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Eelco Dolstra 2020-07-31 15:43:25 +02:00
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122
doc/manual/src/expressions/advanced-attributes.md
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@ -6,7 +6,9 @@ Derivations can declare some infrequently used optional attributes.
The optional attribute `allowedReferences` specifies a list of legal
references (dependencies) of the output of the builder. For example,
allowedReferences = [];
```nix
allowedReferences = [];
```
enforces that the output of a derivation cannot have any runtime
dependencies on its inputs. To allow an output to have a runtime
@ -20,7 +22,9 @@ Derivations can declare some infrequently used optional attributes.
the legal requisites of the whole closure, so all the dependencies
recursively. For example,
allowedRequisites = [ foobar ];
```nix
allowedRequisites = [ foobar ];
```
enforces that the output of a derivation cannot have any other
runtime dependency than `foobar`, and in addition it enforces that
@ -31,7 +35,9 @@ Derivations can declare some infrequently used optional attributes.
illegal references (dependencies) of the output of the builder. For
example,
disallowedReferences = [ foo ];
```nix
disallowedReferences = [ foo ];
```
enforces that the output of a derivation cannot have a direct
runtime dependencies on the derivation `foo`.
@ -41,7 +47,9 @@ Derivations can declare some infrequently used optional attributes.
specifies illegal requisites for the whole closure, so all the
dependencies recursively. For example,
disallowedRequisites = [ foobar ];
```nix
disallowedRequisites = [ foobar ];
```
enforces that the output of a derivation cannot have any runtime
dependency on `foobar` or any other derivation depending recursively
@ -50,20 +58,20 @@ Derivations can declare some infrequently used optional attributes.
- `exportReferencesGraph`
This attribute allows builders access to the references graph of
their inputs. The attribute is a list of inputs in the Nix store
whose references graph the builder needs to know. The value of this
attribute should be a list of pairs `[ name1
path1 name2
path2 ...
]`. The references graph of each *pathN* will be stored in a text
file *nameN* in the temporary build directory. The text files have
the format used by `nix-store
--register-validity` (with the deriver fields left empty). For
example, when the following derivation is built:
whose references graph the builder needs to know. The value of
this attribute should be a list of pairs `[ name1 path1 name2
path2 ... ]`. The references graph of each *pathN* will be stored
in a text file *nameN* in the temporary build directory. The text
files have the format used by `nix-store --register-validity`
(with the deriver fields left empty). For example, when the
following derivation is built:
derivation {
...
exportReferencesGraph = [ "libfoo-graph" libfoo ];
};
```nix
derivation {
...
exportReferencesGraph = [ "libfoo-graph" libfoo ];
};
```
the references graph of `libfoo` is placed in the file
`libfoo-graph` in the temporary build directory.
@ -84,7 +92,9 @@ Derivations can declare some infrequently used optional attributes.
environment variables to be passed unmodified. For example,
`fetchurl` in Nixpkgs has the line
impureEnvVars = [ "http_proxy" "https_proxy" ... ];
```nix
impureEnvVars = [ "http_proxy" "https_proxy" ... ];
```
to make it use the proxy server configuration specified by the user
in the environment variables `http_proxy` and friends.
@ -116,19 +126,23 @@ Derivations can declare some infrequently used optional attributes.
been modified, the caller must also specify a cryptographic hash of
the file. For example,
fetchurl {
url = "http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
}
```nix
fetchurl {
url = "http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
}
```
It sometimes happens that the URL of the file changes, e.g., because
servers are reorganised or no longer available. We then must update
the call to `fetchurl`, e.g.,
fetchurl {
url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
}
```nix
fetchurl {
url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
}
```
If a `fetchurl` derivation was treated like a normal derivation, the
output paths of the derivation and *all derivations depending on it*
@ -147,23 +161,25 @@ Derivations can declare some infrequently used optional attributes.
As an example, here is the (simplified) Nix expression for
`fetchurl`:
{ stdenv, curl }: # The curl program is used for downloading.
{ url, sha256 }:
stdenv.mkDerivation {
name = baseNameOf (toString url);
builder = ./builder.sh;
buildInputs = [ curl ];
# This is a fixed-output derivation; the output must be a regular
# file with SHA256 hash sha256.
outputHashMode = "flat";
outputHashAlgo = "sha256";
outputHash = sha256;
inherit url;
}
```nix
{ stdenv, curl }: # The curl program is used for downloading.
{ url, sha256 }:
stdenv.mkDerivation {
name = baseNameOf (toString url);
builder = ./builder.sh;
buildInputs = [ curl ];
# This is a fixed-output derivation; the output must be a regular
# file with SHA256 hash sha256.
outputHashMode = "flat";
outputHashAlgo = "sha256";
outputHash = sha256;
inherit url;
}
```
The `outputHashAlgo` attribute specifies the hash algorithm used to
compute the hash. It can currently be `"sha1"`, `"sha256"` or
@ -196,21 +212,19 @@ Derivations can declare some infrequently used optional attributes.
A list of names of attributes that should be passed via files rather
than environment variables. For example, if you have
```
```nix
passAsFile = ["big"];
big = "a very long string";
```
then when the builder runs, the environment variable `bigPath` will
contain the absolute path to a temporary file containing `a very
long
string`. That is, for any attribute *x* listed in `passAsFile`, Nix
will pass an environment variable `xPath` holding the path of the
file containing the value of attribute *x*. This is useful when you
need to pass large strings to a builder, since most operating
systems impose a limit on the size of the environment (typically, a
few hundred kilobyte).
then when the builder runs, the environment variable `bigPath`
will contain the absolute path to a temporary file containing `a
very long string`. That is, for any attribute *x* listed in
`passAsFile`, Nix will pass an environment variable `xPath`
holding the path of the file containing the value of attribute
*x*. This is useful when you need to pass large strings to a
builder, since most operating systems impose a limit on the size
of the environment (typically, a few hundred kilobyte).
- `preferLocalBuild`
If this attribute is set to `true` and [distributed building is