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GETENV(3P) POSIX Programmer's Manual GETENV(3P)
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.
getenv — get value of an environment variable
#include <stdlib.h>
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.
#include <stdlib.h>
...
const char *name = "HOME";
char *value;
value = getenv(name);
None.
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.
A future version may add one or more functions to access and modify
the environment in a thread-safe manner.
exec(1p), putenv(3p), setenv(3p), unsetenv(3p)
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables, stdlib.h(0p)
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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 GETENV(3P)
Pages that refer to this page: stdlib.h(0p), exec(3p), putenv(3p), setenv(3p), unsetenv(3p)