#include <stdlib.h> char *getenv(const char *name);
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.
The following sections are informative.
The following example gets the value of the HOME environment variable.
#include <stdlib.h> ... const char *name = "HOME"; char *value; value = getenv(name);
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.
The Base Definitions volume of POSIX.1-2017, Chapter 8, Environment Variables, <stdlib.h>
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