The feature test was incorrect: _POSIX_C_SOURCE=2 "The value 2 or greater additionally exposes definitions for POSIX.2-1992." http://man7.org/linux/man-pages/man7/feature_test_macros.7.html A higher value is needed (atleast 1995): https://pubs.opengroup.org/onlinepubs/9699919799/functions/snprintf.html FreeBSD feature test macro: on https://github.com/freebsd/freebsd/blob/master/include/stdio.h line 297 This was already fixed in dmenu. This fixes a warning on FreeBSD, reported by Plasmoduck on IRC, thanks.main