unsetenv

       #include <stdlib.h>

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

       int unsetenv(const char *name);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       setenv(), unsetenv():
           _BSD_SOURCE || _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600

DESCRIPTION
       The  setenv()  function  adds the variable name to the environment with
       the value value, if name does not already exist.  If name does exist in
       the  environment,  then  its  value is changed to value if overwrite is
       nonzero; if overwrite is zero, then the value of name  is  not  changed
       (and setenv() returns a success status).  This function makes copies of
       the strings pointed to by name and value (by contrast with putenv(3)).

       The unsetenv() function deletes the variable name from the environment.
       If  name does not exist in the environment, then the function succeeds,
       and the environment is unchanged.

RETURN VALUE
       The setenv() function returns zero on success, or  -1  on  error,  with
       errno set to indicate the cause of the error.

       The  unsetenv()  function returns zero on success, or -1 on error, with
       errno set to indicate the cause of the error.

ERRORS
       EINVAL name is NULL, points to a string of length 0, or contains an '='
              character.

       ENOMEM Insufficient memory to add a new variable to the environment.

ATTRIBUTES
       For   an   explanation   of   the  terms  used  in  this  section,  see
       attributes(7).

       +-----------+---------------+---------------------+
       |Interface  | Attribute     | Value               |
       +-----------+---------------+---------------------+
       |setenv(),  | Thread safety | MT-Unsafe const:env |
       |unsetenv() |               |                     |
       +-----------+---------------+---------------------+
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, 4.3BSD.

NOTES
       POSIX.1 does not require setenv() or unsetenv() to be reentrant.

       Prior to glibc 2.2.2, unsetenv() was prototyped as returning void; more
       recent glibc versions follow the POSIX.1-compliant prototype  shown  in
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.

GNU                               2015-08-08                         SETENV(3)
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