Size of malloc'd?

How do I determine the size of a previously malloc'd buffer?

char *buffer;

buffer = calloc(18, sizeof *buffer);

I cannot do a sizeof(buffer) since it would only return the size of the
pointer. I cannot do sizeof *buffer since it would only return the
size of the pointer type. Is there a right way to do this? Do I need
to manage this information in the program?

.

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