Operating Systems - Projects and Exercises

Environment Access Functions

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Each process has an environment associated with it. The environment strings are usually of the form:

name=value (standard NULL terminated strings)

and are referenced by an array of pointers to these strings. This array is made available to a process through the C Run Time library as:

extern char **environ; // NULL terminated array of char *

While an application can access the environment directly through this array, some functions are available to access and manipulate the environment:

#include <stdlib>

char *getenv(const char *name);

Returns pointer to value associated with name, NULL if not found.

getenv returns a pointer to the value of a name=value string. You should always use getenv to fetch a specific value from the environment rather than accessing environ directly.

getenv is supported by both the ANSI C and POSIX standards,

In addition to fetching the value of an environment variable, sometimes it is necessary to set a variable. You mjay want to change the value of an existing variable, or add a new variable to the environment. Unfortunately not all systems support this capability.

#include <stdlib>

int putenv(const char *str);

int setenv(const char *name, const char *value, int rewrite);

Both return: 0 if OK, non-zero on error

void unsetenv(const char *name);

putenv takes a string of the form "name=value" and places it in the environment list. If the name already exists, its old definition is first removed.

N.B. while getenv is supported by both the ANSI C and POSIX standards, putenv is not supported under ANSI C and has only recent support in the POSIX, SVR4 and BSD environments.
setenv sets name to value. If name already exists, its old definition is first removed if rewrite is non-zero. Otherwise the value is not overwritten.
unsetenv removes any definition of name.

N.B. Neither setenv or unsetenv are currently supported under ANSI C or POSIX and are only supported in the BSD environment.

You may need to use putenv with the environment value left blank to unset an environmment object. i.e. putenv("myname=")

The descriptions of the system functions above are drawn from sources that include manuals on the Sun Solaris system and the MAC OS X Darwin/BDS system, and also from 'Advanced Programming in the UNIX Environment', W. Richard Stevens, Addison-Wesley, 1993.

For use only by students and instructors using the supplementary material available with the text book: "Operating Systems - Internals and Design Principles", William Stallings, Prentice Hall, 5th Edition, 2004. Not to be printed out or copied by any other persons or used for any other purpose without written permission of the author(s).