Malloc
The malloc function performs dynamic memory allocation in C and is part of the standard library
Realloc
C library function for reallocating a dynamically allocated memory region.
Faster memcpy free
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"A realloc can occur significantly faster than a malloc memcpy and free" from question Why is there no reallocation functionality in C++ allocators? |
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"That s why realloc can temporarily require more memory than a malloc free pair" from question Why is realloc eating tons of memory? |
Pointer call knows
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"This is evidently what happened in your example since your realloc call asks for less memory than was allocated by calloc and we can see that it didn t do that here since the address of ptr when free is called is the same as the block allocated by calloc;if the existing allocation is much bigger than the request realloc may choose to copy the existing memory block into a smaller allocation possibly acquired with malloc" from question Intel pin: how to detect realloc size |
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"The runtime is complaining about an invalid pointer which indicates that the pointer you are passing to realloc is not a pointer that was created with a call to malloc or to calloc;it knows because whenever you do a malloc there is a memory management header that is part of the data area and the pointer you are given is a pointer to allocated memory after the header" from question Realloc (again) |
Others
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Of course the real realloc is a lot more complex because the real realloc checks the current block to see if the current block can be expanded before the current block allocates new data and probably doesn t call regular malloc but the functionality is roughly this from question Double free or corruption error happens when call free in c |
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However functions like realloc do not have this property as they can return a pointer to storage containing pointers;as you pointed out yourself nonnull and malloc are just macros from question Unfamiliar function declaration in c |
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So your call to realloc may not have resulted in a physical page being returned to the system so it s still mapped to you program and can be used;however a following call to malloc could use that space or it could be reclaimed by the system at any time from question What happens to array elements after the original array is reallocated? |
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But if it works with large datasets the users will notice that using the malloc -only program slows down other programs much more than the realloc -using program with the same data from question Why should we use `realloc` if we need a `tmp buffer` |
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Malloc often gives you more memory that you ask and stores the actual value in a special location that realloc can access at a later time from question Realloc simply not diong anything, not erroring |
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If no storage is allocated or deallocated between the malloc and realloc the size of the realloc is known when the malloc is performed and the realloc size is larger than the malloc size then it may make sense to consolidate the malloc and realloc operations into a single larger allocation from question Are compilers allowed to optimize out realloc? |
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Allocate the block using malloc not new and then use realloc;realloc knows how much free space is available after the block for expansion from question Better method to increase the size of an array in C++ dynamically |
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Also i should note that in same cases you don t have to do a copy even when realloc increases the size for example if the next block in the heap is free;malloc does not initialize memory to zero from question Implementation of Realloc in C |
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For that you have realloc;with malloc you would end up with a fixed size for your table just like if you had only declared it has static but with later initialization well i m a bit agains this sentence because malloc is much more than that but for this purpose this purpose is safe to say this purpose from question Using 2D array of char pointer in C |
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As a result if one wants code to be compatible with aggressive compilers one must refrain from having any pointers point within an object which is going to be realloc ed unless one can guarantee that such pointers will never even be examined after a successful realloc takes place;as far as the c standard is concerned the behavior of realloc in all cases where it succeeds is equivalent to copying the memory block to some arbitrary location calling free upon it calling malloc to create a new block of the requested size and returning a pointer to the new block in cases where the malloc wouldn t succeed realloc is defined to return null without disturbing the original block from question After using realloc the next pointer in array is lost |
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The justification of realloc is that it s faster than 2nd malloc manual copy free from question C++ realloc performance vs malloc |
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If you work with data that doesn t need construction destruction and requires reallocations a large array of ints then i believe malloc free is a good choice as malloc gives you realloc which is way faster than new-memcpy-delete it is on my linux box but i guess this may be platform dependent from question In what cases do I use malloc and/or new? |
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I mean i think realloc is way smarter so why do we even need malloc from question Why am I getting a segmentation fault when using realloc()? |
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Realloc is worse than malloc in that you will need to have the old and new pointers valid during the realloc from question Is realloc() safe in embedded system? |
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At the moment it s possible that there is some space after stringarray into which you are storing the wordlength-arraylength extra pointers when you calloc them and realloc doesn t move stringarray;it s quite probable that 0xb49010 is one of the pointers you calloc d and you re overwritten the memory where malloc keeps its block size from question Reallocate a 2d char array |
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In the case of overflow a free malloc pair costs less than realloc because of its internal hidden memcpy from question Substitute or workaround for asprintf on AIX |