The GNU C Library - Environment Access

Environment Access

The value of an environment variable can be accessed with the getenv function. This is declared in the header file `stdlib.h'.

Function char * getenv (const char *name)

This function returns a string that is the value of the environment variable name. You must not modify this string. In some non-Unix systems not using the GNU library, it might be overwritten by subsequent calls to getenv (but not by any other library function). If the environment variable name is not defined, the value is a null pointer.

Function int putenv (const char *string)

The putenv function adds or removes definitions from the environment. If the string is of the form `name=value', the definition is added to the environment. Otherwise, the string is interpreted as the name of an environment variable, and any definition for this variable in the environment is removed.

The GNU library provides this function for compatibility with SVID; it may not be available in other systems.

You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute; see Executing a File).

Variable char ** environ

The environment is represented as an array of strings. Each string is of the format `name=value'. The order in which strings appear in the environment is not significant, but the same name must not appear more than once. The last element of the array is a null pointer.

This variable is declared in the header file `unistd.h'.

If you just want to get the value of an environment variable, use getenv .

Unix systems, and the GNU system, pass the initial value of environ as the third argument to main . See Program Arguments.