  Fragmentation functions represent a key ingredient to address the proton spin
structure in semi-inclusive deep-inelastic scattering and proton-proton
collisions. They can not be determined from perturbative Quantum Chromodynamics
and have to be extracted from experimental data in different processes. The
COMPASS experiment at CERN provides a large data sample and covers a wide
kinematic range for precise measurement of hadron multiplicities, directly
connected to fragmentation functions. Recent, full-differential results on pion
and kaon multiplicities are presented and discussed
