difference between malloc calloc realloc

can anybody please tell me the exact difference between calloc malloc and realloc

thanx a lot

Code:

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calloc
Allocates an array in memory with elements initialized to 0.
void *calloc( size_t num, size_t size );

malloc
Allocates memory blocks.
void *malloc( size_t size );

realloc
Reallocate memory blocks.
void *realloc( void *memblock, size_t size );

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