char *name, const char
These functions set, unset, and fetch environment variables from the host environment list.
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.
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.
function takes an argument of the form
name=value. The memory pointed
to by string becomes part of the environment and must
not be deallocated by the caller. If the variable already exists, it will be
overwritten. A common source of bugs is to pass a
string argument that is a locally scoped string
buffer. This will result in corruption of the environment after leaving the
scope in which the variable is defined. For this reason, the
setenv() function is preferred over
function deletes all instances of the variable name pointed to by
name from the list.
functions return the value 0 if successful; otherwise the
value -1 is returned and the global variable
errno is set to indicate the error.
getenv() function returns a pointer to
the requested value, or
NULL if it could not be
getenv() is successful, the string
returned should be considered read-only.
unsetenv() function was passed an empty name or a NULL pointer, or was passed a name containing an ‘=’ character.
putenv() function was passed a string that did not contain an ‘=’ character.
putenv() function failed because it was unable to allocate memory for the environment.
csh(1), sh(1), execve(2), issetugid(2), environ(7)
getenv() function conforms to
ANSI X3.159-1989 (“ANSI C89”).
unsetenv() functions conform to
IEEE Std 1003.1-2008 (“POSIX.1”).
getenv() appeared in
Version 7 AT&T UNIX and
3BSD. The functions
unsetenv() appeared in
function appeared in 4.3BSD-Reno.
Library code must be careful about using
getenv() to read untrusted environment variables in
setuid programs. The
issetugid() function is
provided for this purpose.