clipped from www.embedded.com

Using size_t appropriately can improve the portability, efficiency, or readability of your code. Maybe even all three.

Numerous functions in the Standard C library accept arguments or return values that represent object sizes in bytes. For example, the lone argument in malloc(n) specifies the size of the object to be allocated, and the last argument in memcpy(s1, s2, n) specifies the size of the object to be copied. The return value of strlen(s) yields the length of (the number of characters in) null-terminated character array s excluding the null character, which isn't exactly the size of s, but it's in the ballpark.

You might reasonably expect these parameters and return types that represent sizes to be declared with type int (possibly long and/or unsigned), but they aren't. Rather, the C standard declares them as type size_t. According to the standard, the declaration for malloc should appear in <stdlib.h> as something equivalent to:

void *malloc(size_t n);