Let f, g, h, k : $mathbb{R}^n{ ightarrow}mathbb{C}$ be locally integrable functions. We deal with the $L^{infty}$-version of the Hyers-Ulam stability of the quadratic functional inequality and the Pexiderized quadratic functional inequality $${parallel}f(x + y) + f(x - y) -2f(x) - 2f(y){parallel}_{L^{infty}(mathbb{R}^n)}leqvarepsilon$$ $${parallel}f(x + y) + g(x - y) -2h(x) - 2f(y){parallel}_{L^{infty}(mathbb{R}^n)}leqvarepsilon$$ based on the concept of linear functionals on the space of smooth functions with compact support.