These functions set, unset and fetch environment variables from the host
environment list. For compatibility with differing environment conventions, the given arguments
name and
value may be appended and prepended, respectively, with an equal sign “
=”, except for
unsetenv().
The
getenv() function obtains the current value of the environment variable
name. If the variable
name is not in the current environment, a
NULL pointer is returned.
The
getenv_r() function obtains the current value of the environment variable
name and copies it to
buf. If
name is not in the current environment, or the string length of the value of
name is longer than
len characters, then -1 is returned and
errno is set to indicate the error.
The
setenv() function inserts or resets the environment variable
name in the current environment list. If the variable
name does not exist in the list, it is inserted with the given
value. If the variable does exist, the argument
overwrite is tested; if
overwrite is zero, the variable is not reset, otherwise it is reset to the given
value.
The
putenv() function takes an argument of the form “name=value” and it will set the environment variable “name” equal to “value” by altering an existing entry, or creating a new one if an existing one does not exist. The actual string argument passed to
putenv() will become part of the environment. If one changes the string, the environment will also change.
The
unsetenv() function deletes all instances of the variable name pointed to by
name from the list.