$CHECKING - QB64Official/qb64 GitHub Wiki

The $CHECKING metacommand turns C++ event checking ON or OFF.

Syntax

$CHECKING:{ON|OFF}

The Metacommand does not require a comment or REM before it. There is no space after the colon.
The OFF action turns event checking off and should only be used when running stable, errorless code.
The default $CHECKING:ON action is only required when checking has been turned OFF previously.
When $CHECKING:OFF is used, all error code and the reporting code is removed from the EXE program.
Warning: Turning OFF error checking could create a General Protection Fault (or segfault). Use only with 100% stable sections of code.

After every QB64 command is translated to C++, the compiler adds special code sections to check for ON TIMER (n) events and errors that may have occured in the last function call. Disabling error checking with the $CHECKING:OFF directive prevents the compiler from adding the extra code sections.
Setting $CHECKING:OFF is only designed for 100% stable, errorless sections of code, where every CPU cycle saved counts, such as in a software 3D texture mapper, for example.