GETENV(3)                BSD Library Functions Manual                GETENV(3)

NAME
     getenv, putenv, setenv, unsetenv -- environment variable functions

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     char *
     getenv(const char *name);

     int
     setenv(const char *name, const char *value, int overwrite);

     int
     putenv(char *string);

     int
     unsetenv(const char *name);

DESCRIPTION
     These functions set, unset and fetch environment variables from the host
     environment list.

     The getenv() function obtains the current value of the environment vari-
     able, name.  The application should not modify the string pointed to by
     the getenv() function.

     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 is
     equivalent to:

           setenv(name, value, 1);

     The string pointed to by string becomes part of the environment.  A pro-
     gram should not alter or free the string, and should not use stack or
     other transient string variables as arguments to putenv().  The setenv()
     function is strongly preferred to putenv().

     The unsetenv() function deletes all instances of the variable name
     pointed to by name from the list.  Note that only the variable name
     (e.g., "NAME") should be given; "NAME=value" will not work.

RETURN VALUES
     The getenv() function returns the value of the environment variable as a
     NUL-terminated string.  If the variable name is not in the current envi-
     ronment, NULL is returned.

     The setenv(), putenv(), and unsetenv() functions return the value 0 if
     successful; otherwise the value -1 is returned and the global variable
     errno is set to indicate the error.

ERRORS
     [EINVAL]           The function getenv(), setenv() or unsetenv() failed
                        because the name is a NULL pointer, points to an empty
                        string, or points to a string containing an ``=''
                        character.

                        The function putenv() failed because string is a NULL
                        pointer or string is without an ``='' character.

     [ENOMEM]           The function setenv(), unsetenv() or putenv() failed
                        because it was unable to allocate memory for the envi-
                        ronment.

LEGACY SYNOPSIS
     #include <stdlib.h>

     void
     unsetenv(const char *name);

     unsetenv() doesn't return a value.

COMPATIBILITY
     putenv() no longer copies its input buffer.  This often appears in crash
     logs as a crash in getenv().  Avoid passing local buffers or freeing the
     memory that is passed to putenv().  Use setenv(), which still makes an
     internal copy of its buffers.

     unsetenv() no longer parses the variable name; e.g., unsetenv ("FOO=BAR")
     no longer works.  Use unsetenv("FOO").  unsetenv() also now returns a
     status value and will set errno to EINVAL if name is not a defined envi-
     ronment variable.

SEE ALSO
     csh(1), sh(1), execve(2), compat(5), environ(7)

STANDARDS
     The getenv() function conforms to ISO/IEC 9899:1990 (``ISO C90'').  The
     setenv(), putenv() and unsetenv() functions conforms to IEEE Std
     1003.1-2001 (``POSIX.1'').

HISTORY
     The functions setenv() and unsetenv() appeared in Version 7 AT&T UNIX.
     The putenv() function appeared in 4.3BSD-Reno.

BUGS
     Successive calls to setenv() that assign a larger-sized value than any
     previous value to the same name will result in a memory leak.  The
     FreeBSD semantics for this function (namely, that the contents of value
     are copied and that old values remain accessible indefinitely) make this
     bug unavoidable.  Future versions may eliminate one or both of these
     semantic guarantees in order to fix the bug.

BSD                              June 20, 2007                             BSD