The value of an environment variable can be accessed with the
getenv
function. This is declared in the header file
`stdlib.h'. All of the following functions can be safely used in
multi-threaded programs. It is made sure that concurrent modifications
to the environment do not lead to errors.
getenv
(but not by any other library function). If the
environment variable name is not defined, the value is a null
pointer.
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.
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.
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 originall part of the BSD library but is now part of the Unix standard. The BSD version had no return value, though.
There is one more function to modify the whole environment. This function is said to be used in the POSIX.9 (POSIX bindings for Fortran 77) and so one should expect it did made it into POSIX.1. But this never happened. But we still provide this function as a GNU extension to enable writing standard compliant Fortran environments.
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.
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 section Executing a File).
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 section Program Arguments.
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