We present a model atom for C I - C II - C III - C IV using the most up-to-date atomic data and evaluated the non-local thermodynamic equilibrium (NLTE) line formation in classical 1D atmospheric models of O-B-type stars. Our models predict the emission lines of C II 9903~AA and 18535~AA to appear at effective temperature Teff~$geq$~17,500~K, those of C II 6151~AA and 6461~AA to appear at Teff~$>$~25,000~K, and those of C III 5695, 6728--44, 9701--17~AA to appear at Teff~$geq$~35,000~K (log~$g$=4.0). Emission occurs in the lines of minority species, where the photoionization-recombination mechanism provides a depopulation of the lower levels to a greater extent than the upper levels. For C II 9903 and 18535~AA, the upper levels are mainly populated from C III reservoir through the Rydberg states. For C III 5695 and 6728--44~AA, the lower levels are depopulated due to photon losses in UV transitions at 885, 1308, and 1426--28~AA which become optically thin in the photosphere. We analysed the lines of C I, C II, C III, and C IV for twenty-two O-B-type stars with temperature range between 15,800 $leq$~Teff~$leq$ 38,000~K. Abundances from emission lines of C I, C II and C III are in agreement with those from absorption ones for most of the stars. We obtained log~$epsilon_{rm C}$=8.36$pm$0.08 from twenty B-type stars, that is in line with the present-day Cosmic Abundance Standard. The obtained carbon abundances in 15~Mon and HD~42088 from emission and absorption lines are 8.27$pm$0.11 and 8.31$pm$0.11, respectively.
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