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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.

DESCRIPTION top

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. Thegetenv() function shall search the environment of the calling process (see the Base Definitions volume of POSIX.1‐2017,Chapter8,Environment Variables) for the environment variablename 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 thegetenv() function. The returned string pointer might be invalidated or the string content might be overwritten by a subsequent call togetenv(),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. Thegetenv() function need not be thread-safe.

Upon successful completion,getenv() shall return a pointer to a string containing thevalue for the specifiedname. If the specifiedname cannot be found in the environment of the calling process, a null pointer shall be returned.

Getting the Value of an Environment Variable The following example gets the value of theHOME environment variable. #include <stdlib.h> ... const char *name = "HOME"; char *value; value = getenv(name);

RATIONALE top

Theclearenv() function was considered but rejected. Theputenv() function has now been included for alignment with the Single UNIX Specification. Thegetenv() 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 whichenviron points, but to use only thesetenv(),unsetenv(), andputenv() functions, or assignment toenviron 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 inenviron whenunsetenv() is called. A C runtime start-up procedure (that which invokesmain() and perhaps initializesenviron) 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 toenviron, this can be detected bygetenv(),setenv(),unsetenv(), orputenv() 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 assigningenviron to point to it.) In fact, for higher performance ofgetenv(), implementations that do not provideputenv() 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 atenviron whensetenv() orunsetenv() is invoked. On implementations that do provideputenv(), 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 withputenv(). For example, if an environment string found by searching the copy is one that was added usingputenv(), the implementation would need to check that the string inenviron 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 inenviron that was added usingputenv() in case any of them have changed their names and now match. Thus, each use ofputenv() to add to the environment would reduce the speed advantage of having the copy. Performance ofgetenv() 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 asls, which attempts to useCOLUMNS), 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.

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online athttp://www.opengroup.org/unix/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group 2017GETENV(3P)

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