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In low memory environments, "nix-env -qa" failed because the fork to run the pager hit the kernel's overcommit limits. Using posix_spawn gets around this. (Actually, you have to use posix_spawn with the undocumented POSIX_SPAWN_USEVFORK flag, otherwise it just uses fork/exec...)
Nix is a purely functional package manager. For installation and usage instructions, please read the manual, which can be found in `docs/manual/manual.html', and additionally at the Nix website at <http://nixos.org/>. Acknowledgments This product includes software developed by the OpenSSL Project for use in the OpenSSL Toolkit (http://www.OpenSSL.org/).