Undefined Behavior

Zig has many instances of undefined behavior. If undefined behavior is detected at compile-time, Zig emits a compile error and refuses to continue. Most undefined behavior that cannot be detected at compile-time can be detected at runtime. In these cases, Zig has safety checks. Safety checks can be disabled on a per-block basis with @setRuntimeSafety. The ReleaseFast and ReleaseSmall build modes disable all safety checks (except where overridden by @setRuntimeSafety) in order to facilitate optimizations.

When a safety check fails, Zig crashes with a stack trace, like this:

test.zig

test "safety check" {
    unreachable;
}

$ zig test test.zig
1/1 test "safety check"...reached unreachable code
/deps/zig/docgen_tmp/test.zig:2:5: 0x204b9a in test "safety check" (test)
    unreachable;
    ^
/deps/zig/lib/std/special/test_runner.zig:47:28: 0x22ba9e in std.special.main (test)
        } else test_fn.func();
                           ^
/deps/zig/lib/std/start.zig:253:37: 0x2055cd in std.start.posixCallMainAndExit (test)
            const result = root.main() catch |err| {
                                    ^
/deps/zig/lib/std/start.zig:123:5: 0x20530f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
Tests failed. Use the following command to reproduce the failure:
/deps/zig/docgen_tmp/test

Reaching Unreachable Code

At compile-time:

test.zig

comptime {
    assert(false);
}
fn assert(ok: bool) void {
    if (!ok) unreachable; // assertion failure
}

$ zig test test.zig
./docgen_tmp/test.zig:5:14: error: unable to evaluate constant expression
    if (!ok) unreachable; // assertion failure
             ^
./docgen_tmp/test.zig:2:11: note: called from here
    assert(false);
          ^
./docgen_tmp/test.zig:1:10: note: called from here
comptime {
         ^
./docgen_tmp/test.zig:2:11: note: referenced here
    assert(false);
          ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    std.debug.assert(false);
}

$ zig build-exe test.zig
$ ./test
reached unreachable code
/deps/zig/lib/std/debug.zig:228:14: 0x203cbb in std.debug.assert (test)
    if (!ok) unreachable; // assertion failure
             ^
/deps/zig/docgen_tmp/test.zig:4:21: 0x22a7da in main (test)
    std.debug.assert(false);
                    ^
/deps/zig/lib/std/start.zig:243:22: 0x2046df in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x2044bf in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Index out of Bounds

At compile-time:

test.zig

comptime {
    const array: [5]u8 = "hello".*;
    const garbage = array[5];
}

$ zig test test.zig
./docgen_tmp/test.zig:3:26: error: index 5 outside array of size 5
    const garbage = array[5];
                         ^

At runtime:

test.zig

pub fn main() void {
    var x = foo("hello");
}
fn foo(x: []const u8) u8 {
    return x[5];
}

$ zig build-exe test.zig
$ ./test
index out of bounds
/deps/zig/docgen_tmp/test.zig:6:13: 0x22e969 in foo (test)
    return x[5];
            ^
/deps/zig/docgen_tmp/test.zig:2:16: 0x22a7e6 in main (test)
    var x = foo("hello");
               ^
/deps/zig/lib/std/start.zig:243:22: 0x2046df in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x2044bf in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Cast Negative Number to Unsigned Integer

At compile-time:

test.zig

comptime {
    const value: i32 = -1;
    const unsigned = @intCast(u32, value);
}

$ zig test test.zig
./docgen_tmp/test.zig:3:36: error: cannot cast negative value -1 to unsigned integer type 'u32'
    const unsigned = @intCast(u32, value);
                                   ^
./docgen_tmp/test.zig:3:22: note: referenced here
    const unsigned = @intCast(u32, value);
                     ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    var value: i32 = -1;
    var unsigned = @intCast(u32, value);
    std.debug.warn("value: {}\n", .{unsigned});
}

$ zig build-exe test.zig
$ ./test
attempt to cast negative value to unsigned integer
/deps/zig/docgen_tmp/test.zig:5:20: 0x22a867 in main (test)
    var unsigned = @intCast(u32, value);
                   ^
/deps/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

To obtain the maximum value of an unsigned integer, use std.math.maxInt.

Cast Truncates Data

At compile-time:

test.zig

comptime {
    const spartan_count: u16 = 300;
    const byte = @intCast(u8, spartan_count);
}

$ zig test test.zig
./docgen_tmp/test.zig:3:31: error: integer value 300 cannot be coerced to type 'u8'
    const byte = @intCast(u8, spartan_count);
                              ^
./docgen_tmp/test.zig:3:18: note: referenced here
    const byte = @intCast(u8, spartan_count);
                 ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    var spartan_count: u16 = 300;
    const byte = @intCast(u8, spartan_count);
    std.debug.warn("value: {}\n", .{byte});
}

$ zig build-exe test.zig
$ ./test
integer cast truncated bits
/deps/zig/docgen_tmp/test.zig:5:18: 0x22a86c in main (test)
    const byte = @intCast(u8, spartan_count);
                 ^
/deps/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

To truncate bits, use @truncate.

Integer Overflow

Default Operations

The following operators can cause integer overflow:

• + (addition)
• - (subtraction)
• - (negation)
• * (multiplication)
• / (division)
• @divTrunc (division)
• @divFloor (division)
• @divExact (division)

Example with addition at compile-time:

test.zig

comptime {
    var byte: u8 = 255;
    byte += 1;
}

$ zig test test.zig
./docgen_tmp/test.zig:3:10: error: operation caused overflow
    byte += 1;
         ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    var byte: u8 = 255;
    byte += 1;
    std.debug.warn("value: {}\n", .{byte});
}

$ zig build-exe test.zig
$ ./test
integer overflow
/deps/zig/docgen_tmp/test.zig:5:10: 0x22a850 in main (test)
    byte += 1;
         ^
/deps/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Standard Library Math Functions

These functions provided by the standard library return possible errors.

• @import("std").math.add
• @import("std").math.sub
• @import("std").math.mul
• @import("std").math.divTrunc
• @import("std").math.divFloor
• @import("std").math.divExact
• @import("std").math.shl

Example of catching an overflow for addition:

test.zig

const math = @import("std").math;
const warn = @import("std").debug.warn;
pub fn main() !void {
    var byte: u8 = 255;
    byte = if (math.add(u8, byte, 1)) |result| result else |err| {
        warn("unable to add one: {}\n", .{@errorName(err)});
        return err;
    };
    warn("result: {}\n", .{byte});
}

$ zig build-exe test.zig
$ ./test
unable to add one: Overflow
error: Overflow
/deps/zig/lib/std/math.zig:342:5: 0x22ee70 in std.math.add (test)
    return if (@addWithOverflow(T, a, b, &answer)) error.Overflow else answer;
    ^
/deps/zig/docgen_tmp/test.zig:8:9: 0x22ab3b in main (test)
        return err;
        ^

Builtin Overflow Functions

These builtins return a bool of whether or not overflow occurred, as well as returning the overflowed bits:

• @addWithOverflow
• @subWithOverflow
• @mulWithOverflow
• @shlWithOverflow

Example of @addWithOverflow:

test.zig

const warn = @import("std").debug.warn;
pub fn main() void {
    var byte: u8 = 255;
    var result: u8 = undefined;
    if (@addWithOverflow(u8, byte, 10, &result)) {
        warn("overflowed result: {}\n", .{result});
    } else {
        warn("result: {}\n", .{result});
    }
}

$ zig build-exe test.zig
$ ./test
overflowed result: 9

Wrapping Operations

These operations have guaranteed wraparound semantics.

• +% (wraparound addition)
• -% (wraparound subtraction)
• -% (wraparound negation)
• *% (wraparound multiplication)

test.zig

const std = @import("std");
const assert = std.debug.assert;
const minInt = std.math.minInt;
const maxInt = std.math.maxInt;
test "wraparound addition and subtraction" {
    const x: i32 = maxInt(i32);
    const min_val = x +% 1;
    assert(min_val == minInt(i32));
    const max_val = min_val -% 1;
    assert(max_val == maxInt(i32));
}

$ zig test test.zig
1/1 test "wraparound addition and subtraction"...OK
All 1 tests passed.

Exact Left Shift Overflow

At compile-time:

test.zig

comptime {
    const x = @shlExact(@as(u8, 0b01010101), 2);
}

$ zig test test.zig
./docgen_tmp/test.zig:2:15: error: operation caused overflow
    const x = @shlExact(@as(u8, 0b01010101), 2);
              ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    var x: u8 = 0b01010101;
    var y = @shlExact(x, 2);
    std.debug.warn("value: {}\n", .{y});
}

$ zig build-exe test.zig
$ ./test
left shift overflowed bits
/deps/zig/docgen_tmp/test.zig:5:13: 0x22a89d in main (test)
    var y = @shlExact(x, 2);
            ^
/deps/zig/lib/std/start.zig:243:22: 0x20475f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20453f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Exact Right Shift Overflow

At compile-time:

test.zig

comptime {
    const x = @shrExact(@as(u8, 0b10101010), 2);
}

$ zig test test.zig
./docgen_tmp/test.zig:2:15: error: exact shift shifted out 1 bits
    const x = @shrExact(@as(u8, 0b10101010), 2);
              ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    var x: u8 = 0b10101010;
    var y = @shrExact(x, 2);
    std.debug.warn("value: {}\n", .{y});
}

$ zig build-exe test.zig
$ ./test
right shift overflowed bits
/deps/zig/docgen_tmp/test.zig:5:13: 0x22a89d in main (test)
    var y = @shrExact(x, 2);
            ^
/deps/zig/lib/std/start.zig:243:22: 0x20475f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20453f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Division by Zero

At compile-time:

test.zig

comptime {
    const a: i32 = 1;
    const b: i32 = 0;
    const c = a / b;
}

$ zig test test.zig
./docgen_tmp/test.zig:4:17: error: division by zero
    const c = a / b;
                ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    var a: u32 = 1;
    var b: u32 = 0;
    var c = a / b;
    std.debug.warn("value: {}\n", .{c});
}

$ zig build-exe test.zig
$ ./test
division by zero
/deps/zig/docgen_tmp/test.zig:6:15: 0x22a859 in main (test)
    var c = a / b;
              ^
/deps/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Remainder Division by Zero

At compile-time:

test.zig

comptime {
    const a: i32 = 10;
    const b: i32 = 0;
    const c = a % b;
}

$ zig test test.zig
./docgen_tmp/test.zig:4:17: error: division by zero
    const c = a % b;
                ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    var a: u32 = 10;
    var b: u32 = 0;
    var c = a % b;
    std.debug.warn("value: {}\n", .{c});
}

$ zig build-exe test.zig
$ ./test
remainder division by zero or negative value
/deps/zig/docgen_tmp/test.zig:6:15: 0x22a87b in main (test)
    var c = a % b;
              ^
/deps/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Exact Division Remainder

At compile-time:

test.zig

comptime {
    const a: u32 = 10;
    const b: u32 = 3;
    const c = @divExact(a, b);
}

$ zig test test.zig
./docgen_tmp/test.zig:4:15: error: exact division had a remainder
    const c = @divExact(a, b);
              ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    var a: u32 = 10;
    var b: u32 = 3;
    var c = @divExact(a, b);
    std.debug.warn("value: {}\n", .{c});
}

$ zig build-exe test.zig
$ ./test
exact division produced remainder
/deps/zig/docgen_tmp/test.zig:6:13: 0x22a89d in main (test)
    var c = @divExact(a, b);
            ^
/deps/zig/lib/std/start.zig:243:22: 0x20472f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20450f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Attempt to Unwrap Null

At compile-time:

test.zig

comptime {
    const optional_number: ?i32 = null;
    const number = optional_number.?;
}

$ zig test test.zig
./docgen_tmp/test.zig:3:35: error: unable to unwrap null
    const number = optional_number.?;
                                  ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    var optional_number: ?i32 = null;
    var number = optional_number.?;
    std.debug.warn("value: {}\n", .{number});
}

$ zig build-exe test.zig
$ ./test
attempt to use null value
/deps/zig/docgen_tmp/test.zig:5:33: 0x22a86c in main (test)
    var number = optional_number.?;
                                ^
/deps/zig/lib/std/start.zig:243:22: 0x20474f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20452f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

One way to avoid this crash is to test for null instead of assuming non-null, with the if expression:

test.zig

const warn = @import("std").debug.warn;
pub fn main() void {
    const optional_number: ?i32 = null;
    if (optional_number) |number| {
        warn("got number: {}\n", .{number});
    } else {
        warn("it's null\n", .{});
    }
}

$ zig build-exe test.zig
$ ./test
it's null

See also:

• Optionals

Attempt to Unwrap Error

At compile-time:

test.zig

comptime {
    const number = getNumberOrFail() catch unreachable;
}
fn getNumberOrFail() !i32 {
    return error.UnableToReturnNumber;
}

$ zig test test.zig
./docgen_tmp/test.zig:2:38: error: caught unexpected error 'UnableToReturnNumber'
    const number = getNumberOrFail() catch unreachable;
                                     ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    const number = getNumberOrFail() catch unreachable;
    std.debug.warn("value: {}\n", .{number});
}
fn getNumberOrFail() !i32 {
    return error.UnableToReturnNumber;
}

$ zig build-exe test.zig
$ ./test
attempt to unwrap error: UnableToReturnNumber
/deps/zig/docgen_tmp/test.zig:9:5: 0x22ea6c in getNumberOrFail (test)
    return error.UnableToReturnNumber;
    ^
???:?:?: 0x20698c in ??? (???)
/deps/zig/docgen_tmp/test.zig:4:38: 0x22a8cb in main (test)
    const number = getNumberOrFail() catch unreachable;
                                     ^
/deps/zig/lib/std/start.zig:243:22: 0x20476f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20454f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

One way to avoid this crash is to test for an error instead of assuming a successful result, with the if expression:

test.zig

const warn = @import("std").debug.warn;
pub fn main() void {
    const result = getNumberOrFail();
    if (result) |number| {
        warn("got number: {}\n", .{number});
    } else |err| {
        warn("got error: {}\n", .{@errorName(err)});
    }
}
fn getNumberOrFail() !i32 {
    return error.UnableToReturnNumber;
}

$ zig build-exe test.zig
$ ./test
got error: UnableToReturnNumber

See also:

• Errors

Invalid Error Code

At compile-time:

test.zig

comptime {
    const err = error.AnError;
    const number = @errorToInt(err) + 10;
    const invalid_err = @intToError(number);
}

$ zig test test.zig
./docgen_tmp/test.zig:4:25: error: integer value 11 represents no error
    const invalid_err = @intToError(number);
                        ^

At runtime:

test.zig

const std = @import("std");
pub fn main() void {
    var err = error.AnError;
    var number = @errorToInt(err) + 500;
    var invalid_err = @intToError(number);
    std.debug.warn("value: {}\n", .{number});
}

$ zig build-exe test.zig
$ ./test
invalid error code
/deps/zig/docgen_tmp/test.zig:6:23: 0x22a8b4 in main (test)
    var invalid_err = @intToError(number);
                      ^
/deps/zig/lib/std/start.zig:243:22: 0x20475f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20453f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Invalid Enum Cast

At compile-time:

test.zig

const Foo = enum {
    A,
    B,
    C,
};
comptime {
    const a: u2 = 3;
    const b = @intToEnum(Foo, a);
}

$ zig test test.zig
./docgen_tmp/test.zig:8:15: error: enum 'Foo' has no tag matching integer value 3
    const b = @intToEnum(Foo, a);
              ^
./docgen_tmp/test.zig:1:13: note: 'Foo' declared here
const Foo = enum {
            ^

At runtime:

test.zig

const std = @import("std");
const Foo = enum {
    A,
    B,
    C,
};
pub fn main() void {
    var a: u2 = 3;
    var b = @intToEnum(Foo, a);
    std.debug.warn("value: {}\n", .{@tagName(b)});
}

$ zig build-exe test.zig
$ ./test
invalid enum value
/deps/zig/docgen_tmp/test.zig:11:13: 0x22a8cc in main (test)
    var b = @intToEnum(Foo, a);
            ^
/deps/zig/lib/std/start.zig:243:22: 0x20479f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20457f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Invalid Error Set Cast

At compile-time:

test.zig

const Set1 = error{
    A,
    B,
};
const Set2 = error{
    A,
    C,
};
comptime {
    _ = @errSetCast(Set2, Set1.B);
}

$ zig test test.zig
./docgen_tmp/test.zig:10:9: error: error.B not a member of error set 'Set2'
    _ = @errSetCast(Set2, Set1.B);
        ^

At runtime:

test.zig

const std = @import("std");
const Set1 = error{
    A,
    B,
};
const Set2 = error{
    A,
    C,
};
pub fn main() void {
    foo(Set1.B);
}
fn foo(set1: Set1) void {
    const x = @errSetCast(Set2, set1);
    std.debug.warn("value: {}\n", .{x});
}

$ zig build-exe test.zig
$ ./test
invalid error code
/deps/zig/docgen_tmp/test.zig:15:15: 0x22ea4c in foo (test)
    const x = @errSetCast(Set2, set1);
              ^
/deps/zig/docgen_tmp/test.zig:12:8: 0x22a89d in main (test)
    foo(Set1.B);
       ^
/deps/zig/lib/std/start.zig:243:22: 0x20479f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x20457f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Incorrect Pointer Alignment

At compile-time:

test.zig

comptime {
    const ptr = @intToPtr(*align(1) i32, 0x1);
    const aligned = @alignCast(4, ptr);
}

$ zig test test.zig
./docgen_tmp/test.zig:3:35: error: pointer address 0x1 is not aligned to 4 bytes
    const aligned = @alignCast(4, ptr);
                                  ^
./docgen_tmp/test.zig:3:21: note: referenced here
    const aligned = @alignCast(4, ptr);
                    ^

At runtime:

test.zig

const mem = @import("std").mem;
pub fn main() !void {
    var array align(4) = [_]u32{ 0x11111111, 0x11111111 };
    const bytes = mem.sliceAsBytes(array[0..]);
    if (foo(bytes) != 0x11111111) return error.Wrong;
}
fn foo(bytes: []u8) u32 {
    const slice4 = bytes[1..5];
    const int_slice = mem.bytesAsSlice(u32, @alignCast(4, slice4));
    return int_slice[0];
}

$ zig build-exe test.zig
$ ./test
incorrect alignment
/deps/zig/docgen_tmp/test.zig:9:59: 0x22edbb in foo (test)
    const int_slice = mem.bytesAsSlice(u32, @alignCast(4, slice4));
                                                          ^
/deps/zig/docgen_tmp/test.zig:5:12: 0x22aa49 in main (test)
    if (foo(bytes) != 0x11111111) return error.Wrong;
           ^
/deps/zig/lib/std/start.zig:253:37: 0x2047ed in std.start.posixCallMainAndExit (test)
            const result = root.main() catch |err| {
                                    ^
/deps/zig/lib/std/start.zig:123:5: 0x20452f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

Wrong Union Field Access

At compile-time:

test.zig

comptime {
    var f = Foo{ .int = 42 };
    f.float = 12.34;
}
const Foo = union {
    float: f32,
    int: u32,
};

$ zig test test.zig
./docgen_tmp/test.zig:3:6: error: accessing union field 'float' while field 'int' is set
    f.float = 12.34;
     ^

At runtime:

test.zig

const std = @import("std");
const Foo = union {
    float: f32,
    int: u32,
};
pub fn main() void {
    var f = Foo{ .int = 42 };
    bar(&f);
}
fn bar(f: *Foo) void {
    f.float = 12.34;
    std.debug.warn("value: {}\n", .{f.float});
}

$ zig build-exe test.zig
$ ./test
access of inactive union field
/deps/zig/docgen_tmp/test.zig:14:6: 0x23c21a in bar (test)
    f.float = 12.34;
     ^
/deps/zig/docgen_tmp/test.zig:10:8: 0x23806c in main (test)
    bar(&f);
       ^
/deps/zig/lib/std/start.zig:243:22: 0x211f5f in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x211d3f in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)

This safety is not available for extern or packed unions.

To change the active field of a union, assign the entire union, like this:

test.zig

const std = @import("std");
const Foo = union {
    float: f32,
    int: u32,
};
pub fn main() void {
    var f = Foo{ .int = 42 };
    bar(&f);
}
fn bar(f: *Foo) void {
    f.* = Foo{ .float = 12.34 };
    std.debug.warn("value: {}\n", .{f.float});
}

$ zig build-exe test.zig
$ ./test
value: 1.23400001e+01

To change the active field of a union when a meaningful value for the field is not known, use undefined, like this:

test.zig

const std = @import("std");
const Foo = union {
    float: f32,
    int: u32,
};
pub fn main() void {
    var f = Foo{ .int = 42 };
    f = Foo{ .float = undefined };
    bar(&f);
    std.debug.warn("value: {}\n", .{f.float});
}
fn bar(f: *Foo) void {
    f.float = 12.34;
}

$ zig build-exe test.zig
$ ./test
value: 1.23400001e+01

See also:

• union
• extern union

Out of Bounds Float to Integer Cast

TODO

Pointer Cast Invalid Null

This happens when casting a pointer with the address 0 to a pointer which may not have the address 0. For example, C Pointers, Optional Pointers, and allowzero pointers allow address zero, but normal Pointers do not.

At compile-time:

test.zig

comptime {
    const opt_ptr: ?*i32 = null;
    const ptr = @ptrCast(*i32, opt_ptr);
}

$ zig test test.zig
./docgen_tmp/test.zig:3:17: error: null pointer casted to type '*i32'
    const ptr = @ptrCast(*i32, opt_ptr);
                ^

At runtime:

test.zig

pub fn main() void {
    var opt_ptr: ?*i32 = null;
    var ptr = @ptrCast(*i32, opt_ptr);
}

$ zig build-exe test.zig
$ ./test
cast causes pointer to be null
/deps/zig/docgen_tmp/test.zig:3:15: 0x22a800 in main (test)
    var ptr = @ptrCast(*i32, opt_ptr);
              ^
/deps/zig/lib/std/start.zig:243:22: 0x2046df in std.start.posixCallMainAndExit (test)
            root.main();
                     ^
/deps/zig/lib/std/start.zig:123:5: 0x2044bf in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
(process terminated by signal)