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       setenv - change or add an environment variable


       #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


       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.
       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.


       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.


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

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


       4.3BSD, POSIX.1-2001.


       POSIX.1-2001 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.1-2001-compliant prototype shown
       in the SYNOPSIS.


       POSIX.1-2001  specifies  that  if  name contains an '=' character, then
       setenv() should fail with the error EINVAL; however, versions of  glibc
       before 2.3.4 allowed an '=' sign in name.


       clearenv(3), getenv(3), putenv(3), environ(7)


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