Both realloc() and free() are functions of C programming language used for dynamic memory allocation. Both of the functions are defined in the stdlib.h library. 

realloc() : It is a function which is used to dynamically reallocate memory space to an already dynamically allocated memory space to a variable.

free() : It is used to deallocate or free the dynamically allocated memory space allocated to a variable and thus make it available to other variables or other memory functions of the operating system.

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