Problem - Kevin Tafuro

Typically, Unix systems are considerably more dependent on environment variables
than are Windows systems. In fact, the only scenario common to both Unix and
Windows is that there is an environment variable defining the path that the system
should search to find an executable or shared library (although differently named
variables are used on each platform). On Windows, one environment variable controls
the search path for finding both executables and shared libraries. On Unix,
these are controlled by separate environment variables. Generally, you should not
specify a filename and then rely on these variables for determining the full path.
Instead, you should always use absolute paths to known locations. *
Certain variables expected to be present in the environment can cause insecure program
behavior if they are missing or improperly set. Make sure, therefore, that you
never fully purge the environment and leave it empty. Instead, variables that should
exist should be forced to sane values or, at the very least, treated as highly suspect
and examined closely before they’re used. Remove any unknown variables from the
environment altogether.
Discussion
The standard C runtime library defines a global variable, † environ, asaNULL-terminated
array of strings, where each string in the array is of the form “name=value”.
Most systems do not declare the variable in any standard header file, Linux being the
notable exception, providing a declaration in unistd.h. You can gain access to the
variable by including the following extern statement in your code:
extern char **environ;
Several functions defined in stdlib.h, such as getenv( ) and putenv( ), provide access
to environment variables, and they all operate on this variable. You can therefore
make changes to the contents of the array or even build a new array and assign it to
the variable.
This variable also exists in the standard C runtime library on Windows; however, the
C runtime on Windows is not as tightly bound to the operating system as it is on
Unix. Directly manipulating the environ variable on Windows will not necessarily
produce the same effects as it will on Unix; in the majority of Windows programs,
the C runtime is never used at all, instead favoring the Win32 API to perform the
same functions as those provided by the C runtime. Because of this, and because of
Windows’ lack of dependence on environment variables, we do not recommend
* Note that the shared library environment variable can be relatively benign on modern Unix-based operating
systems, because the environment variable will get ignored when a program that can change permissions (i.e.,
a setuid program) is invoked. Nonetheless, it is better to be safe than sorry!
† The use of the term “variable” can quickly become confusing because C defines variables and the environment
defines variables. In this recipe, when we are referring to a C variable, we simply say “variable,” and
when we are referring to an environment variable, we say “environment variable.”
2 | Chapter 1: Safe Initialization
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.