What causes malloc() and calloc() functions to return a NULL pointer?

Answer 1

When there is no more memory available to allocate c/malloc return NULL If they return NULL, errno (see errno.h) will be set to appropriate error. You can get the error text using perror().

Answer 2

these functions return a NULL pointer if the requested memory space is not available. the space is allocated in after the end of the data segment and beginning of the code.
the functions will also return NULL if the size requested is 0.

Answer 3

malloc() or calloc() return null pointer in case of failure in allocating memory