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 correspond-
ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
NAME
getenv -- get value of an environment variable
SYNOPSIS
#include <stdlib.h>
char *getenv(const char *name);
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.1-2008 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.
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.
ERRORS
No errors are defined.
The following sections are informative.
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);
APPLICATION USAGE
None.
RATIONALE
The clearenv() function was considered but rejected. The putenv() function has now been included for alignment with the Single UNIX Speci-
fication.
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 ini-
tializes 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 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 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.
FUTURE DIRECTIONS
A future version may add one or more functions to access and modify the environment in a thread-safe manner.
SEE ALSO
exec, putenv(), setenv(), unsetenv()
The Base Definitions volume of POSIX.1-2008, Chapter 8, Environment Variables, <stdlib.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technol-
ogy -- 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 Stan-
dard 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)