  We study the $\cos 2 \phi$ azimuthal asymmetry in unpolarized semi-inclusive
DIS, taking into account both the perturbative contribution (gluon emission and
splitting) and the non perturbative effects arising from intrinsic transverse
motion and transverse spin of quarks. In particular we explore the possibility
to extract from $<\cos 2 \phi>$ some information about the Boer--Mulders
function $h_1^{\perp}$, which represents a transverse--polarization asymmetry
of quarks inside an unpolarized hadron. Predictions are presented for the
HERMES, COMPASS and JLab kinematics, where $<\cos 2 \phi>$ is dominated by the
kinematical higher--twist contribution, and turns to be of order of few
percent. We show that a larger asymmetry in $\pi^-$ production, compared to
$\pi^+$ production, would represent a signature of the Boer--Mulders effect.
