Return_Values

   0                  Successful completion.
   -1                 Indicates an error. The function sets errno to
                      one of the following values:

                      o  EACCES - The value of the pid argument
                         matches the process ID of a child process
                         of the calling process and the child
                         process has successfully executed one of
                         the exec functions.

                      o  EINVAL - The value of the pgid argument is
                         less than 0, or is not a value supported by
                         the implementation.

                      o  EPERM - The process indicated by the pid
                         argument is a session leader. The value
                         of the pid argument matches the process ID
                         of a child process of the calling process,
                         and the child process is not in the same
                         session as the calling process. The value
                         of the pgid argument is valid but does
                         not match the process ID of the process
                         indicated by the pid argument, and there
                         is no process with a process group ID that
                         matches the value of the pgid argument in
                         the same session as the calling process.

                      o  ESRCH - The value of the pid argument does
                         not match the process ID of the calling
                         process or of a child process of the
                         calling process.