In a typical C or Unix like programming environment, how can a program get and set environment variables at runtime?

Introduction / Context:
Environment variables provide a simple way to pass configuration information from the operating system and shell to programs. Common examples include PATH, HOME, and LANG. Programs often need to read these values and sometimes create or modify environment variables for child processes. Knowing the standard library functions that provide access to the environment is important for systems programming.

Given Data / Assumptions:

• The programming environment is C on a Unix like system.
• The process has an environment block which is an array of strings of the form NAME=VALUE.
• Standard library functions exist to interact with the environment.

Concept / Approach:
In C on Unix like systems, the getenv() function retrieves the value of a named environment variable as a string pointer. If the variable is not set, getenv() returns a null pointer. To modify or create environment variables, functions such as setenv() and putenv() are used. setenv() typically takes a variable name, value, and overwrite flag, while putenv() allows direct insertion of a NAME=VALUE string into the environment. These changes affect the environment of the current process and will be inherited by child processes created afterward, but they do not retroactively change the parent shell.

Step-by-Step Solution:
Step 1: Recall that environment variables are exposed to C programs through global variables such as environ and through helper functions.Step 2: Identify getenv() as the standard way to query the value associated with a given environment variable name.Step 3: Recognize setenv() and putenv() as standard ways to set or update environment variables within the process environment.Step 4: Understand that these functions manipulate data structures in memory, not the compiled executable file or hardware settings.Step 5: Select the option that correctly names getenv(), setenv(), and putenv() as the appropriate functions.

Verification / Alternative check:
Man pages for getenv(), setenv(), and putenv() describe their prototypes and behavior. Example programs that print the value of PATH or set a temporary variable and then execute another program via exec() demonstrate that the child sees the new variable. These experiments confirm that these functions are the correct tools for environment manipulation in C and Unix like systems.

Why Other Options Are Wrong:
Option B: Editing the compiled executable while it is running is not a standard or safe way to change environment variables and is not supported as a normal mechanism.Option C: Refers to changing hardware jumpers, which has nothing to do with environment variables in the operating system.Option D: Suggests that repeatedly calling fork() would change the environment automatically, which is incorrect; fork() simply copies the current environment to child processes.

Common Pitfalls:
One pitfall is assuming that changes made by a program to its environment will affect the shell that launched it. In fact, environment changes propagate only from parent to child, not backward. Another issue is failing to check return values of setenv() or putenv(), which can lead to silent failures if memory is exhausted or invalid input is provided.

Final Answer:
The correct answer is By using library functions such as getenv() to retrieve the value of an environment variable and setenv() or putenv() to set or modify environment variables.