Not quite. nextafterf() is trying to return the smallest float after 0.0 in the direction of infinity (infinity is still valid even when the FPU is configured to flush denormal numbers to 0; infinity isn’t a denormal number). But since the smallest float after zero is denormal, when a shared library built with -ffast-math is loaded (which sets the FPU’s FTZ and DAZ bit) denormal numbers get forced to zero, and so nextafterf returns 0.0 and the comparison becomes (x >= 0.0), which means that x=0.0 passes the check and ceilf returns 0.0 rather than 1.0.