Are some allocators lazy?

I wrote a C program in Linux that mallocs memory, ran it in a loop, and TOP didn't show any memory consumption.

then I've done something with that memory, and TOP did show memory consumption.

When I malloc, do I really "get memory", or is there a "lazy" memory management, that only gives me the memory if/when I use it?

(There is also an option that TOP only know about memory consumption when I use it, so I'm not sure about this..)

Thanks