hell it might even be in swap.

If the contents of the memory is critical initialize it yourself.

Answer 3

The OS will usually clear fresh memory pages it sends to your process so it can't look at an older process' data. This means that the first time you initialize a variable (or malloc something) it will often be zero but if you ever reuse that memory (by freeing it and malloc-ing again, for instance) then all bets are off.

This inconsistence is precisely why uninitialized variables are such a hard to find bug.

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As for the unwanted performance overheads, avoiding unspecified behaviour is probably more important. Whatever small performance boost you could gain in this case won't compensate the hard to find bugs you will have to deal with if someone slightly modifies the codes (breaking previous assumptions) or ports it to another system (where the assumptions might have been invalid in the first place).

Answer 4

Do you know that it is definitely being initialised? Is it possible that the area returned by malloc() just frequently has 0 at the beginning?

Answer 5

The standard does not dictate that malloc() should initialize the values to zero. It just happens at your platform that it might be set to zero, or it might have been zero at the specific moment you read that value.

Answer 6

From gnu.org:

Very large blocks (much larger than a page) are allocated with mmap (anonymous or via /dev/zero) by this implementation.