Difference between malloc and calloc

Question

What is the difference between doing   ptr    char     malloc  MAXELEMS   sizeof char        or   ptr    char     calloc  MAXELEMS  sizeof char       When is it a good idea to use calloc over malloc or vice versa

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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  calloc   - initializes the allocated memory by zero  Speed  malloc   is fast  calloc   is slower than malloc    Arguments  amp  Syntax  malloc   takes 1 argument   bytes  The number of bytes to be allocated    calloc   takes 2 arguments   length  the number of blocks of memory to be allocated   bytes  the number of bytes to be allocated at each block of memory    void  malloc size t bytes            void  calloc size t length  size t bytes          Manner of memory Allocation  The malloc function assigns memory of the desired  size  from the available heap  The calloc function assigns memory that is the size of what   s equal to    num  size     Meaning on name  The name malloc means  quot memory allocation quot   The name calloc means  quot contiguous allocation quot

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The calloc   function that is declared in the  lt stdlib h gt  header offers a couple of advantages over the malloc   function    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

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One often-overlooked advantage of calloc is that  conformant implementations of  it will help protect you against integer overflow vulnerabilities  Compare   size t count   get int32 file   struct foo  bar   malloc count   sizeof  bar     vs   size t count   get int32 file   struct foo  bar   calloc count  sizeof  bar     The former could result in a tiny allocation and subsequent buffer overflows  if count is greater than SIZE MAX sizeof  bar  The latter will automatically fail in this case since an object that large cannot be created   Of course you may have to be on the lookout for non-conformant implementations which simply ignore the possibility of overflow    If this is a concern on platforms you target  you ll have to do a manual test for overflow anyway

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from an article Benchmarking fun with calloc   and zero pages on Georg Hager s Blog      When allocating memory using calloc    the amount of memory requested is not allocated right away  Instead  all pages that belong to the memory block are connected to a single page containing all zeroes by some MMU magic  links below   If such pages are only read  which was true for arrays b  c and d in the original version of the benchmark   the data is provided from the single zero page  which     of course     fits into cache  So much for memory-bound loop kernels  If a page gets written to  no matter how   a fault occurs  the    real    page is mapped and the zero page is copied to memory  This is called copy-on-write  a well-known optimization approach  that I even have taught multiple times in my C   lectures   After that  the zero-read trick does not work any more for that page and this is why performance was so much lower after inserting the     supposedly redundant     init loop

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There s no difference in the size of the memory block allocated  calloc just fills the memory block with physical all-zero-bits pattern  In practice it is often assumed that the objects located in the memory block allocated with calloc have initilial value as if they were initialized with literal 0  i e  integers should have value of 0  floating-point variables - value of 0 0  pointers - the appropriate null-pointer value  and so on   From the pedantic point of view though  calloc  as well as memset      0        is only guaranteed to properly initialize  with zeroes  objects of type unsigned char  Everything else is not guaranteed to be properly initialized and may contain so called trap representation  which causes undefined behavior  In other words  for any type other than unsigned char the aforementioned all-zero-bits patterm might represent an illegal value  trap representation    Later  in one of the Technical Corrigenda to C99 standard  the behavior was defined for all integer types  which makes sense   I e  formally  in the current C language you can initialize only integer types with calloc  and memset      0         Using it to initialize anything else in general case leads to undefined behavior  from the point of view of C language   In practice  calloc works  as we all know     but whether you d want to use it  considering the above  is up to you  I personally prefer to avoid it completely  use malloc instead and perform my own initialization   Finally  another important detail is that calloc is required to calculate the final block size internally  by multiplying element size by number of elements  While doing that  calloc must watch for possible arithmetic overflow  It will result in unsuccessful allocation  null pointer  if the requested block size cannot be correctly calculated  Meanwhile  your malloc version makes no attempt to watch for overflow  It will allocate some  unpredictable  amount of memory in case overflow happens

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malloc   and calloc   are functions from the C standard library that allow dynamic memory allocation  meaning that they both allow memory allocation during runtime   Their prototypes are as follows   void  malloc  size t n   void  calloc  size t n  size t t    There are mainly two differences between the two    Behavior  malloc   allocates a memory block  without initializing it  and reading the contents from this block will result in garbage values  calloc    on the other hand  allocates a memory block and initializes it to zeros  and obviously reading the content of this block will result in zeros  Syntax  malloc   takes 1 argument  the size to be allocated   and calloc   takes two arguments  number of blocks to be allocated and size of each block     The return value from both is a pointer to the allocated block of memory  if successful  Otherwise  NULL will be returned indicating the memory allocation failure   Example   int  arr      allocate memory for 10 integers with garbage values arr    int   malloc 10   sizeof int         allocate memory for 10 integers and sets all of them to 0 arr    int   calloc 10  sizeof int      The same functionality as calloc   can be achieved using malloc   and memset        allocate memory for 10 integers with garbage values    arr   int   malloc 10   sizeof int       set all of them to 0 memset arr  0  10   sizeof int       Note that malloc   is preferably used over calloc   since it s faster  If zero-initializing the values is wanted  use calloc   instead

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calloc   gives you a zero-initialized buffer  while malloc   leaves the memory uninitialized   For large allocations  most calloc implementations under mainstream OSes will get known-zeroed pages from the OS  e g  via POSIX mmap MAP ANONYMOUS  or Windows VirtualAlloc  so it doesn t need to write them in user-space   This is how normal malloc gets more pages from the OS as well  calloc just takes advantage of the OS s guarantee   This means calloc memory can still be  clean  and lazily-allocated  and copy-on-write mapped to a system-wide shared physical page of zeros    Assuming a system with virtual memory    Some compilers even can optimize malloc   memset 0  into calloc for you  but you should use calloc explicitly if you want the memory to read as 0   If you aren t going to ever read memory before writing it  use malloc so it can  potentially  give you dirty memory from its internal free list instead of getting new pages from the OS    Or instead of zeroing a block of memory on the free list for a small allocation      Embedded implementations of calloc may leave it up to calloc itself to zero memory if there s no OS  or it s not a fancy multi-user OS that zeros pages to stop information leaks between processes   On embedded Linux  malloc could mmap MAP UNINITIALIZED MAP ANONYMOUS   which is only enabled for some embedded kernels because it s insecure on a multi-user system

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A difference not yet mentioned  size limit  void  malloc size t size  can only allocate up to SIZE MAX   void  calloc size t nmemb  size t size   can allocate up about SIZE MAX SIZE MAX   This ability is not often used in many platforms with linear addressing   Such systems limit calloc   with nmemb   size  lt   SIZE MAX   Consider a type of 512 bytes called disk sector and code wants to use lots of sectors   Here  code can only use up to SIZE MAX sizeof disk sector sectors   size t count   SIZE MAX sizeof disk sector  disk sector  p   malloc count   sizeof  p     Consider the following which allows an even larger allocation   size t count   something in the range SIZE MAX sizeof disk sector   1  SIZE MAX  disk sector  p   calloc count  sizeof  p       Now if such a system can supply such a large allocation is another matter   Most today will not   Yet it has occurred for many years when SIZE MAX was 65535   Given Moore s law  suspect this will be occurring about 2030 with certain memory models with SIZE MAX    4294967295 and memory pools in the 100 of GBytes

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The documentation makes the calloc look like malloc  which just does zero-initialize the memory  this is not the primary difference  The idea of calloc is to abstract copy-on-write semantics for memory allocation  When you allocate memory with calloc it all maps to same physical page which is initialized to zero  When any of the pages of the allocated memory is written into a physical page is allocated  This is often used to make HUGE hash tables  for example since the parts of hash which are empty aren t backed by any extra memory  pages   they happily point to the single zero-initialized page  which can be even shared between processes  Any write to virtual address is mapped to a page  if that page is the zero-page  another physical page is allocated  the zero page is copied there and the control flow is returned to the client process  This works same way memory mapped files  virtual memory  etc  work   it uses paging  Here is one optimization story about the topic  http   blogs fau de hager 2007 05 08 benchmarking-fun-with-calloc-and-zero-pages

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Difference 1   malloc   usually allocates the memory block and it is initialized memory segment   calloc   allocates the memory block and initialize all the memory block to 0    Difference 2   If you consider malloc   syntax  it will take only 1 argument  Consider the following example below   data type ptr    cast type   malloc  sizeof data type  no of blocks      Ex  If you want to allocate 10 block of memory for int type   int  ptr    int    malloc sizeof int    10      If you consider calloc   syntax  it will take 2 arguments  Consider the following example below   data type ptr    cast type   calloc no of blocks   sizeof data type       Ex  if you want to allocate 10 blocks of memory for int type and Initialize all that to ZERO   int  ptr    int    calloc 10   sizeof int       Similarity   Both malloc   and calloc   will return void  by default if they are not type casted

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There are two differences   First  is in the number of arguments  malloc   takes a single argument  memory required in bytes   while calloc   needs two arguments   Secondly  malloc   does not initialize the memory allocated  while calloc   initializes the allocated memory to ZERO     calloc   allocates a memory area  the length will be the product of its parameters  calloc fills the memory with ZERO s and returns a pointer to first byte  If it fails to locate enough space it returns a NULL pointer    Syntax  ptr var  cast type   calloc no of blocks   size of each block   i e  ptr var  type   calloc n s     malloc   allocates a single block of memory of REQUSTED SIZE and returns a pointer to first byte  If it fails to locate requsted amount of memory it returns a null pointer    Syntax  ptr var  cast type   malloc Size in bytes   The malloc   function take one argument  which is the number of bytes to allocate  while the calloc   function takes two arguments  one being the number of elements  and the other being the number of bytes to allocate for each of those elements  Also  calloc   initializes the allocated space to zeroes  while malloc   does not

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Both malloc and calloc allocate memory  but calloc initialises all the bits to zero whereas malloc doesn t  Calloc could be said to be equivalent to malloc   memset with 0  where memset sets the specified bits of memory to zero   So if initialization to zero is not necessary  then using malloc could be faster

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A less known difference is that in operating systems with optimistic memory allocation  like Linux  the pointer returned by malloc isn t backed by real memory until the program actually touches it   calloc does indeed touch the memory  it writes zeroes on it  and thus you ll be sure the OS is backing the allocation with actual RAM  or swap   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

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calloc is generally malloc memset to 0  It is generally slightly better to use malloc memset explicitly  especially when you are doing something like   ptr malloc sizeof Item    memset ptr  0  sizeof Item      That is better because sizeof Item  is know to the compiler at compile time and the compiler will in most cases replace it with the best possible instructions to zero memory  On the other hand if memset is happening in calloc  the parameter size of the allocation is not compiled in in the calloc code and real memset is often called  which would typically contain code to do byte-by-byte fill up until long boundary  than cycle to fill up memory in sizeof long  chunks and finally byte-by-byte fill up of the remaining space  Even if the allocator is smart enough to call some aligned memset it will still be a generic loop   One notable exception would be when you are doing malloc calloc of a very large chunk of memory  some power of two kilobytes  in which case allocation may be done directly from kernel  As OS kernels will typically zero out all memory they give away for security reasons  smart enough calloc might just return it withoud additional zeroing  Again - if you are just allocating something you know is small  you may be better off with malloc memset performance-wise

[c] Difference between malloc and calloc?

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