Despite much effort in the past decades, the C-burning reaction rate is uncertain by several orders of magnitude, and the relative strength between the different channels 12 C (12 C, α) 20 Ne, 12 C (12 C, p) 23 Na, and 12 C (12 C, n) 23 Mg is poorly determined. Additionally, in C-burning conditions a high 12 C+ 12 C rate may lead to lower central C-burning temperatures and to 13 C (α, n) 16 O emerging as a more dominant neutron source than 22 Ne (α, n) 25 Mg, increasing significantly the s-process production. This is due to the chain 12 C (p, γ) 13 N followed by 13 N (β+) 13 C, where the photodisintegration reverse channel 13 N (γ, p) 12 C is strongly decreasing with increasing temperature. Presented here is the impact of the 12 C+ 12 C reaction uncertainties on the s-process and on explosive p-process nucleosynthesis in massive stars, including also fast rotating massive stars at low metallicity. Using various 12 C+ 12 C rates, in particular an upper and lower rate limit of∼ 50,000 higher and∼ 20 lower than the standard rate at 5× 10 8 K, five 25 M☉ stellar models are calculated. The enhanced s-process signature due to 13 C (α, n) 16 O activation is considered, taking into account the impact of the uncertainty of all three C-burning reaction branches. Consequently, we show that the p-process abundances have an average production factor increased up to about a factor of eight compared with the standard case, efficiently producing the elusive Mo and Ru proton-rich isotopes. We also show that an s-process being driven by 13 C (α, n) 16 O is a secondary process, even though the abundance of 13 C does not depend on the initial metal content. Finally, implications for the Sr-peak elements inventory in the solar system and at low metallicity are discussed.