The getenv() function shall search the environment of the calling process
(see the Base Definitions volume of IEEE Std 1003.1-2001,
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 string pointed to may be overwritten by a subsequent call to
getenv(), setenv(), or unsetenv(), but shall not be
overwritten by a call to any other function in this volume of
IEEE Std 1003.1-2001.
If the application modifies environ or the pointers to which it points,
the behavior of getenv() is undefined.
The getenv() function need not be reentrant. A function that is not
required to be reentrant is not required to 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.
The return value from getenv() may point to static data which may be
overwritten by subsequent calls to getenv(), setenv(), or
On XSI-conformant systems, the return value from getenv() may point to
static data which may also be overwritten by subsequent calls to
The clearenv() function was considered but rejected. The putenv()
function has now been included for alignment with the Single UNIX
The getenv() function is inherently not reentrant because it returns a
value pointing to static data.
Conforming applications are required not to modify environ directly, but
to use only the functions described here to manipulate the process environment
as an abstract object. Thus, the implementation of the environment access
functions has complete control over the data structure used to represent the
environment (subject to the requirement that environ be maintained as a
list of strings with embedded equal signs for applications that wish to scan
the environment). This constraint allows the implementation to properly manage
the memory it allocates, either by using allocated storage for all variables
(copying them on the first invocation of setenv() or
unsetenv()), or keeping track of which strings are currently in
allocated space and which are not, via a separate table or some other means.
This enables the implementation to free any allocated space used by 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.
In fact, for higher performance of getenv(), the implementation 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.
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
The POSIX Threads Extension states that multi-threaded applications must not
modify environ directly, and that IEEE Std 1003.1-2001 is
providing functions which such applications can use in the future to
manipulate the environment in a thread-safe manner. Thus, moving away from
application use of environ is desirable from that standpoint as well.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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 at http://www.opengroup.org/unix/online.html