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diff --git a/doc/manual/src/language/derivations.md b/doc/manual/src/language/derivations.md new file mode 100644 index 000000000..3391ec0d8 --- /dev/null +++ b/doc/manual/src/language/derivations.md @@ -0,0 +1,161 @@ +# Derivations + +The most important built-in function is `derivation`, which is used to +describe a single derivation (a build action). It takes as input a set, +the attributes of which specify the inputs of the build. + + - There must be an attribute named [`system`]{#attr-system} whose value must be a + string specifying a Nix system type, such as `"i686-linux"` or + `"x86_64-darwin"`. (To figure out your system type, run `nix -vv + --version`.) The build can only be performed on a machine and + operating system matching the system type. (Nix can automatically + [forward builds for other + platforms](../advanced-topics/distributed-builds.md) by forwarding + them to other machines.) + + - There must be an attribute named `name` whose value must be a + string. This is used as a symbolic name for the package by + `nix-env`, and it is appended to the output paths of the derivation. + + - There must be an attribute named `builder` that identifies the + program that is executed to perform the build. It can be either a + derivation or a source (a local file reference, e.g., + `./builder.sh`). + + - Every attribute is passed as an environment variable to the builder. + Attribute values are translated to environment variables as follows: + + - Strings and numbers are just passed verbatim. + + - A *path* (e.g., `../foo/sources.tar`) causes the referenced file + to be copied to the store; its location in the store is put in + the environment variable. The idea is that all sources should + reside in the Nix store, since all inputs to a derivation should + reside in the Nix store. + + - A *derivation* causes that derivation to be built prior to the + present derivation; its default output path is put in the + environment variable. + + - Lists of the previous types are also allowed. They are simply + concatenated, separated by spaces. + + - `true` is passed as the string `1`, `false` and `null` are + passed as an empty string. + + - The optional attribute `args` specifies command-line arguments to be + passed to the builder. It should be a list. + + - The optional attribute `outputs` specifies a list of symbolic + outputs of the derivation. By default, a derivation produces a + single output path, denoted as `out`. However, derivations can + produce multiple output paths. This is useful because it allows + outputs to be downloaded or garbage-collected separately. For + instance, imagine a library package that provides a dynamic library, + header files, and documentation. A program that links against the + library doesn’t need the header files and documentation at runtime, + and it doesn’t need the documentation at build time. Thus, the + library package could specify: + + ```nix + outputs = [ "lib" "headers" "doc" ]; + ``` + + This will cause Nix to pass environment variables `lib`, `headers` + and `doc` to the builder containing the intended store paths of each + output. The builder would typically do something like + + ```bash + ./configure \ + --libdir=$lib/lib \ + --includedir=$headers/include \ + --docdir=$doc/share/doc + ``` + + for an Autoconf-style package. You can refer to each output of a + derivation by selecting it as an attribute, e.g. + + ```nix + buildInputs = [ pkg.lib pkg.headers ]; + ``` + + The first element of `outputs` determines the *default output*. + Thus, you could also write + + ```nix + buildInputs = [ pkg pkg.headers ]; + ``` + + since `pkg` is equivalent to `pkg.lib`. + +The function `mkDerivation` in the Nixpkgs standard environment is a +wrapper around `derivation` that adds a default value for `system` and +always uses Bash as the builder, to which the supplied builder is passed +as a command-line argument. See the Nixpkgs manual for details. + +The builder is executed as follows: + + - A temporary directory is created under the directory specified by + `TMPDIR` (default `/tmp`) where the build will take place. The + current directory is changed to this directory. + + - The environment is cleared and set to the derivation attributes, as + specified above. + + - In addition, the following variables are set: + + - `NIX_BUILD_TOP` contains the path of the temporary directory for + this build. + + - Also, `TMPDIR`, `TEMPDIR`, `TMP`, `TEMP` are set to point to the + temporary directory. This is to prevent the builder from + accidentally writing temporary files anywhere else. Doing so + might cause interference by other processes. + + - `PATH` is set to `/path-not-set` to prevent shells from + initialising it to their built-in default value. + + - `HOME` is set to `/homeless-shelter` to prevent programs from + using `/etc/passwd` or the like to find the user's home + directory, which could cause impurity. Usually, when `HOME` is + set, it is used as the location of the home directory, even if + it points to a non-existent path. + + - `NIX_STORE` is set to the path of the top-level Nix store + directory (typically, `/nix/store`). + + - For each output declared in `outputs`, the corresponding + environment variable is set to point to the intended path in the + Nix store for that output. Each output path is a concatenation + of the cryptographic hash of all build inputs, the `name` + attribute and the output name. (The output name is omitted if + it’s `out`.) + + - If an output path already exists, it is removed. Also, locks are + acquired to prevent multiple Nix instances from performing the same + build at the same time. + + - A log of the combined standard output and error is written to + `/nix/var/log/nix`. + + - The builder is executed with the arguments specified by the + attribute `args`. If it exits with exit code 0, it is considered to + have succeeded. + + - The temporary directory is removed (unless the `-K` option was + specified). + + - If the build was successful, Nix scans each output path for + references to input paths by looking for the hash parts of the input + paths. Since these are potential runtime dependencies, Nix registers + them as dependencies of the output paths. + + - After the build, Nix sets the last-modified timestamp on all files + in the build result to 1 (00:00:01 1/1/1970 UTC), sets the group to + the default group, and sets the mode of the file to 0444 or 0555 + (i.e., read-only, with execute permission enabled if the file was + originally executable). Note that possible `setuid` and `setgid` + bits are cleared. Setuid and setgid programs are not currently + supported by Nix. This is because the Nix archives used in + deployment have no concept of ownership information, and because it + makes the build result dependent on the user performing the build. |