# 常量

Core.nothingConstant
nothing

The singleton instance of type Nothing, used by convention when there is no value to return (as in a C void function) or when a variable or field holds no value.

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Base.DEPOT_PATHConstant
DEPOT_PATH

A stack of "depot" locations where the package manager, as well as Julia's code loading mechanisms, look for package registries, installed packages, named environments, repo clones, cached compiled package images, and configuration files. By default it includes:

1. ~/.julia where ~ is the user home as appropriate on the system;
2. an architecture-specific shared system directory, e.g. /usr/local/share/julia;
3. an architecture-independent shared system directory, e.g. /usr/share/julia.

So DEPOT_PATH might be:

[joinpath(homedir(), ".julia"), "/usr/local/share/julia", "/usr/share/julia"]

The first entry is the "user depot" and should be writable by and owned by the current user. The user depot is where: registries are cloned, new package versions are installed, named environments are created and updated, package repos are cloned, newly compiled package image files are saved, log files are written, development packages are checked out by default, and global configuration data is saved. Later entries in the depot path are treated as read-only and are appropriate for registries, packages, etc. installed and managed by system administrators.

DEPOT_PATH is populated based on the JULIA_DEPOT_PATH environment variable if set.

DEPOT_PATH contents

Each entry in DEPOT_PATH is a path to a directory which contains subdirectories used by Julia for various purposes. Here is an overview of some of the subdirectories that may exist in a depot:

• clones: Contains full clones of package repos. Maintained by Pkg.jl and used as a cache.
• compiled: Contains precompiled *.ji files for packages. Maintained by Julia.
• dev: Default directory for Pkg.develop. Maintained by Pkg.jl and the user.
• environments: Default package environments. For instance the global environment for a specific julia version. Maintained by Pkg.jl.
• logs: Contains logs of Pkg and REPL operations. Maintained by Pkg.jl and Julia.
• packages: Contains packages, some of which were explicitly installed and some which are implicit dependencies. Maintained by Pkg.jl.
• registries: Contains package registries. By default only General. Maintained by Pkg.jl.

See also: JULIA_DEPOT_PATH, and Code Loading.

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Base.LOAD_PATHConstant
LOAD_PATH

An array of paths for using and import statements to consider as project environments or package directories when loading code. It is populated based on the JULIA_LOAD_PATH environment variable if set; otherwise it defaults to ["@", "@v#.#", "@stdlib"]. Entries starting with @ have special meanings:

• @ refers to the "current active environment", the initial value of which is initially determined by the JULIA_PROJECT environment variable or the --project command-line option.

• @stdlib expands to the absolute path of the current Julia installation's standard library directory.

• @name refers to a named environment, which are stored in depots (see JULIA_DEPOT_PATH) under the environments subdirectory. The user's named environments are stored in ~/.julia/environments so @name would refer to the environment in ~/.julia/environments/name if it exists and contains a Project.toml file. If name contains # characters, then they are replaced with the major, minor and patch components of the Julia version number. For example, if you are running Julia 1.2 then @v#.# expands to @v1.2 and will look for an environment by that name, typically at ~/.julia/environments/v1.2.

The fully expanded value of LOAD_PATH that is searched for projects and packages can be seen by calling the Base.load_path() function.

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Base.Sys.BINDIRConstant
Sys.BINDIR

A string containing the full path to the directory containing the julia executable.

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Base.Sys.CPU_THREADSConstant
Sys.CPU_THREADS

The number of logical CPU cores available in the system, i.e. the number of threads that the CPU can run concurrently. Note that this is not necessarily the number of CPU cores, for example, in the presence of hyper-threading.

See Hwloc.jl or CpuId.jl for extended information, including number of physical cores.

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Base.Sys.KERNELConstant
Sys.KERNEL

A symbol representing the name of the operating system, as returned by uname of the build configuration.

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