получить настраиваемые системные переменные (get configurable system variables)
Использование в приложениях (Application usage)
As -1 is a permissible return value in a successful situation, an
application wishing to check for error situations should set
errno to 0, then call sysconf(), and, if it returns -1, check to
see if errno is non-zero.
Application developers should check whether an option, such as
_POSIX_TRACE, is supported prior to obtaining and using values
for related variables, such as _POSIX_TRACE_NAME_MAX.
