This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       See exec() for restrictions on changing the environment in multi-threaded applications.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical  and  Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE  and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                        SETENV(3POSIX)

       The  setenv()  function  shall  update  or  add a variable in the environment of the calling process. The
       envname argument points to a string containing the name  of  an  environment  variable  to  be  added  or
       altered.  The  environment  variable shall be set to the value to which envval points. The function shall
       fail if envname points to a string which contains an '=' character. If the environment variable named  by
       envname  already exists and the value of overwrite is non-zero, the function shall return success and the
       environment shall be updated. If the environment variable named by envname already exists and  the  value
       of overwrite is zero, the function shall return success and the environment shall remain unchanged.

       The setenv() function shall update the list of pointers to which environ points.

       The strings described by envname and envval are copied by this function.

       The setenv() function need not be thread-safe.

       The setenv() function shall fail if:

       EINVAL The envname argument points to an empty string or points to a string containing an '=' character.

       ENOMEM Insufficient memory was available to add a variable or its value to the environment.

       Thefollowingsectionsareinformative.

       None.

       None.

       setenv — add or change environment variable

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       Unanticipated results may occur if setenv() changes the external variable environ.  In particular, if the
       optional envp argument to main() is present, it is not changed, and thus may point to an obsolete copy of
       the environment (as may any other copy of environ).  However, other than the aforementioned  restriction,
       the standard developers intended that the traditional method of walking through the environment by way of
       the environ pointer must be supported.

       It  was  decided that setenv() should be required by this version because it addresses a piece of missing
       functionality, and does not impose a significant burden on the implementor.

       There was considerable debate as to whether the System V putenv() function or the BSD  setenv()  function
       should  be  required  as  a mandatory function. The setenv() function was chosen because it permitted the
       implementation of the unsetenv() function  to  delete  environmental  variables,  without  specifying  an
       additional interface. The putenv() function is available as part of the XSI option.

       The  standard  developers  considered  requiring  that setenv() indicate an error when a call to it would
       result in exceeding {ARG_MAX}.  The requirement was rejected since the condition might be temporary, with
       the application eventually reducing the environment size. The ultimate success or failure depends on  the
       size at the time of a call to exec, which returns an indication of this error condition.

       See also the RATIONALE section in getenv().

       Upon  successful  completion,  zero  shall  be  returned.  Otherwise,  -1 shall be returned, errno set to
       indicate the error, and the environment shall be unchanged.

exec, getenv(), putenv(), unsetenv()

       The Base Definitions volume of POSIX.1‐2017, <stdlib.h>, <sys_types.h>, <unistd.h>

       #include <stdlib.h>

       int setenv(const char *envname, const char *envval, int overwrite);