How to find the \textwidth in two-column mode?

Sometimes I find that floats don't fit nicely on a page, so I use

\resizebox{\textwidth}{!}{ <blah blah blah> }

so that the borders match the margins. However, I'm now working on a paper in two-column format (specifically using sigplanconf.cls) and if I use the above command, it stretches the table over both columns.

Is there a way I can find the "proper" \textwidth? As in, a variable that describes the width of a single column.

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