This is also why it is slower than malloc not only does it have to zero it, the OS must also find a suitable memory area by possibly swapping out other processes See for instance this SO question for further discussion about the behavior of malloc. Community Bot 1 1 1 silver badge. Isak Savo Isak Savo If the allocated block consists mostly of new zero pages provided by the operating system, it can leave those untouched. This of course requires calloc to be tuned to the operating system rather than a generic library function on top of malloc.
Or, an implementor could make calloc compare each word against zero before zeroing it. This would not save any time, but it would avoid dirtying the new pages.
But in practice, does such implementations exist in the wild? All dlmalloc -like implementations skip the memset if the chunk was obtained via mmap ing new anonymous pages or equivalent. Usually this kind of allocation is used for larger chunks, starting at k or so. I don't know of any implementations that do the comparison against zero before writing zero aside from my own. Though, extremely primitive calloc implementations differ.
Show 2 more comments. Daren Thomas Apparently the arithmetic overflow was what caused OpenSSH hole in Good article from OpenBSD on the perils of this with memory-related functions: undeadly. Sadly the article you linked has misinformation right at the beginning.
The example with char is not an overflow but rather an implementation-defined conversion when assigning the result back into a char object.
It's there probably for illustration purpose only. Because compiler is likely to optimise that away anyway. Mine compiles into this asm: push 1. Show 4 more comments. Vishnu CS 1 1 gold badge 7 7 silver badges 23 23 bronze badges. AnT AnT k 39 39 gold badges silver badges bronze badges. It would be helpful if there were a standard means by which code could assert that an implementation must use all-bits-zero as a null pointer refusing compilation otherwise , since there exist implementations that use other null-pointer representations, but they are comparatively rare; code that doesn't have to run on such implementations can be faster if it can use calloc or memset to initialize arrays of pointers.
AGS 14k 5 5 gold badges 47 47 silver badges 65 65 bronze badges. There is also a second point that make calloc slower than malloc : the multiplication for the size. If it fails to locate enough space it returns a NULL pointer. If it fails to locate requsted amount of memory it returns a null pointer. Jainendra Jainendra Difference 1: malloc usually allocates the memory block and it is initialized memory segment.
Difference 2: If you consider malloc syntax, it will take only 1 argument. Felipe Augusto 6, 7 7 gold badges 29 29 silver badges 64 64 bronze badges.
Shivaraj Bhat Shivaraj Bhat 1 1 gold badge 8 8 silver badges 19 19 bronze badges. Number of blocks: malloc assigns single block of requested memory, calloc assigns multiple blocks of the requested memory Initialization: malloc - doesn't clear and initialize the allocated memory.
Majbah Habib Majbah Habib 3, 1 1 gold badge 30 30 silver badges 34 34 bronze badges. It allocates memory as a number of elements of a given size, and It initializes the memory that is allocated so that all bits are zero. Vipin Diwakar Vipin Diwakar 1 1 silver badge 3 3 bronze badges.
It certainly does not require that there be any circumstance under which such an allocation might succeed; I'm not sure there's any particular benefit from mandating that implementations that cannot handle such allocations must return NULL especially given that it's common for some implementations to have malloc return pointers to space that's not yet committed and might not be available when code actually tries to use it.
Further, where there may have been systems in the past whose available addressing range exceeded the largest representable integer, I do not see any realistic possibility of that ever again occurring, since that would require a storage capacity of billions of gigabytes.
Even if Moore's Law continued to hold, going from the point where 32 bits ceases to be enough to the point where 64 bits ceased to be enough would take twice as long as getting from the point where 16 bits was enough to the point where 32 wasn't.
Show 3 more comments. So if initialization to zero is not necessary, then using malloc could be faster. Roberto Caboni 6, 10 10 gold badges 22 22 silver badges 37 37 bronze badges. Akshay V Akshay V 13 3 3 bronze badges.
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