— 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 (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 difference to the setenv function is that the exact string given as the parameter string is put into the environment. If the user should change the string after the putenv call this will reflect in automatically in the environment. This also requires that string is no automatic variable which scope is left before the variable is removed from the environment. The same applies of course to dynamically allocated variables which are freed later.

This function is part of the extended Unix interface. Since it was also available in old SVID libraries you should define either _XOPEN_SOURCE or _SVID_SOURCE before including any header.

— Function: int setenv (const char *name, const char *value, int replace)

The setenv function can be used to add a new definition to the environment. The entry with the name name is replaced by the value ‘name=value’. Please note that this is also true if value is the empty string. To do this a new string is created and the strings name and value are copied. A null pointer for the value parameter is illegal. If the environment already contains an entry with key name the replace parameter controls the action. If replace is zero, nothing happens. Otherwise the old entry is replaced by the new one.

Please note that you cannot remove an entry completely using this function.

This function was originally part of the BSD library but is now part of the Unix standard.

— Function: int unsetenv (const char *name)

Using this function one can remove an entry completely from the environment. If the environment contains an entry with the key name this whole entry is removed. A call to this function is equivalent to a call to putenv when the value part of the string is empty.

The function return -1 if name is a null pointer, points to an empty string, or points to a string containing a = character. It returns 0 if the call succeeded.

This function was originally part of the BSD library but is now part of the Unix standard. The BSD version had no return value, though.

— Function: int clearenv (void)

The clearenv function removes all entries from the environment. Using putenv and setenv new entries can be added again later.

If the function is successful it returns 0. Otherwise the return value is nonzero.

— 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.