Generally, malloc, realloc and free are all part of the same library. One of the things this allows is some 'behind the scenes' meta-data chicanery.

For example if you wanted to call malloc(16), the memory library might allocate 20 bytes of space, with the first 4 bytes containing the length of the allocation and then returning a pointer to 4 bytes past the start of the block.

When calling free, the library will subtract 4 bytes from the pointer passed in, to find the pointer to the length of the allocation.

realloc can also use this trick to find out how long the original allocation was.

So, in answer to your question, you can't /only/ implement realloc, you must implement malloc and free also.

Also, if you have an original K&R, I believe you would find some realloc source in there too.

Generally, malloc, realloc and free are all part of the same library. One of the things this allows is some 'behind the scenes' meta-data chicanery.

For example if you wanted to call malloc(16), the memory library might allocate 20 bytes of space, with the first 4 bytes containing the length of the allocation and then returning a pointer to 4 bytes past the start of the block.

When calling free, the library will subtract 4 bytes from the pointer passed in, to find the pointer to the length of the allocation.

realloc can also use this trick to find out how long the original allocation was.

So, in answer to your question, you can't only implement realloc, you must implement malloc and free also.

Also, if you have an original K&R, I believe you would find some realloc source in there too.