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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
       (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.


       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.


       For   an   explanation   of   the  terms  used  in  this  section,  see

       │InterfaceAttributeValue               │
       │setenv(),  │ Thread safety │ MT-Unsafe const:env │
       │unsetenv() │               │                     │


       POSIX.1-2001, POSIX.1-2008, 4.3BSD.


       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
       the SYNOPSIS.


       POSIX.1 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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