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.
— 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.
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.
— 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.
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.
Published under the terms of the GNU General Public License
