contents/using-lttng/instrumenting/c-application/lttng-ust-environment-variables-compiler-flags.md

   1 ---
   2 id: lttng-ust-environment-variables-compiler-flags
   3 ---
   4
   5 A few special environment variables and compile flags may affect the
   6 behavior of LTTng-UST.
   7
   8 LTTng-UST's debugging can be activated by setting the environment
   9 variable `LTTNG_UST_DEBUG` to `1` when launching the application. It
  10 can also be enabled at compile time by defining `LTTNG_UST_DEBUG` when
  11 compiling LTTng-UST (using the `-DLTTNG_UST_DEBUG` compiler option).
  12
  13 The environment variable `LTTNG_UST_REGISTER_TIMEOUT` can be used to
  14 specify how long the application should wait for the
  15 [session daemon](#doc-lttng-sessiond)'s _registration done_ command
  16 before proceeding to execute the main program. The timeout value is
  17 specified in milliseconds. 0 means _don't wait_. -1 means
  18 _wait forever_. Setting this environment variable to 0 is recommended
  19 for applications with time contraints on the process startup time.
  20
  21 The default value of `LTTNG_UST_REGISTER_TIMEOUT` (when not defined)
  22 is **3000&nbsp;ms**.
  23
  24 The compilation definition `LTTNG_UST_DEBUG_VALGRIND` should be enabled
  25 at build time (`-DLTTNG_UST_DEBUG_VALGRIND`) to allow `liblttng-ust`
  26 to be used with <a href="http://valgrind.org/" class="ext">Valgrind</a>.
  27 The side effect of defining `LTTNG_UST_DEBUG_VALGRIND` is that per-CPU
  28 buffering is disabled.