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process/doc/reference/environment.adoc
2025-05-06 10:11:41 +08:00

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== `environment.hpp`
[#environment]
=== `environment`
The `environment` header provides facilities to manipulate the current environment and set it for new processes.
An environment is a a `range` of `T` fulfilling these requirements:
For `T value`
* - std::get<0>(value) must return a type comparable to `key_view`.
* - std::get<1>(value) must return a type convertible to `value_view`.
[source,cpp]
----
// Namespace for information and functions regarding the calling process.
namespace environment
{
// A char traits type that reflects the OS rules for string representing environment keys.
/* Can be an alias of std::char_traits. May only be defined for `char` and `wchar_t`.
*
* Windows treats keys as case-insensitive yet preserving. The char traits are made to reflect
* that behaviour.
*/
template<typename Char>
using key_char_traits = implementation_defined ;
// A char traits type that reflects the OS rules for string representing environment values.
/* Can be an alias of std::char_traits. May only be defined for `char` and `wchar_t`.
*/
template<typename Char>
using value_char_traits = implementation_defined ;
// The character type used by the environment. Either `char` or `wchar_t`.
using char_type = implementation_defined ;
// The equal character in an environment string used to separate key and value.
constexpr char_type equality_sign = implementation_defined;
// The delimiter in environemtn lists. Commonly used by the `PATH` variable.
constexpr char_type delimiter = implementation_defined;
// The native handle of an environment. Note that this can be an owning pointer and is generally not thread safe.
using native_handle = implementation_defined;
// A forward iterator over string_view used by a value or value_view to iterator through environments that are lists.
struct value_iterator;
// A view type for a key of an environment
struct key_view
{
using value_type = char_type;
using traits_type = key_char_traits<char_type>;
using string_view_type = basic_string_view<char_type, traits_type>;
using string_type = std::basic_string<char_type, key_char_traits<char_type>>;
key_view() noexcept = default;
key_view( const key_view& p ) = default;
key_view( key_view&& p ) noexcept = default;
template<typename Source>
key_view( const Source& source );
key_view( const char_type * p);
key_view( char_type * p);
~key_view() = default;
key_view& operator=( const key_view& p ) = default;
key_view& operator=( key_view&& p ) noexcept = default;
key_view& operator=( string_view_type source );
void swap( key_view& other ) noexcept;
string_view_type native() const noexcept;
operator string_view_type() const;
int compare( const key_view& p ) const noexcept;
int compare( string_view_type str ) const;
int compare( const value_type* s ) const;
template< class CharT, class Traits = std::char_traits<CharT>,
class Alloc = std::allocator<CharT> >
std::basic_string<CharT,Traits,Alloc>
basic_string( const Alloc& alloc = Alloc()) const;
std::string string() const;
std::wstring wstring() const;
string_type native_string() const;
friend bool operator==(key_view l, key_view r);
friend bool operator!=(key_view l, key_view r);
friend bool operator<=(key_view l, key_view r);
friend bool operator>=(key_view l, key_view r);
friend bool operator< (key_view l, key_view r);
friend bool operator> (key_view l, key_view r);
bool empty() const;
template< class CharT, class Traits >
friend std::basic_ostream<CharT,Traits>&
operator<<( std::basic_ostream<CharT,Traits>& os, const key_view& p );
template< class CharT, class Traits >
friend std::basic_istream<CharT,Traits>&
operator>>( std::basic_istream<CharT,Traits>& is, key_view& p );
const value_type * data() const;
std::size_t size() const;
};
// A view for a value in an environment
struct value_view
{
using value_type = char_type;
using string_view_type = basic_cstring_ref<char_type, value_char_traits<char_type>>;
using string_type = std::basic_string<char_type, value_char_traits<char_type>>;
using traits_type = value_char_traits<char_type>;
value_view() noexcept = default;
value_view( const value_view& p ) = default;
value_view( value_view&& p ) noexcept = default;
template<typename Source>
value_view( const Source& source );
value_view( const char_type * p);
value_view( char_type * p);
~value_view() = default;
value_view& operator=( const value_view& p ) = default;
value_view& operator=( value_view&& p ) noexcept = default;
value_view& operator=( string_view_type source );
void swap( value_view& other ) noexcept;
string_view_type native() const noexcept ;
operator string_view_type() const;
operator typename string_view_type::string_view_type() const;
int compare( const value_view& p ) const noexcept;
int compare( string_view_type str ) const;
int compare( const value_type* s ) const;
template< class CharT, class Traits = std::char_traits<CharT>,
class Alloc = std::allocator<CharT> >
std::basic_string<CharT,Traits,Alloc>
basic_string( const Alloc& alloc = Alloc() ) const;
std::string string() const;
std::wstring wstring() const;
string_type native_string() const;
bool empty() const;
friend bool operator==(value_view l, value_view r);
friend bool operator!=(value_view l, value_view r);
friend bool operator<=(value_view l, value_view r);
friend bool operator>=(value_view l, value_view r);
friend bool operator< (value_view l, value_view r);
friend bool operator> (value_view l, value_view r);
template< class CharT, class Traits >
friend std::basic_ostream<CharT,Traits>&
operator<<( std::basic_ostream<CharT,Traits>& os, const value_view& p );
template< class CharT, class Traits >
friend std::basic_istream<CharT,Traits>&
operator>>( std::basic_istream<CharT,Traits>& is, value_view& p );
value_iterator begin() const;
value_iterator end() const;
const char_type * c_str();
const value_type * data() const;
std::size_t size() const;
};
// A view for a key value pair in an environment
struct key_value_pair_view
{
using value_type = char_type;
using string_type = std::basic_string<char_type>;
using string_view_type = basic_cstring_ref<char_type>;
using traits_type = std::char_traits<char_type>;
key_value_pair_view() noexcept = default;
key_value_pair_view( const key_value_pair_view& p ) = default;
key_value_pair_view( key_value_pair_view&& p ) noexcept = default;
template<typename Source,
typename = typename std::enable_if<is_constructible<string_view_type, Source>::value>::type>
key_value_pair_view( const Source& source );
key_value_pair_view( const char_type * p);
key_value_pair_view( char_type * p);
~key_value_pair_view() = default;
key_value_pair_view& operator=( const key_value_pair_view& p ) = default;
key_value_pair_view& operator=( key_value_pair_view&& p ) noexcept = default;
void swap( key_value_pair_view& other ) noexcept;
string_view_type native() const noexcept;
operator string_view_type() const;
operator typename string_view_type::string_view_type() const;
int compare( key_value_pair_view p ) const noexcept;
int compare( const string_type& str ) const;
int compare( string_view_type str ) const;
int compare( const value_type* s ) const;
template< class CharT, class Traits = std::char_traits<CharT>, class Alloc = std::allocator<CharT> >
std::basic_string<CharT,Traits,Alloc>
basic_string( const Alloc& alloc = Alloc()) const;
std::string string() const;
std::wstring wstring() const;
string_type native_string() const;
bool empty() const;
key_view key() const;
value_view value() const;
friend bool operator==(key_value_pair_view l, key_value_pair_view r);
friend bool operator!=(key_value_pair_view l, key_value_pair_view r);
friend bool operator<=(key_value_pair_view l, key_value_pair_view r);
friend bool operator>=(key_value_pair_view l, key_value_pair_view r);
friend bool operator< (key_value_pair_view l, key_value_pair_view r);
friend bool operator> (key_value_pair_view l, key_value_pair_view r);
template< class CharT, class Traits >
friend std::basic_ostream<CharT,Traits>&
operator<<( std::basic_ostream<CharT,Traits>& os, const key_value_pair_view& p );
template< class CharT, class Traits >
friend std::basic_istream<CharT,Traits>&
operator>>( std::basic_istream<CharT,Traits>& is, key_value_pair_view& p );
template<std::size_t Idx>
inline auto get() const -> typename conditional<Idx == 0u, key_view,
value_view>::type;
const value_type * c_str() const noexcept;
const value_type * data() const;
std::size_t size() const;
};
// Allow tuple-likg getters & structured bindings.
template<> key_view key_value_pair_view::get<0u>() const;
template<> value_view key_value_pair_view::get<1u>() const;
// A class representing a key within an environment.
struct key
{
using value_type = char_type;
using traits_type = key_char_traits<char_type>;
using string_type = std::basic_string<char_type, traits_type>;
using string_view_type = basic_string_view<char_type, traits_type>;
key();
key( const key& p ) = default;
key( key&& p ) noexcept = default;
key( const string_type& source );
key( string_type&& source );
key( const value_type * raw );
key( value_type * raw );
explicit key(key_view kv);
template< class Source >
key( const Source& source);
key(const typename conditional<is_same<value_type, char>::value, wchar_t, char>::type * raw);
template< class InputIt >
key( InputIt first, InputIt last);
~key() = default;
key& operator=( const key& p ) = default;
key& operator=( key&& p );
key& operator=( string_type&& source );
template< class Source >
key& operator=( const Source& source );
key& assign( string_type&& source );
template< class Source >
key& assign( const Source& source );
template< class InputIt >
key& assign( InputIt first, InputIt last );
void clear();
void swap( key& other ) noexcept;
const value_type* c_str() const noexcept;
const string_type& native() const noexcept;
string_view_type native_view() const noexcept;
operator string_type() const;
operator string_view_type() const;
int compare( const key& p ) const noexcept;
int compare( const string_type& str ) const;
int compare( string_view_type str ) const;
int compare( const value_type* s ) const;
template< class CharT, class Traits = std::char_traits<CharT>,
class Alloc = std::allocator<CharT> >
std::basic_string<CharT,Traits,Alloc>
basic_string( const Alloc& alloc = Alloc()) const;
std::string string() const;
std::wstring wstring() const;
const string_type & native_string() const;
bool empty() const;
friend bool operator==(const key & l, const key & r);
friend bool operator!=(const key & l, const key & r);
friend bool operator<=(const key & l, const key & r);
friend bool operator>=(const key & l, const key & r);
friend bool operator< (const key & l, const key & r);
friend bool operator> (const key & l, const key & r);
template< class CharT, class Traits >
friend std::basic_ostream<CharT,Traits>&
operator<<( std::basic_ostream<CharT,Traits>& os, const key& p );
template< class CharT, class Traits >
friend std::basic_istream<CharT,Traits>&
operator>>( std::basic_istream<CharT,Traits>& is, key& p );
const value_type * data() const;
std::size_t size() const;
};
bool operator==(const value_view &, const value_view);
bool operator!=(const value_view &, const value_view);
bool operator<=(const value_view &, const value_view);
bool operator< (const value_view &, const value_view);
bool operator> (const value_view &, const value_view);
bool operator>=(const value_view &, const value_view);
struct value
{
using value_type = char_type;
using traits_type = value_char_traits<char_type>;
using string_type = std::basic_string<char_type, traits_type>;
using string_view_type = basic_cstring_ref<char_type, traits_type>;
value();
value( const value& p ) = default;
value( const string_type& source );
value( string_type&& source );
value( const value_type * raw );
value( value_type * raw );
explicit value(value_view kv);
template< class Source >
value( const Source& source );
value(const typename conditional<is_same<value_type, char>::value, wchar_t, char>::type * raw);
template< class InputIt >
value( InputIt first, InputIt last);
~value() = default;
value& operator=( const value& p ) = default;
value& operator=( value&& p );
value& operator=( string_type&& source );
template< class Source >
value& operator=( const Source& source );
value& assign( string_type&& source );
template< class Source >
value& assign( const Source& source );
template< class InputIt >
value& assign( InputIt first, InputIt last );
void push_back(const value & sv);
void clear() {value_.clear();}
void swap( value& other ) noexcept;
const value_type* c_str() const noexcept;
const string_type& native() const noexcept;
string_view_type native_view() const noexcept;
operator string_type() const;
operator string_view_type() const;
operator typename string_view_type::string_view_type() const;
int compare( const value& p ) const noexcept;
int compare( const string_type& str ) const;
int compare( string_view_type str ) const;
int compare( const value_type* s ) const;
template< class CharT, class Traits = std::char_traits<CharT>,
class Alloc = std::allocator<CharT> >
std::basic_string<CharT,Traits,Alloc>
basic_string( const Alloc& alloc = Alloc()) const;
std::string string() const;
std::wstring wstring() const;
const string_type & native_string() const;
bool empty() const;
friend bool operator==(const value & l, const value & r);
friend bool operator!=(const value & l, const value & r);
friend bool operator<=(const value & l, const value & r);
friend bool operator>=(const value & l, const value & r);
friend bool operator< (const value & l, const value & r);
friend bool operator> (const value & l, const value & r);
template< class CharT, class Traits >
friend std::basic_ostream<CharT,Traits>&
operator<<( std::basic_ostream<CharT,Traits>& os, const value& p );
template< class CharT, class Traits >
friend std::basic_istream<CharT,Traits>&
operator>>( std::basic_istream<CharT,Traits>& is, value& p );
value_iterator begin() const;
value_iterator end() const;
const value_type * data() const;
std::size_t size() const;
};
bool operator==(const value_view &, const value_view);
bool operator!=(const value_view &, const value_view);
bool operator<=(const value_view &, const value_view);
bool operator< (const value_view &, const value_view);
bool operator> (const value_view &, const value_view);
bool operator>=(const value_view &, const value_view);
struct key_value_pair
{
using value_type = char_type;
using traits_type = std::char_traits<char_type>;
using string_type = std::basic_string<char_type>;
using string_view_type = basic_cstring_ref<char_type>;
key_value_pair()l
key_value_pair( const key_value_pair& p ) = default;
key_value_pair( key_value_pair&& p ) noexcept = default;
key_value_pair(key_view key, value_view value);
key_value_pair(key_view key, std::initializer_list<basic_string_view<char_type>> values);
key_value_pair( const string_type& source );
key_value_pair( string_type&& source );
key_value_pair( const value_type * raw );
key_value_pair( value_type * raw );
explicit key_value_pair(key_value_pair_view kv) : value_(kv.c_str()) {}
template< class Source >
key_value_pair( const Source& source);
template< typename Key,
typename Value >
key_value_pair(
const std::pair<Key, Value> & kv);
key_value_pair(const typename conditional<is_same<value_type, char>::value, wchar_t, char>::type * raw);
template< class InputIt , typename std::iterator_traits<InputIt>::iterator_category>
key_value_pair( InputIt first, InputIt last );
~key_value_pair() = default;
key_value_pair& operator=( const key_value_pair& p ) = default;
key_value_pair& operator=( key_value_pair&& p );
key_value_pair& operator=( string_type&& source );
template< class Source >
key_value_pair& operator=( const Source& source );
key_value_pair& assign( string_type&& source );
template< class Source >
key_value_pair& assign( const Source& source );
template< class InputIt >
key_value_pair& assign( InputIt first, InputIt last );
void clear();
void swap( key_value_pair& other ) noexcept;
const value_type* c_str() const noexcept;
const string_type& native() const noexcept;
string_view_type native_view() const noexcept;
operator string_type() const;
operator string_view_type() const;
operator typename string_view_type::string_view_type() const;
operator key_value_pair_view() const;
int compare( const key_value_pair& p ) const noexcept;
int compare( const string_type& str ) const;
int compare( string_view_type str ) const;
int compare( const value_type* s ) const;
template< class CharT, class Traits = std::char_traits<CharT>, class Alloc = std::allocator<CharT> >
std::basic_string<CharT,Traits,Alloc>
basic_string( const Alloc& alloc = Alloc() ) const;
std::string string() const {return basic_string<char>();}
std::wstring wstring() const {return basic_string<wchar_t>();}
const string_type & native_string() const;
friend bool operator==(const key_value_pair & l, const key_value_pair & r);
friend bool operator!=(const key_value_pair & l, const key_value_pair & r);
friend bool operator<=(const key_value_pair & l, const key_value_pair & r);
friend bool operator>=(const key_value_pair & l, const key_value_pair & r);
friend bool operator< (const key_value_pair & l, const key_value_pair & r);
friend bool operator> (const key_value_pair & l, const key_value_pair & r);
bool empty() const;
struct key_view key() const;
struct value_view value() const;
template< class CharT, class Traits >
friend std::basic_ostream<CharT,Traits>&
operator<<( std::basic_ostream<CharT,Traits>& os, const key_value_pair& p );
template< class CharT, class Traits >
friend std::basic_istream<CharT,Traits>&
operator>>( std::basic_istream<CharT,Traits>& is, key_value_pair& p );
const value_type * data() const;
std::size_t size() const;
template<std::size_t Idx>
inline auto get() const -> typename conditional<Idx == 0u,environment::key_view, environment::value_view>::type;
};
bool operator==(const key_value_pair_view &, const key_value_pair_view);
bool operator!=(const key_value_pair_view &, const key_value_pair_view);
bool operator<=(const key_value_pair_view &, const key_value_pair_view);
bool operator< (const key_value_pair_view &, const key_value_pair_view);
bool operator> (const key_value_pair_view &, const key_value_pair_view);
bool operator>=(const key_value_pair_view &, const key_value_pair_view);
// Allow tuple-likg getters & structured bindings.
template<>
key_view key_value_pair::get<0u>() const;
template<>
value_view key_value_pair::get<1u>() const;
// A view object for the current environment of this process.
/*
* The view might (windows) or might not (posix) be owning;
* if it owns it will deallocate the on destruction, like a unique_ptr.
*
* Note that accessing the environment in this way is not thread-safe.
*
* @code
*
* void dump_my_env(current_view env = current())
* {
* for (auto & [k, v] : env)
* std::cout << k.string() << " = " << v.string() << std::endl;
* }
*
* @endcode
*
*
*/
struct current_view
{
using native_handle_type = environment::native_handle_type;
using value_type = key_value_pair_view;
current_view() = default;
current_view(current_view && nt) = default;
native_handle_type native_handle() { return handle_.get(); }
struct iterator
{
using value_type = key_value_pair_view;
using difference_type = int;
using reference = key_value_pair_view;
using pointer = key_value_pair_view;
using iterator_category = std::forward_iterator_tag;
iterator() = default;
iterator(const iterator & ) = default;
iterator(const native_iterator &native_handle) : iterator_(native_handle) {}
iterator & operator++()
{
iterator_ = detail::next(iterator_);
return *this;
}
iterator operator++(int)
{
auto last = *this;
iterator_ = detail::next(iterator_);
return last;
}
key_value_pair_view operator*() const
{
return detail::dereference(iterator_);
}
friend bool operator==(const iterator & l, const iterator & r) {return l.iterator_ == r.iterator_;}
friend bool operator!=(const iterator & l, const iterator & r) {return l.iterator_ != r.iterator_;}
private:
environment::native_iterator iterator_;
};
iterator begin() const {return iterator(handle_.get());}
iterator end() const {return iterator(detail::find_end(handle_.get()));}
private:
std::unique_ptr<typename remove_pointer<native_handle_type>::type,
detail::native_handle_deleter> handle_{environment::detail::load_native_handle()};
};
// Obtain a handle to the current environment
inline current_view current() {return current_view();}
// Find the home folder in an environment-like type.
/*
* @param env The environment to search. Defaults to the current environment of this process
*
* The environment type passed in must be a range with value T that fulfills the following requirements:
*
* For `T value`
*
* - std::get<0>(value) must return a type comparable to `key_view`.
* - std::get<1>(value) must return a type convertible to filesystem::path.
*
* @return A filesystem::path to the home directory or an empty path if it cannot be found.
*
*/
template<typename Environment = current_view>
inline filesystem::path home(Environment && env = current());
// Find the executable `name` in an environment-like type.
template<typename Environment = current_view>
filesystem::path find_executable(BOOST_PROCESS_V2_NAMESPACE::filesystem::path name,
Environment && env = current());
// Get an environment variable from the current process.
value get(const key & k, error_code & ec);
value get(const key & k);
value get(basic_cstring_ref<char_type, key_char_traits<char_type>> k, error_code & ec);
value get(basic_cstring_ref<char_type, key_char_traits<char_type>> k);
value get(const char_type * c, error_code & ec);
value get(const char_type * c);
// Set an environment variable for the current process.
void set(const key & k, value_view vw, error_code & ec);
void set(const key & k, value_view vw);
void set(basic_cstring_ref<char_type, key_char_traits<char_type>> k, value_view vw, error_code & ec);
void set(basic_cstring_ref<char_type, key_char_traits<char_type>> k, value_view vw);
void set(const char_type * k, value_view vw, error_code & ec);
void set(const char_type * k, value_view vw);
template<typename Char>
void set(const key & k, const Char * vw, error_code & ec);
template<typename Char>
void set(const key & k, const Char * vw);
template<typename Char>
void set(basic_cstring_ref<char_type, key_char_traits<char_type>> k, const Char * vw, error_code & ec);
template<typename Char>
void set(basic_cstring_ref<char_type, key_char_traits<char_type>> k, const Char * vw);
template<typename Char>
void set(const char_type * k, const Char * vw, error_code & ec);
template<typename Char>
void set(const char_type * k, const Char * vw);
// Remove an environment variable from the current process.
void unset(const key & k, error_code & ec);
void unset(const key & k);
void unset(basic_cstring_ref<char_type, key_char_traits<char_type>> k, error_code & ec);
void unset(basic_cstring_ref<char_type, key_char_traits<char_type>> k);
void unset(const char_type * c, error_code & ec);
void unset(const char_type * c);
}
----
=== `process_environment`
In order to set the environment of a child process, `process_environment` can be used.
[source, cpp]
.This will set the environment in a subprocess:
----
process proc{executor, find_executable("printenv"), {"foo"}, process_environment{"foo=bar"}};
----
The environment initializer will persist it's state, so that it can
be used multiple times. Do however note the the Operating System is
allowed to modify the internal state.
[source,cpp]
----
auto exe = find_executable("printenv");
process_environment env = {"FOO=BAR", "BAR=FOO"};
process proc1(executor, exe, {"FOO"}, env);
process proc2(executor, exe, {"BAR"}, env);
----