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_archive/rhai_engine/rhaibook/safety/checked.md

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+Turning Off Safety Checks
+=========================
+
+{{#include ../links.md}}
+
+
+Checked Arithmetic
+------------------
+
+```admonish bug.side "Don't Panic"
+
+Scripts under normal builds of Rhai never crash the host system – any panic is a bug.
+```
+
+By default, all arithmetic calculations in Rhai are _checked_, meaning that the script terminates
+with a runtime error whenever it detects a numeric over-flow/under-flow condition or an invalid
+floating-point operation.
+
+This checking can be turned off via the [`unchecked`] feature for higher performance (but higher
+risks as well).
+
+```rust
+let x = 1_000_000_000_000;
+
+x * x;      // Normal build - runtime error: multiplication overflow
+
+x * x;      // 'unchecked' debug build - panic!
+            // 'unchecked' release build - overflow with no error
+
+x / 0;      // Normal build - runtime error: division by zero
+
+x / 0;      // 'unchecked' build - panic!
+```
+
+
+Other Safety Checks
+-------------------
+
+In addition to overflows, there are many other safety checks performed by Rhai at runtime.
+[`unchecked`] turns them **all** off as well, such as...
+
+### [Infinite loops][maximum number of operations]
+
+```rust
+// Normal build - runtime error: exceeds maximum number of operations
+loop {
+    foo();
+}
+
+// 'unchecked' build - never terminates!
+loop {
+    foo();
+}
+```
+
+### [Infinite recursion][maximum call stack depth]
+
+```rust
+fn foo() {
+    foo();
+}
+
+foo();      // Normal build - runtime error: exceeds maximum stack depth
+
+foo();      // 'unchecked' build - panic due to stack overflow!
+```
+
+### [Gigantic data structures][maximum size of arrays]
+
+```rust
+let x = [];
+
+// Normal build - runtime error: array exceeds maximum size
+loop {
+    x += 42;
+}
+
+// 'unchecked' build - panic due to out-of-memory!
+loop {
+    x += 42;
+}
+```
+
+### Improper range iteration
+
+```rust
+// Normal build - runtime error: zero step
+for x in range(0, 10, 0) { ... }
+
+// 'unchecked' build - never terminates!
+for x in range(0, 10, 0) { ... }
+
+// Normal build - empty range
+for x in range(0, 10, -1) { ... }
+
+// 'unchecked' build - panic due to numeric underflow!
+for x in range(0, 10, -1) { ... }
+```