GETENV(3P) POSIX Programmer's Manual GETENV(3P)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.
NAMEgetenv — get value of an environment variable
char *getenv(const char *name);
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.1‐2008
defers to the ISO C standard.
The getenv() function shall search the environment of the calling
process (see the Base Definitions volume of POSIX.1‐2008, 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.1‐2008.
The getenv() function need not be thread-safe.
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.
No errors are defined.
The following sections are informative.
Getting the Value of an Environment Variable
The following example gets the value of the HOME environment variable.
const char *name = "HOME";
value = getenv(name);
The clearenv() function was considered but rejected. The putenv() func‐
tion has now been included for alignment with the Single UNIX Specifi‐
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 point‐
ers 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 implementa‐
tion to free any space it has allocated to strings (and perhaps the
pointers to them) stored in environ when unsetenv() is called. A C run‐
time start-up procedure (that which invokes main() and perhaps initial‐
izes environ) can also initialize a flag indicating that none of the
environment has yet been copied to allocated storage, or that the sepa‐
rate 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 environ‐
ment. (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 envi‐
ronment 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
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.
A future version may add one or more functions to access and modify the
environment in a thread-safe manner.
exec, putenv(), setenv(), unsetenv()
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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
at http://www.unix.org/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, see https://www.ker‐
IEEE/The Open Group 2013 GETENV(3P)