How can I view the size of memory allocated via malloc in gdb?

I am debugging a program with gdb and I have a ptr to memory allocated using malloc. It is dynamically allocatted based off of some math so it would be useful to be able to type into gdb a command and specify the ptr, and be able to get back the exact actual size of memory allocated. Is this possible? Are there any alternative methods? I'm just trying to debug and make sure my math isn't off at runtime.

Thanks.

Why not put the result of the math into a variable, then examine that?

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Originally Posted by laserlight

Why not put the result of the math into a variable, then examine that?

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That's a possibility for sure but was wondering if the functionality was in gdb that way I could examine other buffers as well. I'm guessing gdb does not do this? I've used ptype for stack buffers but doesn't work on malloc'd ones.