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File: libc.info, Node: Environment Access, Next: Standard Environment, Up: Environment Variables
25.4.1 Environment Access
-------------------------
The value of an environment variable can be accessed with the 'getenv'
function. This is declared in the header file 'stdlib.h'.
Libraries should use 'secure_getenv' instead of 'getenv', so that
they do not accidentally use untrusted environment variables.
Modifications of environment variables are not allowed in multi-threaded
programs. The 'getenv' and 'secure_getenv' functions can be safely used
in multi-threaded programs.
-- Function: char * getenv (const char *NAME)
Preliminary: | MT-Safe env | AS-Safe | AC-Safe | *Note POSIX Safety
Concepts::.
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 C 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: char * secure_getenv (const char *NAME)
Preliminary: | MT-Safe env | AS-Safe | AC-Safe | *Note POSIX Safety
Concepts::.
This function is similar to 'getenv', but it returns a null pointer
if the environment is untrusted. This happens when the program
file has SUID or SGID bits set. General-purpose libraries should
always prefer this function over 'getenv' to avoid vulnerabilities
if the library is referenced from a SUID/SGID program.
This function is a GNU extension.
-- Function: int putenv (char *STRING)
Preliminary: | MT-Unsafe const:env | AS-Unsafe heap lock |
AC-Unsafe corrupt lock mem | *Note POSIX Safety Concepts::.
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.
If the function is successful it returns '0'. Otherwise the return
value is nonzero and 'errno' is set to indicate the error.
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 automatically in the environment. This also requires that
STRING not be an automatic variable whose 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. You should
define _XOPEN_SOURCE before including any header.
-- Function: int setenv (const char *NAME, const char *VALUE, int
REPLACE)
Preliminary: | MT-Unsafe const:env | AS-Unsafe heap lock |
AC-Unsafe corrupt lock mem | *Note POSIX Safety Concepts::.
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.
If the function is successful it returns '0'. Otherwise the
environment is unchanged and the return value is '-1' and 'errno'
is set.
This function was originally part of the BSD library but is now
part of the Unix standard.
-- Function: int unsetenv (const char *NAME)
Preliminary: | MT-Unsafe const:env | AS-Unsafe lock | AC-Unsafe
lock | *Note POSIX Safety Concepts::.
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)
Preliminary: | MT-Unsafe const:env | AS-Unsafe heap lock |
AC-Unsafe lock mem | *Note POSIX Safety Concepts::.
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;
*note 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 GNU systems, pass the initial value of 'environ' as
the third argument to 'main'. *Note Program Arguments::.
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