получить значение переменной окружения (get value of an environment variable)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
getenv — get value of an environment variable
Синопсис (Synopsis)
#include <stdlib.h>
char *getenv(const char *name);
Описание (Description)
The functionality described on this reference page is aligned
with the ISO C standard. Any conflict between the requirements
described here and the ISO C standard is unintentional. This
volume of POSIX.1‐2017 defers to the ISO C standard.
The getenv() function shall search the environment of the calling
process (see the Base Definitions volume of POSIX.1‐2017, Chapter
8, Environment Variables) for the environment variable name if it
exists and return a pointer to the value of the environment
variable. If the specified environment variable cannot be found,
a null pointer shall be returned. The application shall ensure
that it does not modify the string pointed to by the getenv()
function.
The returned string pointer might be invalidated or the string
content might be overwritten by a subsequent call to getenv(),
setenv(), unsetenv(),
or (if supported) putenv() but they shall not be affected by a
call to any other function in this volume of POSIX.1‐2017.
The returned string pointer might also be invalidated if the
calling thread is terminated.
The getenv() function need not be thread-safe.
Возвращаемое значение (Return value)
Upon successful completion, getenv() shall return a pointer to a
string containing the value for the specified name. If the
specified name cannot be found in the environment of the calling
process, a null pointer shall be returned.
Ошибки (Error)
No errors are defined.
The following sections are informative.
Примеры (Examples)
Getting the Value of an Environment Variable
The following example gets the value of the HOME environment
variable.
#include <stdlib.h>
...
const char *name = "HOME";
char *value;
value = getenv(name);
Использование в приложениях (Application usage)
None.
Обоснование (Rationale)
The clearenv() function was considered but rejected. The putenv()
function has now been included for alignment with the Single UNIX
Specification.
The getenv() function is inherently not thread-safe because it
returns a value pointing to static data.
Conforming applications are required not to directly modify the
pointers to which environ points, but to use only the setenv(),
unsetenv(), and putenv() functions, or assignment to environ
itself, to manipulate the process environment. This constraint
allows the implementation to properly manage the memory it
allocates. This enables the implementation to free any space it
has allocated to strings (and perhaps the pointers to them)
stored in environ when unsetenv() is called. A C runtime start-up
procedure (that which invokes main() and perhaps initializes
environ) can also initialize a flag indicating that none of the
environment has yet been copied to allocated storage, or that the
separate table has not yet been initialized. If the application
switches to a complete new environment by assigning a new value
to environ, this can be detected by getenv(), setenv(),
unsetenv(), or putenv() and the implementation can at that point
reinitialize based on the new environment. (This may include
copying the environment strings into a new array and assigning
environ to point to it.)
In fact, for higher performance of getenv(), implementations that
do not provide putenv() could also maintain a separate copy of
the environment in a data structure that could be searched much
more quickly (such as an indexed hash table, or a binary tree),
and update both it and the linear list at environ when setenv()
or unsetenv() is invoked. On implementations that do provide
putenv(), such a copy might still be worthwhile but would need to
allow for the fact that applications can directly modify the
content of environment strings added with putenv(). For example,
if an environment string found by searching the copy is one that
was added using putenv(), the implementation would need to check
that the string in environ still has the same name (and value, if
the copy includes values), and whenever searching the copy
produces no match the implementation would then need to search
each environment string in environ that was added using putenv()
in case any of them have changed their names and now match. Thus,
each use of putenv() to add to the environment would reduce the
speed advantage of having the copy.
Performance of getenv() can be important for applications which
have large numbers of environment variables. Typically,
applications like this use the environment as a resource database
of user-configurable parameters. The fact that these variables
are in the user's shell environment usually means that any other
program that uses environment variables (such as ls, which
attempts to use COLUMNS), or really almost any utility (LANG,
LC_ALL, and so on) is similarly slowed down by the linear search
through the variables.
An implementation that maintains separate data structures, or
even one that manages the memory it consumes, is not currently
required as it was thought it would reduce consensus among
implementors who do not want to change their historical
implementations.
Будущие направления (Future directions)
A future version may add one or more functions to access and
modify the environment in a thread-safe manner.
Смотри также (See also)
exec(1p), putenv(3p), setenv(3p), unsetenv(3p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, stdlib.h(0p)
