environ

Environ is a global variable representing the environment of the current process on many POSIX-compliant systems. It is typically declared as extern char **environ in C and points to an array of strings, each in the form NAME=VALUE, terminated by a NULL pointer. The strings reflect environment variables provided by the host system or set by the program, and the array is maintained by the runtime.

The contents are conventionally formatted as NAME=VALUE pairs, such as PATH=/usr/bin:/bin or HOME=/home/user. Each element is a null-terminated string, and the array ends with a NULL pointer. Reading and interpreting these strings is the responsibility of the program, while writing or updating them is governed by available interfaces and portability considerations.

Access to and modification of the environment is commonly performed through standard APIs. The function getenv(name) retrieves the value associated with a name. To add or change variables, programs typically use setenv(name, value, overwrite) or putenv(string). To remove a variable, unsetenv(name) is used. Directly altering the environ array or its strings is generally discouraged on portable code, as such manipulation can be implementation-dependent and error-prone.

Inheritance and portability: When a process creates a new program via the exec family of functions, the current environment is usually passed to the new program, making environ the source for the child’s environment unless overridden. The exact mechanism can vary by platform; in POSIX systems the environment is closely tied to environ and the envp parameter used by execve.

Scope and standards: The environ variable is defined by POSIX but is not part of the ISO C standard. Windows and other platforms provide their own environment mechanisms, such as per-process environment blocks, with different APIs (for example, GetEnvironmentVariable).