GETENV(3)              FreeBSD 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
     variable, 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
     puts it directly into the current environment, so altering the argument
     shall change the environment.  If the variable name does not exist in the
     list, it is inserted with the given value.  If the variable name does
     exist, it is reset to the given value.

     The unsetenv() function deletes all instances of the variable name
     pointed to by name from the list.

     If corruption (e.g., a name without a value) is detected while making a
     copy of environ for internal usage, then setenv(), unsetenv() and
     putenv() will output a warning to stderr about the issue, drop the
     corrupt entry and complete the task without error.

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
     environment, 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, string is without an "=" character or "=" is
                        the first character in string.  This does not follow
                        the POSIX specification.

     [ENOMEM]           The function setenv(), unsetenv() or putenv() failed
                        because they were unable to allocate memory for the
                        environment.

SEE ALSO
     csh(1), sh(1), execve(2), 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.

     Until FreeBSD 7.0, putenv() would make a copy of string and insert it
     into the environment using setenv().  This was changed to use string as
     the memory location of the ``name=value'' pair to follow the POSIX
     specification.

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.

FreeBSD 13.1-RELEASE-p6          June 20, 2007         FreeBSD 13.1-RELEASE-p6